lang: ast, gapi, interfaces, parser: Print line numbers on error

This adds an initial implementation of printing line numbers on type
unification errors. It also attempts to print a visual position
indicator for most scenarios.

This patch was started by Felix Frank and finished by James Shubin.

Co-authored-by: Felix Frank <Felix.Frank.de@gmail.com>

lang/ast/structs.go

@@ -180,6 +180,9 @@ var (
 // StmtBind is a representation of an assignment, which binds a variable to an
 // expression.
 type StmtBind struct {
+	Textarea
+	data *interfaces.Data
+
 	Ident string
 	Value interfaces.Expr
 	Type  *types.Type
@@ -205,6 +208,9 @@ func (obj *StmtBind) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtBind) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if obj.Ident == "" {
 		return fmt.Errorf("bind ident is empty")
 	}
@@ -221,6 +227,8 @@ func (obj *StmtBind) Interpolate() (interfaces.Stmt, error) {
 		return nil, err
 	}
 	return &StmtBind{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		Ident:    obj.Ident,
 		Value:    interpolated,
 		Type:     obj.Type,
@@ -242,6 +250,8 @@ func (obj *StmtBind) Copy() (interfaces.Stmt, error) {
 		return obj, nil
 	}
 	return &StmtBind{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		Ident:    obj.Ident,
 		Value:    value,
 		Type:     obj.Type,
@@ -319,6 +329,7 @@ func (obj *StmtBind) TypeCheck() ([]*interfaces.UnificationInvariant, error) {
 	}
 
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj.Value,
 		Expect: typExpr, // obj.Type
 		Actual: typ,
@@ -359,6 +370,7 @@ func (obj *StmtBind) Output(map[interfaces.Func]types.Value) (*interfaces.Output
 // TODO: Consider expanding Name to have this return a list of Res's in the
 // Output function if it is a map[name]struct{}, or even a map[[]name]struct{}.
 type StmtRes struct {
+	Textarea
 	data *interfaces.Data
 
 	Kind     string            // kind of resource, eg: pkg, file, svc, etc...
@@ -393,6 +405,9 @@ func (obj *StmtRes) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtRes) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if obj.Kind == "" {
 		return fmt.Errorf("res kind is empty")
 	}
@@ -401,7 +416,6 @@ func (obj *StmtRes) Init(data *interfaces.Data) error {
 		return fmt.Errorf("kind must not contain underscores")
 	}
 
-	obj.data = data
 	if err := obj.Name.Init(data); err != nil {
 		return err
 	}
@@ -457,6 +471,7 @@ func (obj *StmtRes) Interpolate() (interfaces.Stmt, error) {
 	}
 
 	return &StmtRes{
+		Textarea: obj.Textarea,
 		data:     obj.data,
 		Kind:     obj.Kind,
 		Name:     name,
@@ -497,6 +512,7 @@ func (obj *StmtRes) Copy() (interfaces.Stmt, error) {
 		return obj, nil
 	}
 	return &StmtRes{
+		Textarea: obj.Textarea,
 		data:     obj.data,
 		Kind:     obj.Kind,
 		Name:     name,
@@ -628,6 +644,7 @@ func (obj *StmtRes) TypeCheck() ([]*interfaces.UnificationInvariant, error) {
 	}
 
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj.Name,
 		Expect: typExpr, // the name
 		Actual: typ,
@@ -1201,6 +1218,9 @@ type StmtResContents interface {
 // StmtResField represents a single field in the parsed resource representation.
 // This does not satisfy the Stmt interface.
 type StmtResField struct {
+	Textarea
+	data *interfaces.Data
+
 	Field        string
 	Value        interfaces.Expr
 	valuePtr     interfaces.Func // ptr for table lookup
@@ -1234,6 +1254,9 @@ func (obj *StmtResField) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtResField) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if obj.Field == "" {
 		return fmt.Errorf("res field name is empty")
 	}
@@ -1264,6 +1287,8 @@ func (obj *StmtResField) Interpolate() (StmtResContents, error) {
 		}
 	}
 	return &StmtResField{
+		Textarea:  obj.Textarea,
+		data:      obj.data,
 		Field:     obj.Field,
 		Value:     interpolated,
 		Condition: condition,
@@ -1297,6 +1322,8 @@ func (obj *StmtResField) Copy() (StmtResContents, error) {
 		return obj, nil
 	}
 	return &StmtResField{
+		Textarea:  obj.Textarea,
+		data:      obj.data,
 		Field:     obj.Field,
 		Value:     value,
 		Condition: condition,
@@ -1388,6 +1415,7 @@ func (obj *StmtResField) TypeCheck(kind string) ([]*interfaces.UnificationInvari
 
 		// XXX: Is this needed?
 		invar := &interfaces.UnificationInvariant{
+			Node:   obj,
 			Expr:   obj.Condition,
 			Expect: types.TypeBool,
 			Actual: typ,
@@ -1428,6 +1456,7 @@ func (obj *StmtResField) TypeCheck(kind string) ([]*interfaces.UnificationInvari
 
 	// regular scenario
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj.Value,
 		Expect: typExpr,
 		Actual: typ,
@@ -1474,6 +1503,9 @@ func (obj *StmtResField) Graph() (*pgraph.Graph, error) {
 // StmtResEdge represents a single edge property in the parsed resource
 // representation. This does not satisfy the Stmt interface.
 type StmtResEdge struct {
+	Textarea
+	data *interfaces.Data
+
 	Property     string // TODO: iota constant instead?
 	EdgeHalf     *StmtEdgeHalf
 	Condition    interfaces.Expr // the value will be used if nil or true
@@ -1506,6 +1538,9 @@ func (obj *StmtResEdge) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtResEdge) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if obj.Property == "" {
 		return fmt.Errorf("res edge property is empty")
 	}
@@ -1539,6 +1574,8 @@ func (obj *StmtResEdge) Interpolate() (StmtResContents, error) {
 		}
 	}
 	return &StmtResEdge{
+		Textarea:  obj.Textarea,
+		data:      obj.data,
 		Property:  obj.Property,
 		EdgeHalf:  interpolated,
 		Condition: condition,
@@ -1571,6 +1608,8 @@ func (obj *StmtResEdge) Copy() (StmtResContents, error) {
 		return obj, nil
 	}
 	return &StmtResEdge{
+		Textarea:  obj.Textarea,
+		data:      obj.data,
 		Property:  obj.Property,
 		EdgeHalf:  edgeHalf,
 		Condition: condition,
@@ -1663,6 +1702,7 @@ func (obj *StmtResEdge) TypeCheck(kind string) ([]*interfaces.UnificationInvaria
 
 		// XXX: Is this needed?
 		invar := &interfaces.UnificationInvariant{
+			Node:   obj,
 			Expr:   obj.Condition,
 			Expect: types.TypeBool,
 			Actual: typ,
@@ -1713,6 +1753,9 @@ func (obj *StmtResEdge) Graph() (*pgraph.Graph, error) {
 // correspond to the particular meta parameter specified. This does not satisfy
 // the Stmt interface.
 type StmtResMeta struct {
+	Textarea
+	data *interfaces.Data
+
 	Property     string // TODO: iota constant instead?
 	MetaExpr     interfaces.Expr
 	metaExprPtr  interfaces.Func // ptr for table lookup
@@ -1746,6 +1789,9 @@ func (obj *StmtResMeta) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtResMeta) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if obj.Property == "" {
 		return fmt.Errorf("res meta property is empty")
 	}
@@ -1799,6 +1845,8 @@ func (obj *StmtResMeta) Interpolate() (StmtResContents, error) {
 		}
 	}
 	return &StmtResMeta{
+		Textarea:  obj.Textarea,
+		data:      obj.data,
 		Property:  obj.Property,
 		MetaExpr:  interpolated,
 		Condition: condition,
@@ -1831,6 +1879,8 @@ func (obj *StmtResMeta) Copy() (StmtResContents, error) {
 		return obj, nil
 	}
 	return &StmtResMeta{
+		Textarea:  obj.Textarea,
+		data:      obj.data,
 		Property:  obj.Property,
 		MetaExpr:  metaExpr,
 		Condition: condition,
@@ -1924,6 +1974,7 @@ func (obj *StmtResMeta) TypeCheck(kind string) ([]*interfaces.UnificationInvaria
 
 		// XXX: Is this needed?
 		invar := &interfaces.UnificationInvariant{
+			Node:   obj,
 			Expr:   obj.Condition,
 			Expect: types.TypeBool,
 			Actual: typ,
@@ -2002,6 +2053,7 @@ func (obj *StmtResMeta) TypeCheck(kind string) ([]*interfaces.UnificationInvaria
 	}
 
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj.MetaExpr,
 		Expect: typExpr,
 		Actual: typ,
@@ -2055,6 +2107,9 @@ func (obj *StmtResMeta) Graph() (*pgraph.Graph, error) {
 // names are compatible and listed. In this case of Send/Recv, only lists of
 // length two are legal.
 type StmtEdge struct {
+	Textarea
+	data *interfaces.Data
+
 	EdgeHalfList []*StmtEdgeHalf // represents a chain of edges
 
 	// TODO: should notify be an Expr?
@@ -2083,6 +2138,9 @@ func (obj *StmtEdge) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtEdge) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	for _, x := range obj.EdgeHalfList {
 		if err := x.Init(data); err != nil {
 			return err
@@ -2109,6 +2167,8 @@ func (obj *StmtEdge) Interpolate() (interfaces.Stmt, error) {
 	}
 
 	return &StmtEdge{
+		Textarea:     obj.Textarea,
+		data:         obj.data,
 		EdgeHalfList: edgeHalfList,
 		Notify:       obj.Notify,
 	}, nil
@@ -2133,6 +2193,8 @@ func (obj *StmtEdge) Copy() (interfaces.Stmt, error) {
 		return obj, nil
 	}
 	return &StmtEdge{
+		Textarea:     obj.Textarea,
+		data:         obj.data,
 		EdgeHalfList: edgeHalfList,
 		Notify:       obj.Notify,
 	}, nil
@@ -2394,6 +2456,9 @@ func (obj *StmtEdge) Output(table map[interfaces.Func]types.Value) (*interfaces.
 // is assumed that a list of strings should be expected. More mechanisms to
 // determine if the value is static may be added over time.
 type StmtEdgeHalf struct {
+	Textarea
+	data *interfaces.Data
+
 	Kind     string          // kind of resource, eg: pkg, file, svc, etc...
 	Name     interfaces.Expr // unique name for the res of this kind
 	namePtr  interfaces.Func // ptr for table lookup
@@ -2421,6 +2486,8 @@ func (obj *StmtEdgeHalf) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtEdgeHalf) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
 	if obj.Kind == "" {
 		return fmt.Errorf("edge half kind is empty")
 	}
@@ -2443,6 +2510,7 @@ func (obj *StmtEdgeHalf) Interpolate() (*StmtEdgeHalf, error) {
 	}
 
 	return &StmtEdgeHalf{
+		Textarea: obj.Textarea,
 		Kind:     obj.Kind,
 		Name:     name,
 		SendRecv: obj.SendRecv,
@@ -2464,6 +2532,7 @@ func (obj *StmtEdgeHalf) Copy() (*StmtEdgeHalf, error) {
 		return obj, nil
 	}
 	return &StmtEdgeHalf{
+		Textarea: obj.Textarea,
 		Kind:     obj.Kind,
 		Name:     name,
 		SendRecv: obj.SendRecv,
@@ -2523,6 +2592,7 @@ func (obj *StmtEdgeHalf) TypeCheck() ([]*interfaces.UnificationInvariant, error)
 	}
 
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj.Name,
 		Expect: typExpr, // the name
 		Actual: typ,
@@ -2557,6 +2627,9 @@ func (obj *StmtEdgeHalf) Graph() (*pgraph.Graph, error) {
 // optional, it is the else branch, although this struct allows either to be
 // optional, even if it is not commonly used.
 type StmtIf struct {
+	Textarea
+	data *interfaces.Data
+
 	Condition    interfaces.Expr
 	conditionPtr interfaces.Func // ptr for table lookup
 	ThenBranch   interfaces.Stmt // optional, but usually present
@@ -2604,6 +2677,9 @@ func (obj *StmtIf) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtIf) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if err := obj.Condition.Init(data); err != nil {
 		return err
 	}
@@ -2643,6 +2719,8 @@ func (obj *StmtIf) Interpolate() (interfaces.Stmt, error) {
 		}
 	}
 	return &StmtIf{
+		Textarea:   obj.Textarea,
+		data:       obj.data,
 		Condition:  condition,
 		ThenBranch: thenBranch,
 		ElseBranch: elseBranch,
@@ -2685,6 +2763,8 @@ func (obj *StmtIf) Copy() (interfaces.Stmt, error) {
 		return obj, nil
 	}
 	return &StmtIf{
+		Textarea:   obj.Textarea,
+		data:       obj.data,
 		Condition:  condition,
 		ThenBranch: thenBranch,
 		ElseBranch: elseBranch,
@@ -2807,6 +2887,7 @@ func (obj *StmtIf) TypeCheck() ([]*interfaces.UnificationInvariant, error) {
 
 	typExpr := types.TypeBool // default
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj.Condition,
 		Expect: typExpr, // the condition
 		Actual: typ,
@@ -2915,6 +2996,7 @@ func (obj *StmtIf) Output(table map[interfaces.Func]types.Value) (*interfaces.Ou
 // the bind statement's are correctly applied in this scope, and irrespective of
 // their order of definition.
 type StmtProg struct {
+	Textarea
 	data  *interfaces.Data
 	scope *interfaces.Scope // store for use by imports
 
@@ -3038,6 +3120,7 @@ func (obj *StmtProg) Interpolate() (interfaces.Stmt, error) {
 		body = append(body, interpolated)
 	}
 	return &StmtProg{
+		Textarea:    obj.Textarea,
 		data:        obj.data,
 		scope:       obj.scope,
 		importProgs: obj.importProgs, // TODO: do we even need this here?
@@ -3065,6 +3148,7 @@ func (obj *StmtProg) Copy() (interfaces.Stmt, error) {
 		return obj, nil
 	}
 	return &StmtProg{
+		Textarea:    obj.Textarea,
 		data:        obj.data,
 		scope:       obj.scope,
 		importProgs: obj.importProgs, // TODO: do we even need this here?
@@ -3646,6 +3730,7 @@ func (obj *StmtProg) importScopeWithParsedInputs(input *inputs.ParsedInput, scop
 		LexParser:       obj.data.LexParser,
 		Downloader:      obj.data.Downloader,
 		StrInterpolater: obj.data.StrInterpolater,
+		SourceFinder:    obj.data.SourceFinder,
 		//World: obj.data.World, // TODO: do we need this?
 
 		//Prefix: obj.Prefix, // TODO: add a path on?
@@ -4326,6 +4411,9 @@ func (obj *StmtProg) IsModuleUnsafe() error { // TODO: rename this function?
 // the supplied function in the current scope and irrespective of the order of
 // definition.
 type StmtFunc struct {
+	Textarea
+	data *interfaces.Data
+
 	Name string
 	Func interfaces.Expr
 	Type *types.Type
@@ -4351,11 +4439,13 @@ func (obj *StmtFunc) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtFunc) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if obj.Name == "" {
 		return fmt.Errorf("func name is empty")
 	}
 
-	//obj.data = data // TODO: ???
 	if err := obj.Func.Init(data); err != nil {
 		return err
 	}
@@ -4373,6 +4463,8 @@ func (obj *StmtFunc) Interpolate() (interfaces.Stmt, error) {
 	}
 
 	return &StmtFunc{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		Name:     obj.Name,
 		Func:     interpolated,
 		Type:     obj.Type,
@@ -4394,6 +4486,8 @@ func (obj *StmtFunc) Copy() (interfaces.Stmt, error) {
 		return obj, nil
 	}
 	return &StmtFunc{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		Name:     obj.Name,
 		Func:     fn,
 		Type:     obj.Type,
@@ -4487,6 +4581,7 @@ func (obj *StmtFunc) TypeCheck() ([]*interfaces.UnificationInvariant, error) {
 	}
 
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj.Func,
 		Expect: typExpr, // obj.Type
 		Actual: typ,
@@ -4530,6 +4625,8 @@ func (obj *StmtFunc) Output(map[interfaces.Func]types.Value) (*interfaces.Output
 // TODO: We don't currently support defining polymorphic classes (eg: different
 // signatures for the same class name) but it might be something to consider.
 type StmtClass struct {
+	Textarea
+	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 
 	Name string
@@ -4557,6 +4654,9 @@ func (obj *StmtClass) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtClass) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if obj.Name == "" {
 		return fmt.Errorf("class name is empty")
 	}
@@ -4579,6 +4679,8 @@ func (obj *StmtClass) Interpolate() (interfaces.Stmt, error) {
 	}
 
 	return &StmtClass{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		Name:     obj.Name,
 		Args:     args, // ensure this has length == 0 instead of nil
@@ -4606,6 +4708,8 @@ func (obj *StmtClass) Copy() (interfaces.Stmt, error) {
 		return obj, nil
 	}
 	return &StmtClass{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		Name:     obj.Name,
 		Args:     args, // ensure this has length == 0 instead of nil
@@ -4731,6 +4835,9 @@ func (obj *StmtClass) Output(table map[interfaces.Func]types.Value) (*interfaces
 // to call a class except that it produces output instead of a value. Most of
 // the interesting logic for classes happens here or in StmtProg.
 type StmtInclude struct {
+	Textarea
+	data *interfaces.Data
+
 	class *StmtClass   // copy of class that we're using
 	orig  *StmtInclude // original pointer to this
 
@@ -4770,6 +4877,9 @@ func (obj *StmtInclude) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *StmtInclude) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if obj.Name == "" {
 		return fmt.Errorf("include name is empty")
 	}
@@ -4800,6 +4910,8 @@ func (obj *StmtInclude) Interpolate() (interfaces.Stmt, error) {
 		orig = obj.orig
 	}
 	return &StmtInclude{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		//class: obj.class, // TODO: is this necessary?
 		orig:  orig,
 		Name:  obj.Name,
@@ -4834,6 +4946,8 @@ func (obj *StmtInclude) Copy() (interfaces.Stmt, error) {
 		return obj, nil
 	}
 	return &StmtInclude{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		//class: obj.class, // TODO: is this necessary?
 		orig:  orig,
 		Name:  obj.Name,
@@ -5051,6 +5165,7 @@ func (obj *StmtInclude) TypeCheck() ([]*interfaces.UnificationInvariant, error)
 		// add invariants between the args and the class
 		if typExpr := obj.class.Args[i].Type; typExpr != nil {
 			invar := &interfaces.UnificationInvariant{
+				Node:   obj,
 				Expr:   x,
 				Expect: typExpr, // type of arg
 				Actual: typ,
@@ -5100,6 +5215,9 @@ func (obj *StmtInclude) Output(table map[interfaces.Func]types.Value) (*interfac
 // file. As with any statement, it produces output, but that output is empty. To
 // benefit from its inclusion, reference the scope definitions you want.
 type StmtImport struct {
+	Textarea
+	data *interfaces.Data
+
 	Name  string
 	Alias string
 }
@@ -5118,7 +5236,9 @@ func (obj *StmtImport) Apply(fn func(interfaces.Node) error) error { return fn(o
 
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
-func (obj *StmtImport) Init(*interfaces.Data) error {
+func (obj *StmtImport) Init(data *interfaces.Data) error {
+	obj.Textarea.Setup(data)
+
 	if obj.Name == "" {
 		return fmt.Errorf("import name is empty")
 	}
@@ -5130,6 +5250,8 @@ func (obj *StmtImport) Init(*interfaces.Data) error {
 // on any child elements and builds the new node with those new node contents.
 func (obj *StmtImport) Interpolate() (interfaces.Stmt, error) {
 	return &StmtImport{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		Name:     obj.Name,
 		Alias:    obj.Alias,
 	}, nil
@@ -5202,6 +5324,8 @@ func (obj *StmtImport) Output(map[interfaces.Func]types.Value) (*interfaces.Outp
 // formatting) but so that they can exist anywhere in the code. Currently these
 // are dropped by the lexer.
 type StmtComment struct {
+	Textarea
+
 	Value string
 }
 
@@ -5219,7 +5343,9 @@ func (obj *StmtComment) Apply(fn func(interfaces.Node) error) error { return fn(
 
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
-func (obj *StmtComment) Init(*interfaces.Data) error {
+func (obj *StmtComment) Init(data *interfaces.Data) error {
+	obj.Textarea.Setup(data)
+
 	return nil
 }
 
@@ -5280,6 +5406,8 @@ func (obj *StmtComment) Output(map[interfaces.Func]types.Value) (*interfaces.Out
 
 // ExprBool is a representation of a boolean.
 type ExprBool struct {
+	Textarea
+	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 
 	V bool
@@ -5297,7 +5425,11 @@ func (obj *ExprBool) Apply(fn func(interfaces.Node) error) error { return fn(obj
 
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
-func (obj *ExprBool) Init(*interfaces.Data) error { return nil }
+func (obj *ExprBool) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+	return nil
+}
 
 // Interpolate returns a new node (aka a copy) once it has been expanded. This
 // generally increases the size of the AST when it is used. It calls Interpolate
@@ -5305,6 +5437,8 @@ func (obj *ExprBool) Init(*interfaces.Data) error { return nil }
 // Here it simply returns itself, as no interpolation is possible.
 func (obj *ExprBool) Interpolate() (interfaces.Expr, error) {
 	return &ExprBool{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		V:        obj.V,
 	}, nil
@@ -5359,6 +5493,7 @@ func (obj *ExprBool) Infer() (*types.Type, []*interfaces.UnificationInvariant, e
 	// This adds the obj ptr, so it's seen as an expr that we need to solve.
 	return types.TypeBool, []*interfaces.UnificationInvariant{
 		{
+			Node:   obj,
 			Expr:   obj,
 			Expect: types.TypeBool,
 			Actual: types.TypeBool,
@@ -5424,6 +5559,7 @@ func (obj *ExprBool) Value() (types.Value, error) {
 
 // ExprStr is a representation of a string.
 type ExprStr struct {
+	Textarea
 	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 
@@ -5444,6 +5580,7 @@ func (obj *ExprStr) Apply(fn func(interfaces.Node) error) error { return fn(obj)
 // validate.
 func (obj *ExprStr) Init(data *interfaces.Data) error {
 	obj.data = data
+	obj.Textarea.Setup(data)
 	return nil
 }
 
@@ -5455,6 +5592,7 @@ func (obj *ExprStr) Init(data *interfaces.Data) error {
 // a function which returns a string as its root. Otherwise it returns itself.
 func (obj *ExprStr) Interpolate() (interfaces.Expr, error) {
 	pos := &interfaces.Pos{
+		// XXX: populate this?
 		// column/line number, starting at 1
 		//Column: -1, // TODO
 		//Line: -1, // TODO
@@ -5474,6 +5612,7 @@ func (obj *ExprStr) Interpolate() (interfaces.Expr, error) {
 		LexParser:       obj.data.LexParser,
 		Downloader:      obj.data.Downloader,
 		StrInterpolater: obj.data.StrInterpolater,
+		SourceFinder:    obj.data.SourceFinder,
 		//World: obj.data.World, // TODO: do we need this?
 
 		Prefix: obj.data.Prefix,
@@ -5489,6 +5628,7 @@ func (obj *ExprStr) Interpolate() (interfaces.Expr, error) {
 	}
 	if result == nil {
 		return &ExprStr{
+			Textarea: obj.Textarea,
 			data:     obj.data,
 			scope:    obj.scope,
 			V:        obj.V,
@@ -5556,6 +5696,7 @@ func (obj *ExprStr) Infer() (*types.Type, []*interfaces.UnificationInvariant, er
 	// This adds the obj ptr, so it's seen as an expr that we need to solve.
 	return types.TypeStr, []*interfaces.UnificationInvariant{
 		{
+			Node:   obj,
 			Expr:   obj,
 			Expect: types.TypeStr,
 			Actual: types.TypeStr,
@@ -5620,6 +5761,8 @@ func (obj *ExprStr) Value() (types.Value, error) {
 
 // ExprInt is a representation of an int.
 type ExprInt struct {
+	Textarea
+	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 
 	V int64
@@ -5637,7 +5780,11 @@ func (obj *ExprInt) Apply(fn func(interfaces.Node) error) error { return fn(obj)
 
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
-func (obj *ExprInt) Init(*interfaces.Data) error { return nil }
+func (obj *ExprInt) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+	return nil
+}
 
 // Interpolate returns a new node (aka a copy) once it has been expanded. This
 // generally increases the size of the AST when it is used. It calls Interpolate
@@ -5645,6 +5792,8 @@ func (obj *ExprInt) Init(*interfaces.Data) error { return nil }
 // Here it simply returns itself, as no interpolation is possible.
 func (obj *ExprInt) Interpolate() (interfaces.Expr, error) {
 	return &ExprInt{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		V:        obj.V,
 	}, nil
@@ -5699,6 +5848,7 @@ func (obj *ExprInt) Infer() (*types.Type, []*interfaces.UnificationInvariant, er
 	// This adds the obj ptr, so it's seen as an expr that we need to solve.
 	return types.TypeInt, []*interfaces.UnificationInvariant{
 		{
+			Node:   obj,
 			Expr:   obj,
 			Expect: types.TypeInt,
 			Actual: types.TypeInt,
@@ -5763,6 +5913,8 @@ func (obj *ExprInt) Value() (types.Value, error) {
 
 // ExprFloat is a representation of a float.
 type ExprFloat struct {
+	Textarea
+	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 
 	V float64
@@ -5782,7 +5934,11 @@ func (obj *ExprFloat) Apply(fn func(interfaces.Node) error) error { return fn(ob
 
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
-func (obj *ExprFloat) Init(*interfaces.Data) error { return nil }
+func (obj *ExprFloat) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+	return nil
+}
 
 // Interpolate returns a new node (aka a copy) once it has been expanded. This
 // generally increases the size of the AST when it is used. It calls Interpolate
@@ -5790,6 +5946,8 @@ func (obj *ExprFloat) Init(*interfaces.Data) error { return nil }
 // Here it simply returns itself, as no interpolation is possible.
 func (obj *ExprFloat) Interpolate() (interfaces.Expr, error) {
 	return &ExprFloat{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		V:        obj.V,
 	}, nil
@@ -5844,6 +6002,7 @@ func (obj *ExprFloat) Infer() (*types.Type, []*interfaces.UnificationInvariant,
 	// This adds the obj ptr, so it's seen as an expr that we need to solve.
 	return types.TypeFloat, []*interfaces.UnificationInvariant{
 		{
+			Node:   obj,
 			Expr:   obj,
 			Expect: types.TypeFloat,
 			Actual: types.TypeFloat,
@@ -5908,6 +6067,8 @@ func (obj *ExprFloat) Value() (types.Value, error) {
 
 // ExprList is a representation of a list.
 type ExprList struct {
+	Textarea
+	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 	typ   *types.Type
 
@@ -5941,6 +6102,9 @@ func (obj *ExprList) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *ExprList) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	for _, x := range obj.Elements {
 		if err := x.Init(data); err != nil {
 			return err
@@ -5962,6 +6126,8 @@ func (obj *ExprList) Interpolate() (interfaces.Expr, error) {
 		elements = append(elements, interpolated)
 	}
 	return &ExprList{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
 		Elements: elements,
@@ -5987,6 +6153,8 @@ func (obj *ExprList) Copy() (interfaces.Expr, error) {
 		return obj, nil
 	}
 	return &ExprList{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
 		Elements: elements,
@@ -6136,6 +6304,7 @@ func (obj *ExprList) Infer() (*types.Type, []*interfaces.UnificationInvariant, e
 	}
 	// This must be added even if redundant, so that we collect the obj ptr.
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj,
 		Expect: typExpr, // This is the type that we return.
 		Actual: typType,
@@ -6262,6 +6431,8 @@ func (obj *ExprList) Value() (types.Value, error) {
 
 // ExprMap is a representation of a (dictionary) map.
 type ExprMap struct {
+	Textarea
+	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 	typ   *types.Type
 
@@ -6297,6 +6468,9 @@ func (obj *ExprMap) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *ExprMap) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	// XXX: Can we check that there aren't any duplicate keys? Can we Cmp?
 	for _, x := range obj.KVs {
 		if err := x.Key.Init(data); err != nil {
@@ -6330,6 +6504,8 @@ func (obj *ExprMap) Interpolate() (interfaces.Expr, error) {
 		kvs = append(kvs, kv)
 	}
 	return &ExprMap{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
 		KVs:      kvs,
@@ -6373,6 +6549,8 @@ func (obj *ExprMap) Copy() (interfaces.Expr, error) {
 		return obj, nil
 	}
 	return &ExprMap{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
 		KVs:      kvs,
@@ -6577,6 +6755,7 @@ func (obj *ExprMap) Infer() (*types.Type, []*interfaces.UnificationInvariant, er
 	}
 	// This must be added even if redundant, so that we collect the obj ptr.
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj,
 		Expect: typExpr, // This is the type that we return.
 		Actual: typType,
@@ -6741,12 +6920,16 @@ func (obj *ExprMap) Value() (types.Value, error) {
 // ExprMapKV represents a key and value pair in a (dictionary) map. This does
 // not satisfy the Expr interface.
 type ExprMapKV struct {
+	Textarea
+
 	Key interfaces.Expr // keys can be strings, int's, etc...
 	Val interfaces.Expr
 }
 
 // ExprStruct is a representation of a struct.
 type ExprStruct struct {
+	Textarea
+	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 	typ   *types.Type
 
@@ -6779,6 +6962,9 @@ func (obj *ExprStruct) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *ExprStruct) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	fields := make(map[string]struct{})
 	for _, x := range obj.Fields {
 		// Validate field names and ensure no duplicates!
@@ -6811,6 +6997,8 @@ func (obj *ExprStruct) Interpolate() (interfaces.Expr, error) {
 		fields = append(fields, field)
 	}
 	return &ExprStruct{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
 		Fields:   fields,
@@ -6842,6 +7030,8 @@ func (obj *ExprStruct) Copy() (interfaces.Expr, error) {
 		return obj, nil
 	}
 	return &ExprStruct{
+		Textarea: obj.Textarea,
+		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
 		Fields:   fields,
@@ -7001,6 +7191,7 @@ func (obj *ExprStruct) Infer() (*types.Type, []*interfaces.UnificationInvariant,
 	}
 	// This must be added even if redundant, so that we collect the obj ptr.
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj,
 		Expect: typExpr, // This is the type that we return.
 		Actual: typType,
@@ -7133,6 +7324,8 @@ func (obj *ExprStruct) Value() (types.Value, error) {
 // ExprStructField represents a name value pair in a struct field. This does not
 // satisfy the Expr interface.
 type ExprStructField struct {
+	Textarea
+
 	Name  string
 	Value interfaces.Expr
 }
@@ -7147,6 +7340,7 @@ type ExprStructField struct {
 // 4. A pure built-in function (set Values to a singleton)
 // 5. A pure polymorphic built-in function (set Values to a list)
 type ExprFunc struct {
+	Textarea
 	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 	typ   *types.Type
@@ -7310,6 +7504,7 @@ func (obj *ExprFunc) Interpolate() (interfaces.Expr, error) {
 	}
 
 	return &ExprFunc{
+		Textarea: obj.Textarea,
 		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
@@ -7405,6 +7600,7 @@ func (obj *ExprFunc) Copy() (interfaces.Expr, error) {
 		return obj, nil
 	}
 	return &ExprFunc{
+		Textarea: obj.Textarea,
 		data:     obj.data,
 		scope:    obj.scope, // TODO: copy?
 		typ:      obj.typ,
@@ -7749,6 +7945,7 @@ func (obj *ExprFunc) Infer() (*types.Type, []*interfaces.UnificationInvariant, e
 	}
 	// This must be added even if redundant, so that we collect the obj ptr.
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj,
 		Expect: typExpr, // This is the type that we return.
 		Actual: typType,
@@ -7912,6 +8109,7 @@ func (obj *ExprFunc) Value() (types.Value, error) {
 // declaration or implementation of a new function value. This struct has an
 // analogous symmetry with ExprVar.
 type ExprCall struct {
+	Textarea
 	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 	typ   *types.Type
@@ -7971,6 +8169,7 @@ func (obj *ExprCall) Apply(fn func(interfaces.Node) error) error {
 // validate.
 func (obj *ExprCall) Init(data *interfaces.Data) error {
 	obj.data = data
+	obj.Textarea.Setup(data)
 
 	if obj.Name == "" && obj.Anon == nil {
 		return fmt.Errorf("missing call name")
@@ -8016,6 +8215,7 @@ func (obj *ExprCall) Interpolate() (interfaces.Expr, error) {
 	}
 
 	return &ExprCall{
+		Textarea: obj.Textarea,
 		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
@@ -8088,6 +8288,7 @@ func (obj *ExprCall) Copy() (interfaces.Expr, error) {
 		return obj, nil
 	}
 	return &ExprCall{
+		Textarea: obj.Textarea,
 		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
@@ -8537,6 +8738,7 @@ func (obj *ExprCall) Infer() (*types.Type, []*interfaces.UnificationInvariant, e
 	}
 	// This must be added even if redundant, so that we collect the obj ptr.
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj,
 		Expect: typExpr, // This is the type that we return.
 		Actual: typType,
@@ -8607,6 +8809,7 @@ func (obj *ExprCall) Infer() (*types.Type, []*interfaces.UnificationInvariant, e
 		}
 
 		invar := &interfaces.UnificationInvariant{
+			Node:   obj,
 			Expr:   obj.expr, // this should NOT be obj
 			Expect: typFunc,  // TODO: are these two reversed here?
 			Actual: typFn,
@@ -8743,6 +8946,7 @@ func (obj *ExprCall) Value() (types.Value, error) {
 // ExprVar is a representation of a variable lookup. It returns the expression
 // that that variable refers to.
 type ExprVar struct {
+	Textarea
 	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 	typ   *types.Type
@@ -8775,6 +8979,7 @@ func (obj *ExprVar) Init(data *interfaces.Data) error {
 // support variable, variables or anything crazy like that.
 func (obj *ExprVar) Interpolate() (interfaces.Expr, error) {
 	return &ExprVar{
+		Textarea: obj.Textarea,
 		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
@@ -8789,6 +8994,7 @@ func (obj *ExprVar) Interpolate() (interfaces.Expr, error) {
 // and they won't be able to have different values.
 func (obj *ExprVar) Copy() (interfaces.Expr, error) {
 	return &ExprVar{
+		Textarea: obj.Textarea,
 		data:     obj.data,
 		scope:    obj.scope,
 		typ:      obj.typ,
@@ -8926,6 +9132,7 @@ func (obj *ExprVar) Infer() (*types.Type, []*interfaces.UnificationInvariant, er
 
 	// This adds the obj ptr, so it's seen as an expr that we need to solve.
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj,
 		Expect: typ,
 		Actual: typ,
@@ -9153,6 +9360,7 @@ func (obj *ExprParam) Infer() (*types.Type, []*interfaces.UnificationInvariant,
 
 		// This adds the obj ptr, so it's seen as an expr that we need to solve.
 		invar := &interfaces.UnificationInvariant{
+			Node:   obj,
 			Expr:   obj,
 			Expect: typ,
 			Actual: typ,
@@ -9460,6 +9668,7 @@ func (obj *ExprTopLevel) Infer() (*types.Type, []*interfaces.UnificationInvarian
 
 	// This adds the obj ptr, so it's seen as an expr that we need to solve.
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj,
 		Expect: typ,
 		Actual: typ,
@@ -9654,6 +9863,7 @@ func (obj *ExprSingleton) Infer() (*types.Type, []*interfaces.UnificationInvaria
 		// This adds the obj ptr, so it's seen as an expr that we need
 		// to solve.
 		invar := &interfaces.UnificationInvariant{
+			Node:   obj,
 			Expr:   obj,
 			Expect: typ,
 			Actual: typ,
@@ -9717,6 +9927,8 @@ func (obj *ExprSingleton) Value() (types.Value, error) {
 // returns a value. As a result, it has a type. This is different from a StmtIf,
 // which does not need to have both branches, and which does not return a value.
 type ExprIf struct {
+	Textarea
+	data  *interfaces.Data
 	scope *interfaces.Scope // store for referencing this later
 	typ   *types.Type
 
@@ -9754,6 +9966,9 @@ func (obj *ExprIf) Apply(fn func(interfaces.Node) error) error {
 // Init initializes this branch of the AST, and returns an error if it fails to
 // validate.
 func (obj *ExprIf) Init(data *interfaces.Data) error {
+	obj.data = data
+	obj.Textarea.Setup(data)
+
 	if err := obj.Condition.Init(data); err != nil {
 		return err
 	}
@@ -9785,6 +10000,8 @@ func (obj *ExprIf) Interpolate() (interfaces.Expr, error) {
 		return nil, errwrap.Wrapf(err, "could not interpolate ElseBranch")
 	}
 	return &ExprIf{
+		Textarea:   obj.Textarea,
+		data:       obj.data,
 		scope:      obj.scope,
 		typ:        obj.typ,
 		Condition:  condition,
@@ -9823,6 +10040,8 @@ func (obj *ExprIf) Copy() (interfaces.Expr, error) {
 		return obj, nil
 	}
 	return &ExprIf{
+		Textarea:   obj.Textarea,
+		data:       obj.data,
 		scope:      obj.scope,
 		typ:        obj.typ,
 		Condition:  condition,
@@ -10021,6 +10240,7 @@ func (obj *ExprIf) Infer() (*types.Type, []*interfaces.UnificationInvariant, err
 	}
 	// This must be added even if redundant, so that we collect the obj ptr.
 	invar := &interfaces.UnificationInvariant{
+		Node:   obj,
 		Expr:   obj,
 		Expect: typExpr, // This is the type that we return.
 		Actual: typType,

lang/ast/util.go

@@ -31,6 +31,7 @@ package ast
 
 import (
 	"fmt"
+	"os"
 	"sort"
 	"strings"
 	"sync"
@@ -40,6 +41,7 @@ import (
 	"github.com/purpleidea/mgmt/lang/funcs/vars"
 	"github.com/purpleidea/mgmt/lang/interfaces"
 	"github.com/purpleidea/mgmt/lang/types"
+	"github.com/purpleidea/mgmt/util"
 	"github.com/purpleidea/mgmt/util/errwrap"
 )
 
@@ -393,3 +395,156 @@ func lambdaScopeFeedback(scope *interfaces.Scope, logf func(format string, v ...
 		logf("$%s(...)", name)
 	}
 }
+
+// Textarea stores the coordinates of a statement or expression in the form of a
+// starting line/column and ending line/column.
+type Textarea struct {
+	// debug represents if we're running in debug mode or not.
+	debug bool
+
+	// logf is a logger which should be used.
+	logf func(format string, v ...interface{})
+
+	// sf is the SourceFinder function implementation that maps a filename
+	// to the source.
+	sf interfaces.SourceFinderFunc
+
+	// path is the full path/filename where this text area exists.
+	path string
+
+	// This data is zero-based. (Eg: first line of file is 0)
+	startLine   int // first
+	startColumn int // left
+	endLine     int // last
+	endColumn   int // right
+
+	isSet bool
+
+	// Bug5819 works around issue https://github.com/golang/go/issues/5819
+	Bug5819 interface{} // XXX: workaround
+}
+
+// Setup is used during AST initialization in order to store in each AST node
+// the name of the source file from which it was generated.
+func (obj *Textarea) Setup(data *interfaces.Data) {
+	obj.debug = data.Debug
+	obj.logf = data.Logf
+	obj.sf = data.SourceFinder
+	obj.path = data.AbsFilename()
+}
+
+// IsSet returns if the position was already set with Locate already.
+func (obj *Textarea) IsSet() bool {
+	return obj.isSet
+}
+
+// Locate is used by the parser to store the token positions in AST nodes. The
+// path will be filled during AST node initialization usually, because the
+// parser does not know the name of the file it is processing.
+func (obj *Textarea) Locate(line int, col int, endline int, endcol int) {
+	obj.startLine = line
+	obj.startColumn = col
+	obj.endLine = endline
+	obj.endColumn = endcol
+	obj.isSet = true
+}
+
+// Pos returns the starting line/column of an AST node.
+func (obj *Textarea) Pos() (int, int) {
+	return obj.startLine, obj.startColumn
+}
+
+// End returns the end line/column of an AST node.
+func (obj *Textarea) End() (int, int) {
+	return obj.endLine, obj.endColumn
+}
+
+// Path returns the name of the source file that holds the code for an AST node.
+func (obj *Textarea) Path() string {
+	return obj.path
+}
+
+// Filename returns the printable filename that we'd like to display. It tries
+// to return a relative version if possible.
+func (obj *Textarea) Filename() string {
+	if obj.path == "" {
+		return "<unknown>" // TODO: should this be <stdin> ?
+	}
+
+	wd, _ := os.Getwd() // ignore error since "" would just pass through
+	wd += "/"           // it's a dir
+	if s, err := util.RemoveBasePath(obj.path, wd); err == nil {
+		return s
+	}
+
+	return obj.path
+}
+
+// Byline gives a succinct representation of the Textarea, but is useful only in
+// debugging. In order to generate pretty error messages, see HighlightText.
+func (obj *Textarea) Byline() string {
+	// We convert to 1-based for user display.
+	return fmt.Sprintf("%s @ %d:%d-%d:%d", obj.Filename(), obj.startLine+1, obj.startColumn+1, obj.endLine+1, obj.endColumn+1)
+}
+
+// HighlightText generates a generic description that just visually indicates
+// part of the line described by a Textarea. If the coordinates that are passed
+// span multiple lines, don't show those lines, but just a description of the
+// area. If it can't generate a valid snippet, then it returns the empty string.
+func (obj *Textarea) HighlightText() string {
+	b, err := obj.sf(obj.path) // source finder!
+	if err != nil {
+		return ""
+	}
+	contents := string(b)
+
+	result := &strings.Builder{}
+
+	result.WriteString(obj.Byline())
+
+	lines := strings.Split(contents, "\n")
+	if len(lines) < obj.endLine-1 {
+		// XXX: out of bounds?
+		return ""
+	}
+
+	result.WriteString("\n--\n")
+
+	if obj.startLine == obj.endLine {
+		line := lines[obj.startLine] + "\n"
+		text := strings.TrimLeft(line, " \t")
+		indent := strings.TrimSuffix(line, text)
+		offset := len(indent)
+
+		result.WriteString(line)
+		result.WriteString(indent)
+		result.WriteString(strings.Repeat(" ", obj.startColumn-offset))
+		// TODO: add on the width of the second element as well
+		result.WriteString(strings.Repeat("^", obj.endColumn-obj.startColumn+1))
+		result.WriteString("\n")
+
+		return result.String()
+	}
+
+	line := lines[obj.startLine] + "\n"
+	text := strings.TrimLeft(line, " \t")
+	indent := strings.TrimSuffix(line, text)
+	offset := len(indent)
+
+	result.WriteString(line)
+	result.WriteString(indent)
+	result.WriteString(strings.Repeat(" ", obj.startColumn-offset))
+	result.WriteString("^ from here ...\n")
+
+	line = lines[obj.endLine] + "\n"
+	text = strings.TrimLeft(line, " \t")
+	indent = strings.TrimSuffix(line, text)
+	offset = len(indent)
+
+	result.WriteString(line)
+	result.WriteString(indent)
+	result.WriteString(strings.Repeat(" ", obj.startColumn-offset))
+	result.WriteString("^ ... to here\n")
+
+	return result.String()
+}

lang/gapi/gapi.go

@@ -214,6 +214,7 @@ func (obj *GAPI) Cli(info *gapi.Info) (*gapi.Deploy, error) {
 		LexParser:       parser.LexParse,
 		Downloader:      downloader,
 		StrInterpolater: interpolate.StrInterpolate,
+		SourceFinder:    os.ReadFile,
 		//Local: obj.Local, // TODO: do we need this?
 		//World: obj.World, // TODO: do we need this?
 

lang/interfaces/ast.go

@@ -230,6 +230,15 @@ type Data struct {
 	// cycles.
 	StrInterpolater func(string, *Pos, *Data) (Expr, error)
 
+	// SourceFinder is a function that returns the contents of a source file
+	// when requested by filename. This data is used to annotate error
+	// messages with some context from the source, and as a result is
+	// optional. This function is passed in this way so that the different
+	// consumers of this can use different methods to find the source. The
+	// three main users are: (1) normal GAPI CLI, before the bundle is
+	// created, (2) the main bundled execution, and (3) the tests.
+	SourceFinder SourceFinderFunc
+
 	//World engine.World // TODO: do we need this?
 
 	// Prefix provides a unique path prefix that we can namespace in. It is
@@ -244,6 +253,16 @@ type Data struct {
 	Logf func(format string, v ...interface{})
 }
 
+// AbsFilename returns the absolute filename path to the code this Data struct
+// is running. This is used to pull out a filename for error messages.
+func (obj *Data) AbsFilename() string {
+	// TODO: is this correct? Do we want to check if Metadata is nil?
+	if obj == nil || obj.Metadata == nil { // for tests
+		return ""
+	}
+	return obj.Base + obj.Metadata.Main
+}
+
 // Scope represents a mapping between a variables identifier and the
 // corresponding expression it is bound to. Local scopes in this language exist
 // and are formed by nesting within if statements. Child scopes can shadow
@@ -424,3 +443,39 @@ func EmptyOutput() *Output {
 		Edges:     []*Edge{},
 	}
 }
+
+// PositionableNode is the interface implemented by AST nodes that store their
+// code position. It is implemented by node types that embed Textarea.
+type PositionableNode interface {
+	// IsSet returns if the position was already set with Locate already.
+	IsSet() bool
+
+	// Locate sets the position in zero-based (start line, start column, end
+	// line, end column) format.
+	Locate(int, int, int, int)
+
+	// Pos returns the zero-based start line and then start column position.
+	Pos() (int, int)
+
+	// End returns the zero-based end line and then end column position.
+	End() (int, int)
+
+	// String returns a friendly representation of the positions.
+	String() string
+}
+
+// TextDisplayer is a graph node that is aware of its position in the source
+// code, and can emit a textual representation of that part of the source.
+type TextDisplayer interface {
+	// Byline returns a simple version of the error location.
+	Byline() string
+
+	// HighlightText returns a textual representation of this definition
+	// for this node in source.
+	HighlightText() string
+}
+
+// SourceFinderFunc is the function signature used to return the contents of a
+// source file when requested by filename. This data is used to annotate error
+// messages with some context from the source, and as a result is optional.
+type SourceFinderFunc = func(string) ([]byte, error)

lang/interfaces/unification.go

@@ -39,8 +39,12 @@ import (
 // solution. Those two types are symmetrical in that it doesn't matter which is
 // used where, it only affects how we print out error messages.
 type UnificationInvariant struct { // formerly the SamInvariant
-	// Expr is the expression we are determining the type for. This improves
-	// our error messages.
+	// Node is the AST node holding the expression. This improves our error
+	// messages.
+	Node Node
+
+	// Expr is the expression we are determining the type for. This is what
+	// we are unifying. This improves our error messages.
 	Expr Expr
 
 	// Expect is one of the two types to unify.
@@ -65,6 +69,7 @@ func GenericCheck(obj Expr, typ *types.Type) ([]*UnificationInvariant, error) {
 
 	invar := &UnificationInvariant{
 		Expr:   obj,
+		Node:   obj,
 		Expect: typ, // sam says not backwards
 		Actual: actual,
 	}

lang/interpolate/interpolate.go

@@ -155,6 +155,7 @@ func HilInterpolate(str string, pos *interfaces.Pos, data *interfaces.Data) (int
 		LexParser:       data.LexParser,
 		Downloader:      data.Downloader,
 		StrInterpolater: data.StrInterpolater,
+		SourceFinder:    data.SourceFinder,
 		//World: data.World, // TODO: do we need this?
 
 		Prefix: data.Prefix,

lang/interpolate/interpolate_test.go

@@ -34,6 +34,7 @@ package interpolate
 import (
 	"fmt"
 	"reflect"
+	"regexp"
 	"strings"
 	"testing"
 
@@ -228,6 +229,9 @@ func TestInterpolate0(t *testing.T) {
 			data := &interfaces.Data{
 				// TODO: add missing fields here if/when needed
 				StrInterpolater: StrInterpolate,
+				SourceFinder: func(string) ([]byte, error) {
+					return nil, fmt.Errorf("not implemented")
+				},
 
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {
@@ -271,7 +275,7 @@ func TestInterpolate0(t *testing.T) {
 				StripPackageNames: true,
 				HidePrivateFields: true,
 				HideZeroValues:    true,
-				//FieldExclusions: regexp.MustCompile(`^(data)$`),
+				FieldExclusions:   regexp.MustCompile(`^(Textarea)$`),
 				//FieldFilter       func(reflect.StructField, reflect.Value) bool
 				//HomePackage       string
 				//Separator         string
@@ -480,6 +484,9 @@ func TestInterpolateBasicStmt(t *testing.T) {
 			data := &interfaces.Data{
 				// TODO: add missing fields here if/when needed
 				StrInterpolater: StrInterpolate,
+				SourceFinder: func(string) ([]byte, error) {
+					return nil, fmt.Errorf("not implemented")
+				},
 
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {
@@ -787,6 +794,9 @@ func TestInterpolateBasicExpr(t *testing.T) {
 			data := &interfaces.Data{
 				// TODO: add missing fields here if/when needed
 				StrInterpolater: StrInterpolate,
+				SourceFinder: func(string) ([]byte, error) {
+					return nil, fmt.Errorf("not implemented")
+				},
 
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {

lang/interpret_test.go

@@ -181,11 +181,19 @@ func TestAstFunc1(t *testing.T) {
 				t.Logf("comment: %s\n", comment)
 			}
 
+			sources := map[string][]byte{}
+			sourceFinder := func(path string) ([]byte, error) {
+				if b, exists := sources[path]; exists {
+					return b, nil
+				}
+				return nil, os.ErrNotExist
+			}
 			// copy files out into the test temp directory
 			var testOutput []byte
 			var testConfig []byte
 			found := false
 			for _, file := range archive.Files {
+				sources["/"+file.Name] = file.Data // store!
 				if file.Name == "OUTPUT" {
 					testOutput = file.Data
 					found = true
@@ -247,6 +255,7 @@ func TestAstFunc1(t *testing.T) {
 			if strings.HasPrefix(expstr, magicError) {
 				errStr = strings.TrimPrefix(expstr, magicError)
 				expstr = errStr
+				t.Logf("errStr has length %d", len(errStr))
 
 				if strings.HasPrefix(expstr, magicErrorLexParse) {
 					errStr = strings.TrimPrefix(expstr, magicErrorLexParse)
@@ -398,6 +407,7 @@ func TestAstFunc1(t *testing.T) {
 
 				LexParser:       parser.LexParse,
 				StrInterpolater: interpolate.StrInterpolate,
+				SourceFinder:    sourceFinder,
 
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {
@@ -675,11 +685,19 @@ func TestAstFunc2(t *testing.T) {
 				t.Logf("comment: %s\n", comment)
 			}
 
+			sources := map[string][]byte{}
+			sourceFinder := func(path string) ([]byte, error) {
+				if b, exists := sources[path]; exists {
+					return b, nil
+				}
+				return nil, os.ErrNotExist
+			}
 			// copy files out into the test temp directory
 			var testOutput []byte
 			var testConfig []byte
 			found := false
 			for _, file := range archive.Files {
+				sources["/"+file.Name] = file.Data // store!
 				if file.Name == "OUTPUT" {
 					testOutput = file.Data
 					found = true
@@ -939,6 +957,7 @@ func TestAstFunc2(t *testing.T) {
 
 				LexParser:       parser.LexParse,
 				StrInterpolater: interpolate.StrInterpolate,
+				SourceFinder:    sourceFinder,
 
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {
@@ -1495,11 +1514,19 @@ func TestAstFunc3(t *testing.T) {
 				t.Logf("comment: %s\n", comment)
 			}
 
+			sources := map[string][]byte{}
+			sourceFinder := func(path string) ([]byte, error) {
+				if b, exists := sources[path]; exists {
+					return b, nil
+				}
+				return nil, os.ErrNotExist
+			}
 			// copy files out into the test temp directory
 			var testOutput []byte
 			var testConfig []byte
 			found := false
 			for _, file := range archive.Files {
+				sources["/"+file.Name] = file.Data // store!
 				if file.Name == "OUTPUT" {
 					testOutput = file.Data
 					found = true
@@ -1759,6 +1786,7 @@ func TestAstFunc3(t *testing.T) {
 
 				LexParser:       parser.LexParse,
 				StrInterpolater: interpolate.StrInterpolate,
+				SourceFinder:    sourceFinder,
 
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {

lang/interpret_test/TestAstFunc1/polydoubleincludewithtype.txtar

@@ -12,4 +12,4 @@ class c1($a, $b []str) {
 	}
 }
 -- OUTPUT --
-# err: errUnify: unify error with: str("hello"): type error: list != str
+# err: errUnify: type error: list != str: /main.mcl @ 4:1-4:25

lang/interpret_test/TestAstFunc2/fortytwoerror.txtar

@@ -15,4 +15,4 @@ test ["x",] {
 	float32 => $b,
 }
 -- OUTPUT --
-# err: errUnify: unify error with: topLevel(func() { <built-in:_operator> }): type error: int != float
+# err: errUnify: type error: int != float: /main.mcl @ 10:6-10:22

lang/interpret_test/TestAstFunc2/printfunificationerr0.txtar

@@ -2,4 +2,4 @@
 import "fmt"
 test fmt.printf("%d%d", 42) {} # should not pass, missing second int
 -- OUTPUT --
-# err: errUnify: unify error with: call:fmt.printf(str("%d%d"), int(42)): type error: str != list
+# err: errUnify: type error: str != list: /main.mcl @ 2:1-2:30

lang/interpret_test/TestAstFunc2/res1.txtar

@@ -3,4 +3,4 @@ test "t1" {
 	stringptr => 42, # int, not str
 }
 -- OUTPUT --
-# err: errUnify: unify error with: int(42): type error: str != int
+# err: errUnify: type error: str != int: /main.mcl @ 2:2-2:17

lang/interpret_test/TestAstFunc2/unify-interpolate-edge1-fail.txtar

@@ -7,4 +7,4 @@ test "test" {}
 Test["${name}"] -> Test["test"] # must fail
 
 -- OUTPUT --
-# err: errUnify: unify error with: var(name): type error: str != list
+# err: errUnify: type error: str != list: /main.mcl @ 6:1-6:15

lang/interpret_test/TestAstFunc2/unify-interpolate-edge2-fail.txtar

@@ -7,4 +7,4 @@ test "test" {}
 Test["test"] -> Test["${name}"] # must fail
 
 -- OUTPUT --
-# err: errUnify: unify error with: var(name): type error: str != list
+# err: errUnify: type error: str != list: /main.mcl @ 6:17-6:31

lang/interpret_test/TestAstFunc2/unify-interpolate-res-fail.txtar

@@ -5,4 +5,4 @@ $name = ["a", "bb", "ccc",]
 test "${name}" {} # must fail
 
 -- OUTPUT --
-# err: errUnify: unify error with: var(name): type error: str != list
+# err: errUnify: type error: str != list: /main.mcl @ 4:1-4:17

lang/lang.go

@@ -189,6 +189,10 @@ func (obj *Lang) Init(ctx context.Context) error {
 		LexParser:       parser.LexParse,
 		Downloader:      nil, // XXX: is this used here?
 		StrInterpolater: interpolate.StrInterpolate,
+		SourceFinder: func(string) ([]byte, error) {
+			// We're running a bundle as part of a deploy.
+			return nil, fmt.Errorf("not implemented") // XXX: read from the fs?
+		},
 		//Local: obj.Local, // TODO: do we need this?
 		//World: obj.World, // TODO: do we need this?
 

lang/parser/lexparse_test.go

@@ -35,6 +35,7 @@ import (
 	"fmt"
 	"io"
 	"reflect"
+	"regexp"
 	"strings"
 	"testing"
 
@@ -45,8 +46,9 @@ import (
 	langUtil "github.com/purpleidea/mgmt/lang/util"
 	"github.com/purpleidea/mgmt/util"
 
-	"github.com/davecgh/go-spew/spew"
+	godiff "github.com/kylelemons/godebug/diff"
 	"github.com/kylelemons/godebug/pretty"
+	"github.com/sanity-io/litter"
 )
 
 func TestLexParse0(t *testing.T) {
@@ -2221,6 +2223,36 @@ func TestLexParse0(t *testing.T) {
 			}
 			// double check because DeepEqual is different since the func exists
 
+			lo := &litter.Options{
+				//Compact: false,
+				StripPackageNames: true,
+				HidePrivateFields: true,
+				HideZeroValues:    true,
+				FieldExclusions:   regexp.MustCompile(`^(Textarea)$`),
+				//FieldFilter       func(reflect.StructField, reflect.Value) bool
+				//HomePackage       string
+				//Separator         string
+				DisablePointerReplacement: true,
+			}
+
+			// The litter package adds pointer comments everywhere,
+			// which make it not diff correctly. Clean them here!
+			pattern := regexp.MustCompile(`\ \/\/\ p[0-9]+$`) // the p0, p1 comments...
+			clean := func(s string) string {
+				lines := []string{}
+				for _, line := range strings.Split(s, "\n") {
+					s := pattern.ReplaceAllLiteralString(line, "")
+					lines = append(lines, s)
+				}
+				return strings.Join(lines, "\n")
+			}
+
+			lo1 := clean(lo.Sdump(exp))
+			lo2 := clean(lo.Sdump(xast))
+			if lo1 == lo2 { // simple diff
+				return
+			}
+
 			// more details, for tricky cases:
 			diffable := &pretty.Config{
 				Diffable:          true,
@@ -2228,18 +2260,22 @@ func TestLexParse0(t *testing.T) {
 				//PrintStringers: false, // always false!
 				//PrintTextMarshalers: false,
 				SkipZeroFields: true,
+				//Formatter: map[reflect.Type]interface{}{
+				//	reflect.TypeOf(ast.Textarea{}): func(x ast.Textarea) string {
+				//		return ""
+				//	},
+				//},
 			}
 			diff := diffable.Compare(exp, xast)
 			if diff == "" { // bonus
 				return
 			}
+			diff = godiff.Diff(lo1, lo2) // for printing
+
 			t.Errorf("test #%d: AST did not match expected", index)
 			// TODO: consider making our own recursive print function
-			t.Logf("test #%d:   actual: \n\n%s\n", index, spew.Sdump(xast))
-			t.Logf("test #%d: expected: \n\n%s", index, spew.Sdump(exp))
-
-			t.Logf("test #%d:   actual: \n\n%s\n", index, diffable.Sprint(xast))
-			t.Logf("test #%d: expected: \n\n%s", index, diffable.Sprint(exp))
+			t.Logf("test #%d:   actual: \n\n%s\n", index, lo1)
+			t.Logf("test #%d: expected: \n\n%s", index, lo2)
 			t.Logf("test #%d: diff:\n%s", index, diff)
 		})
 	}

lang/parser/parser.y

@@ -166,54 +166,55 @@ prog:
 			$$.stmt = &ast.StmtProg{
 				Body: stmts,
 			}
+			locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 		}
 	}
 ;
 stmt:
 	COMMENT
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtComment{
 			Value: $1.str,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 |	bind
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = $1.stmt
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 |	panic
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = $1.stmt
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 |	resource
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = $1.stmt
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 |	edge
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = $1.stmt
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 |	IF expr OPEN_CURLY prog CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtIf{
 			Condition:  $2.expr,
 			ThenBranch: $4.stmt,
 			//ElseBranch: nil,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 |	IF expr OPEN_CURLY prog CLOSE_CURLY ELSE OPEN_CURLY prog CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtIf{
 			Condition:  $2.expr,
 			ThenBranch: $4.stmt,
 			ElseBranch: $8.stmt,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// this is the named version, iow, a user-defined function (statement)
 	// `func name() { <expr> }`
@@ -221,7 +222,6 @@ stmt:
 	// `func name(<arg>, <arg>) { <expr> }`
 |	FUNC_IDENTIFIER IDENTIFIER OPEN_PAREN args CLOSE_PAREN OPEN_CURLY expr CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtFunc{
 			Name: $2.str,
 			Func: &ast.ExprFunc{
@@ -230,11 +230,11 @@ stmt:
 				Body: $7.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `func name(...) <type> { <expr> }`
 |	FUNC_IDENTIFIER IDENTIFIER OPEN_PAREN args CLOSE_PAREN type OPEN_CURLY expr CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		fn := &ast.ExprFunc{
 			Args:   $4.args,
 			Return: $6.typ, // return type is known
@@ -271,191 +271,192 @@ stmt:
 			Func: fn,
 			Type: typ, // sam says add the type here instead...
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `class name { <prog> }`
 |	CLASS_IDENTIFIER colon_identifier OPEN_CURLY prog CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtClass{
 			Name: $2.str,
 			Args: nil,
 			Body: $4.stmt,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `class name(<arg>) { <prog> }`
 	// `class name(<arg>, <arg>) { <prog> }`
 |	CLASS_IDENTIFIER colon_identifier OPEN_PAREN args CLOSE_PAREN OPEN_CURLY prog CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtClass{
 			Name: $2.str,
 			Args: $4.args,
 			Body: $7.stmt,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `include name`
 |	INCLUDE_IDENTIFIER dotted_identifier
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtInclude{
 			Name: $2.str,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `include name(...)`
 |	INCLUDE_IDENTIFIER dotted_identifier OPEN_PAREN call_args CLOSE_PAREN
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtInclude{
 			Name: $2.str,
 			Args: $4.exprs,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `include name as foo`
 	// TODO: should we support: `include name as *`
 |	INCLUDE_IDENTIFIER dotted_identifier AS_IDENTIFIER IDENTIFIER
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtInclude{
 			Name:  $2.str,
 			Alias: $4.str,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `include name(...) as foo`
 	// TODO: should we support: `include name(...) as *`
 |	INCLUDE_IDENTIFIER dotted_identifier OPEN_PAREN call_args CLOSE_PAREN AS_IDENTIFIER IDENTIFIER
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtInclude{
 			Name:  $2.str,
 			Args:  $4.exprs,
 			Alias: $7.str,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `import "name"`
 |	IMPORT_IDENTIFIER STRING
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtImport{
 			Name: $2.str,
 			//Alias: "",
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `import "name" as alias`
 |	IMPORT_IDENTIFIER STRING AS_IDENTIFIER IDENTIFIER
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtImport{
 			Name:  $2.str,
 			Alias: $4.str,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `import "name" as *`
 |	IMPORT_IDENTIFIER STRING AS_IDENTIFIER MULTIPLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtImport{
 			Name:  $2.str,
 			Alias: $4.str,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 /*
 	// resource bind
 |	rbind
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = $1.stmt
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 */
 ;
 expr:
 	BOOL
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprBool{
 			V: $1.bool,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	STRING
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprStr{
 			V: $1.str,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	INTEGER
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprInt{
 			V: $1.int,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	FLOAT
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprFloat{
 			V: $1.float,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	list
 	{
-		posLast(yylex, yyDollar) // our pos
 		// TODO: list could be squashed in here directly...
 		$$.expr = $1.expr
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	map
 	{
-		posLast(yylex, yyDollar) // our pos
 		// TODO: map could be squashed in here directly...
 		$$.expr = $1.expr
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	struct
 	{
-		posLast(yylex, yyDollar) // our pos
 		// TODO: struct could be squashed in here directly...
 		$$.expr = $1.expr
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	call
 	{
-		posLast(yylex, yyDollar) // our pos
 		// TODO: call could be squashed in here directly...
 		$$.expr = $1.expr
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	var
 	{
-		posLast(yylex, yyDollar) // our pos
 		// TODO: var could be squashed in here directly...
 		$$.expr = $1.expr
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	func
 	{
-		posLast(yylex, yyDollar) // our pos
 		// TODO: var could be squashed in here directly...
 		$$.expr = $1.expr
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	IF expr OPEN_CURLY expr CLOSE_CURLY ELSE OPEN_CURLY expr CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprIf{
 			Condition:  $2.expr,
 			ThenBranch: $4.expr,
 			ElseBranch: $8.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 	// parenthesis wrap an expression for precedence
 |	OPEN_PAREN expr CLOSE_PAREN
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = $2.expr
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 ;
 list:
 	// `[42, 0, -13]`
 	OPEN_BRACK list_elements CLOSE_BRACK
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprList{
 			Elements: $2.exprs,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 ;
 list_elements:
@@ -473,18 +474,18 @@ list_elements:
 list_element:
 	expr COMMA
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = $1.expr
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 ;
 map:
 	// `{"hello" => "there", "world" => "big",}`
 	OPEN_CURLY map_kvs CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprMap{
 			KVs: $2.mapKVs,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 ;
 map_kvs:
@@ -513,7 +514,6 @@ struct:
 	// `struct{answer => 0, truth => false, hello => "world",}`
 	STRUCT_IDENTIFIER OPEN_CURLY struct_fields CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprStruct{
 			Fields: $3.structFields,
 		}
@@ -546,18 +546,17 @@ call:
 	// iter.map(...)
 	dotted_identifier OPEN_PAREN call_args CLOSE_PAREN
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: $1.str,
 			Args: $3.exprs,
 			//Var: false, // default
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 	// calling a function that's stored in a variable (a lambda)
 	// `$foo(4, "hey")` # call function value
 |	dotted_var_identifier OPEN_PAREN call_args CLOSE_PAREN
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: $1.str,
 			Args: $3.exprs,
@@ -565,20 +564,20 @@ call:
 			// prefix to the Name, but I felt this was more elegant.
 			Var: true, // lambda
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 	// calling an inline function
 |	func OPEN_PAREN call_args CLOSE_PAREN
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: "", // anonymous!
 			Args: $3.exprs,
 			Anon: $1.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr PLUS expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -589,10 +588,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr MINUS expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -603,10 +602,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr MULTIPLY expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -617,10 +616,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr DIVIDE expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -631,10 +630,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr EQ expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -645,10 +644,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr NEQ expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -659,10 +658,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr LT expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -673,10 +672,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr GT expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -687,10 +686,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr LTE expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -701,10 +700,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr GTE expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -715,10 +714,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr AND expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -729,10 +728,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr OR expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -743,10 +742,10 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	NOT expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: operators.OperatorFuncName,
 			Args: []interfaces.Expr{
@@ -756,13 +755,13 @@ call:
 				$2.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 	// lookup an index in a list or a key in a map
 	// lookup($foo, $key)
 	// `$foo[$key]` // no default specifier
 |	expr OPEN_BRACK expr CLOSE_BRACK
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: funcs.LookupFuncName,
 			Args: []interfaces.Expr{
@@ -771,13 +770,13 @@ call:
 				//$6.expr, // the default
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 	// lookup an index in a list or a key in a map with a default
 	// lookup_default($foo, $key, $default)
 	// `$foo[$key] || "default"`
 |	expr OPEN_BRACK expr CLOSE_BRACK DEFAULT expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: funcs.LookupDefaultFuncName,
 			Args: []interfaces.Expr{
@@ -786,13 +785,13 @@ call:
 				$6.expr, // the default
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 	// lookup a field in a struct
 	// _struct_lookup($foo, "field")
 	// $foo->field
 |	expr ARROW IDENTIFIER
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: funcs.StructLookupFuncName,
 			Args: []interfaces.Expr{
@@ -803,13 +802,13 @@ call:
 				//$5.expr, // the default
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 	// lookup a field in a struct with a default
 	// _struct_lookup_optional($foo, "field", "default")
 	// $foo->field || "default"
 |	expr ARROW IDENTIFIER DEFAULT expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: funcs.StructLookupOptionalFuncName,
 			Args: []interfaces.Expr{
@@ -820,10 +819,10 @@ call:
 				$5.expr, // the default
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 |	expr IN expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprCall{
 			Name: funcs.ContainsFuncName,
 			Args: []interfaces.Expr{
@@ -831,6 +830,7 @@ call:
 				$3.expr,
 			},
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 ;
 // list order gets us the position of the arg, but named params would work too!
@@ -856,10 +856,10 @@ call_args:
 var:
 	dotted_var_identifier
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprVar{
 			Name: $1.str,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 ;
 func:
@@ -869,17 +869,16 @@ func:
 	// `func(<arg>, <arg>) { <expr> }`
 	FUNC_IDENTIFIER OPEN_PAREN args CLOSE_PAREN OPEN_CURLY expr CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprFunc{
 			Args: $3.args,
 			//Return: nil,
 			Body: $6.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 	// `func(...) <type> { <expr> }`
 |	FUNC_IDENTIFIER OPEN_PAREN args CLOSE_PAREN type OPEN_CURLY expr CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.expr = &ast.ExprFunc{
 			Args:   $3.args,
 			Return: $5.typ, // return type is known
@@ -909,6 +908,7 @@ func:
 				yylex.Error(fmt.Sprintf("%s: %+v", ErrParseSetType, err))
 			}
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 ;
 args:
@@ -949,17 +949,16 @@ bind:
 	// `$s = "hey"`
 	var_identifier EQUALS expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtBind{
 			Ident: $1.str,
 			Value: $3.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// `$x bool = true`
 	// `$x int = if true { 42 } else { 13 }`
 |	var_identifier type EQUALS expr
 	{
-		posLast(yylex, yyDollar) // our pos
 		var expr interfaces.Expr = $4.expr
 		// XXX: We still need to do this for now it seems...
 		if err := expr.SetType($2.typ); err != nil {
@@ -971,6 +970,7 @@ bind:
 			Value: expr,
 			Type:  $2.typ, // sam says add the type here instead...
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 ;
 panic:
@@ -981,7 +981,6 @@ panic:
 	//}
 	PANIC_IDENTIFIER OPEN_PAREN call_args CLOSE_PAREN
 	{
-		posLast(yylex, yyDollar) // our pos
 		call := &ast.ExprCall{
 			Name: $1.str, // the function name
 			Args: $3.exprs,
@@ -1000,6 +999,7 @@ panic:
 			ThenBranch: res,
 			//ElseBranch: nil,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 ;
 /* TODO: do we want to include this?
@@ -1007,7 +1007,6 @@ panic:
 rbind:
 	var_identifier EQUALS resource
 	{
-		posLast(yylex, yyDollar) // our pos
 		// XXX: this kind of bind is different than the others, because
 		// it can only really be used for send->recv stuff, eg:
 		// foo.SomeString -> bar.SomeOtherString
@@ -1015,6 +1014,7 @@ rbind:
 			Ident: $1.str,
 			Value: $3.stmt,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.expr)
 	}
 ;
 */
@@ -1022,12 +1022,12 @@ resource:
 	// `file "/tmp/hello" { ... }` or `aws:ec2 "/tmp/hello" { ... }`
 	colon_identifier expr OPEN_CURLY resource_body CLOSE_CURLY
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtRes{
 			Kind:     $1.str,
 			Name:     $2.expr,
 			Contents: $4.resContents,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 ;
 resource_body:
@@ -1080,53 +1080,52 @@ resource_body:
 resource_field:
 	IDENTIFIER ROCKET expr COMMA
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.resField = &ast.StmtResField{
 			Field: $1.str,
 			Value: $3.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.resField)
 	}
 ;
 conditional_resource_field:
 	// content => $present ?: "hello",
 	IDENTIFIER ROCKET expr ELVIS expr COMMA
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.resField = &ast.StmtResField{
 			Field:     $1.str,
 			Value:     $5.expr,
 			Condition: $3.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.resField)
 	}
 ;
 resource_edge:
 	// Before => Test["t1"],
 	CAPITALIZED_IDENTIFIER ROCKET edge_half COMMA
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.resEdge = &ast.StmtResEdge{
 			Property: $1.str,
 			EdgeHalf: $3.edgeHalf,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.resEdge)
 	}
 ;
 conditional_resource_edge:
 	// Before => $present ?: Test["t1"],
 	CAPITALIZED_IDENTIFIER ROCKET expr ELVIS edge_half COMMA
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.resEdge = &ast.StmtResEdge{
 			Property:  $1.str,
 			EdgeHalf:  $5.edgeHalf,
 			Condition: $3.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.resEdge)
 	}
 ;
 resource_meta:
 	// Meta:noop => true,
 	CAPITALIZED_IDENTIFIER COLON IDENTIFIER ROCKET expr COMMA
 	{
-		posLast(yylex, yyDollar) // our pos
 		if strings.ToLower($1.str) != strings.ToLower(ast.MetaField) {
 			// this will ultimately cause a parser error to occur...
 			yylex.Error(fmt.Sprintf("%s: %s", ErrParseResFieldInvalid, $1.str))
@@ -1135,6 +1134,7 @@ resource_meta:
 			Property: $3.str,
 			MetaExpr: $5.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.resMeta)
 	}
 ;
 conditional_resource_meta:
@@ -1151,13 +1151,13 @@ conditional_resource_meta:
 			MetaExpr:  $7.expr,
 			Condition: $5.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.resMeta)
 	}
 ;
 resource_meta_struct:
 	// Meta => struct{meta => true, retry => 3,},
 	CAPITALIZED_IDENTIFIER ROCKET expr COMMA
 	{
-		posLast(yylex, yyDollar) // our pos
 		if strings.ToLower($1.str) != strings.ToLower(ast.MetaField) {
 			// this will ultimately cause a parser error to occur...
 			yylex.Error(fmt.Sprintf("%s: %s", ErrParseResFieldInvalid, $1.str))
@@ -1166,13 +1166,13 @@ resource_meta_struct:
 			Property: $1.str,
 			MetaExpr: $3.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.resMeta)
 	}
 ;
 conditional_resource_meta_struct:
 	// Meta => $present ?: struct{poll => 60, sema => ["foo:1", "bar:3",],},
 	CAPITALIZED_IDENTIFIER ROCKET expr ELVIS expr COMMA
 	{
-		posLast(yylex, yyDollar) // our pos
 		if strings.ToLower($1.str) != strings.ToLower(ast.MetaField) {
 			// this will ultimately cause a parser error to occur...
 			yylex.Error(fmt.Sprintf("%s: %s", ErrParseResFieldInvalid, $1.str))
@@ -1182,6 +1182,7 @@ conditional_resource_meta_struct:
 			MetaExpr:  $5.expr,
 			Condition: $3.expr,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.resMeta)
 	}
 ;
 edge:
@@ -1190,16 +1191,15 @@ edge:
 	// Test["t1"] -> Test["t2"] -> Test["t3"] # chain or pair
 	edge_half_list
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtEdge{
 			EdgeHalfList: $1.edgeHalfList,
 			//Notify: false, // unused here
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 	// Test["t1"].foo_send -> Test["t2"].blah_recv # send/recv
 |	edge_half_sendrecv ARROW edge_half_sendrecv
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.stmt = &ast.StmtEdge{
 			EdgeHalfList: []*ast.StmtEdgeHalf{
 				$1.edgeHalf,
@@ -1207,6 +1207,7 @@ edge:
 			},
 			//Notify: false, // unused here, it is implied (i think)
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.stmt)
 	}
 ;
 edge_half_list:
@@ -1225,24 +1226,24 @@ edge_half:
 	// eg: Test["t1"]
 	capitalized_res_identifier OPEN_BRACK expr CLOSE_BRACK
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.edgeHalf = &ast.StmtEdgeHalf{
 			Kind: $1.str,
 			Name: $3.expr,
 			//SendRecv: "", // unused
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.edgeHalf)
 	}
 ;
 edge_half_sendrecv:
 	// eg: Test["t1"].foo_send
 	capitalized_res_identifier OPEN_BRACK expr CLOSE_BRACK DOT IDENTIFIER
 	{
-		posLast(yylex, yyDollar) // our pos
 		$$.edgeHalf = &ast.StmtEdgeHalf{
 			Kind: $1.str,
 			Name: $3.expr,
 			SendRecv: $6.str,
 		}
+		locate(yylex, $1, yyDollar[len(yyDollar)-1], $$.edgeHalf)
 	}
 ;
 type:
@@ -1496,7 +1497,21 @@ func cast(y yyLexer) *lexParseAST {
 	return x.(*lexParseAST)
 }
 
-// posLast pulls out the "last token" and does a pos with that. This is a hack!
+// locate should be called after creating AST nodes from lexer tokens to store
+// the positions of the involved tokens in the AST node.
+func locate(y yyLexer, first yySymType, last yySymType, node interface{}) {
+	pos(y, last)
+	// Only run Locate on nodes that look like they have not received
+	// locations yet otherwise the parser will come back and overwrite them
+	// with invalid ending positions.
+	if pn, ok := node.(interfaces.PositionableNode); !ok {
+		return
+	} else if !pn.IsSet() {
+		pn.Locate(first.row, first.col, last.row, last.col)
+	}
+}
+
+// posLast runs pos on the last token of the current stmt/expr.
 func posLast(y yyLexer, dollars []yySymType) {
 	// pick the last token in the set matched by the parser
 	pos(y, dollars[len(dollars)-1]) // our pos

lang/unification/fastsolver/fastsolver.go

@@ -134,9 +134,16 @@ func (obj *FastInvariantSolver) Solve(ctx context.Context, data *unification.Dat
 		if err := unificationUtil.Unify(x.Expect, x.Actual); err != nil {
 			// Storing the Expr with this invariant is so that we
 			// can generate this more helpful error message here.
-			// TODO: Improve this error message!
+			displayer, ok := x.Node.(interfaces.TextDisplayer)
+			if !ok {
+				obj.Logf("not displayable: %v\n", x.Node)
 				return nil, errwrap.Wrapf(err, "unify error with: %s", x.Expr)
 			}
+			if highlight := displayer.HighlightText(); highlight != "" {
+				obj.Logf("%s: %s", err.Error(), highlight)
+			}
+			return nil, fmt.Errorf("%s: %s", err.Error(), displayer.Byline())
+		}
 		if obj.Debug {
 			e1, e2 := unificationUtil.Extract(x.Expect), unificationUtil.Extract(x.Actual)
 			obj.Logf("#%"+pad+"d extract(%s): %s -- %s", i, x.Expr, u(e1), u(e2))

lang/unification/solvers/unification_test.go

@@ -859,6 +859,11 @@ func TestUnification1(t *testing.T) {
 
 			data := &interfaces.Data{
 				// TODO: add missing fields here if/when needed
+				Metadata: &interfaces.Metadata{},
+				SourceFinder: func(string) ([]byte, error) {
+					return nil, fmt.Errorf("not implemented")
+				},
+
 				Debug: testing.Verbose(), // set via the -test.v flag to `go test`
 				Logf: func(format string, v ...interface{}) {
 					t.Logf(fmt.Sprintf("test #%d", index)+": ast: "+format, v...)