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/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,
 	}