lang: funcs, parser: Add improved panic magic

This is a newer implementation of the panic magic. I kept the old commit
in for posterity and to show the difference. The two versions are
identical to the end-user with one exception: the newer version doesn't
include a useless panic resource in the graph when there is no panic. In
this version, the panic function returns false and the if statement it's
the condition of, doesn't produce the resource within. On error, we
still consume the function in the if expression, and doing so causes
everything to shutdown.

The other benefit is that the implementation is much cleaner and doesn't
need the interpolate hack.

examples/lang/panic0.mcl

@@ -12,7 +12,6 @@ test "test" {
 }
 
 # this is what we're simulating:
-#$_panic1 = panic("whatever1")	# this is a function
-#_panic $_panic1 {}		# this is a resource
-#$_panic2 = panic("whatever2")
-#_panic $_panic2 {}
+#if panic("whatever1") {	# this is a function (and an if statement)
+#	_panic "panic" {}	# this is a resource
+#}

lang/ast/structs.go

@@ -291,8 +291,6 @@ type StmtRes struct {
 	Name     interfaces.Expr   // unique name for the res of this kind
 	namePtr  interfaces.Func   // ptr for table lookup
 	Contents []StmtResContents // list of fields/edges in parsed order
-
-	allowUnderscores bool // secret flag to bypass some validation
 }
 
 // String returns a short representation of this statement.
@@ -321,10 +319,7 @@ 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 {
-
-	isPanic := obj.allowUnderscores && obj.Kind == interfaces.PanicResKind
-
-	if strings.Contains(obj.Kind, "_") && !isPanic {
+	if strings.Contains(obj.Kind, "_") && obj.Kind != interfaces.PanicResKind {
 		return fmt.Errorf("kind must not contain underscores")
 	}
 
@@ -388,7 +383,6 @@ func (obj *StmtRes) Interpolate() (interfaces.Stmt, error) {
 		Kind:     obj.Kind,
 		Name:     name,
 		Contents: contents,
-		allowUnderscores: obj.allowUnderscores,
 	}, nil
 }
 
@@ -429,7 +423,6 @@ func (obj *StmtRes) Copy() (interfaces.Stmt, error) {
 		Kind:     obj.Kind,
 		Name:     name,
 		Contents: contents,
-		allowUnderscores: obj.allowUnderscores,
 	}, nil
 }
 
@@ -2840,8 +2833,6 @@ type StmtProg struct {
 	importProgs []*StmtProg // list of child programs after running SetScope
 	importFiles []string    // list of files seen during the SetScope import
 
-	panicCounter uint // number of possible different panics
-
 	Body []interfaces.Stmt
 }
 
@@ -2896,36 +2887,6 @@ func (obj *StmtProg) Interpolate() (interfaces.Stmt, error) {
 			return nil, err
 		}
 		body = append(body, interpolated)
-
-		// If we have the magic bind statement, then add the res.
-		// NOTE: We could have used a custom StmtPanic instead here...
-		if bind, ok := interpolated.(*StmtBind); ok && bind.Ident == interfaces.PanicVarName {
-			// TODO: does it still work if we have multiple StmtProg's?
-			obj.panicCounter++
-			name := fmt.Sprintf("%s%d", interfaces.PanicVarName, obj.panicCounter)
-			bind.Ident = name // change name to magic name
-			exprVar := &ExprVar{
-				Name: name, // use magic name to match
-
-				allowUnderscores: true, // allow our magic name
-			}
-			if err := exprVar.Init(obj.data); err != nil {
-				return nil, errwrap.Wrapf(err, "special panic ExprVar Init error during interpolate")
-			}
-			stmtRes := &StmtRes{
-				Kind:     interfaces.PanicResKind, // special resource kind
-				Name:     exprVar,
-				Contents: []StmtResContents{},
-
-				allowUnderscores: true, // allow our magic kind
-			}
-			if err := stmtRes.Init(obj.data); err != nil {
-				return nil, errwrap.Wrapf(err, "special panic StmtRes Init error during interpolate")
-			}
-
-			body = append(body, stmtRes)
-			continue
-		}
 	}
 	return &StmtProg{
 		data:        obj.data,
@@ -8289,8 +8250,6 @@ type ExprVar struct {
 	typ   *types.Type
 
 	Name string // name of the variable
-
-	allowUnderscores bool // secret flag to bypass some validation
 }
 
 // String returns a short representation of this expression.
@@ -8306,9 +8265,6 @@ func (obj *ExprVar) 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 *ExprVar) Init(*interfaces.Data) error {
-	if obj.allowUnderscores && strings.HasPrefix(obj.Name, interfaces.PanicVarName) {
-		return nil
-	}
 	return langutil.ValidateVarName(obj.Name)
 }
 
@@ -8322,7 +8278,6 @@ func (obj *ExprVar) Interpolate() (interfaces.Expr, error) {
 		scope: obj.scope,
 		typ:   obj.typ,
 		Name:  obj.Name,
-		allowUnderscores: obj.allowUnderscores,
 	}, nil
 }
 
@@ -8336,7 +8291,6 @@ func (obj *ExprVar) Copy() (interfaces.Expr, error) {
 		scope: obj.scope,
 		typ:   obj.typ,
 		Name:  obj.Name,
-		allowUnderscores: obj.allowUnderscores,
 	}, nil
 }
 

lang/funcs/core/panic_func.go

@@ -26,21 +26,18 @@ import (
 
 func init() {
 	simple.Register("panic", &types.FuncValue{
-		T: types.NewType("func(x str) str"),
+		T: types.NewType("func(x str) bool"),
 		V: Panic,
 	})
 }
 
 // Panic returns an error when it receives a non-empty string. The error should
-// cause the function engine to shutdown.
+// cause the function engine to shutdown. If there's no error, it returns false.
 func Panic(input []types.Value) (types.Value, error) {
 	if s := input[0].Str(); s != "" {
-		// This StrValue not really used here, since we error...
-		return &types.StrValue{
-			V: s,
-		}, fmt.Errorf("panic occurred: %s", s)
+		return nil, fmt.Errorf("panic occurred: %s", s)
 	}
-	return &types.StrValue{
-		V: "panic", // name can't be empty
+	return &types.BoolValue{
+		V: false,
 	}, nil
 }

lang/interfaces/const.go

@@ -28,7 +28,4 @@ const (
 
 	// PanicResKind is the kind string used for the panic resource.
 	PanicResKind = "_panic"
-
-	// PanicVarName is the magic name used for the panic output var.
-	PanicVarName = "_panic"
 )

lang/interpret_test/TestAstFunc2/panic0.txtar

@@ -2,5 +2,5 @@
 # This should not panic.
 panic("")
 panic("")
+# Empty graph, no output.
 -- OUTPUT --
-Vertex: _panic[panic]

lang/interpret_test/TestAstFunc2/panic1.txtar

@@ -3,5 +3,4 @@
 panic("")
 test "hello" {}
 -- OUTPUT --
-Vertex: _panic[panic]
 Vertex: test[hello]

lang/parser/parser.y

@@ -927,17 +927,30 @@ bind:
 ;
 panic:
 	// panic("some error")
+	// generates:
+	// if panic("some error") {
+	//	_panic "_panic" {} # resource
+	//}
 	PANIC_IDENTIFIER OPEN_PAREN call_args CLOSE_PAREN
 	{
 		posLast(yylex, yyDollar) // our pos
 		call := &ast.ExprCall{
-			Name: $1.str,
+			Name: $1.str, // the function name
 			Args: $3.exprs,
 			//Var: false, // default
 		}
-		$$.stmt = &ast.StmtBind{
-			Ident: interfaces.PanicVarName, // make up a placeholder var
-			Value: call,
+		name := &ast.ExprStr{
+			V: $1.str, // any constant, non-empty name
+		}
+		res := &ast.StmtRes{
+			Kind:     interfaces.PanicResKind,
+			Name:     name,
+			Contents: []ast.StmtResContents{},
+		}
+		$$.stmt = &ast.StmtIf{
+			Condition:  call,
+			ThenBranch: res,
+			//ElseBranch: nil,
 		}
 	}
 ;