Merge pull request #802 from hashicorp/f-ast-transforms

config/lang: AST transforms into the visitor pattern

config/lang/ast/ast.go

@@ -6,8 +6,9 @@ import (
 
 // Node is the interface that all AST nodes must implement.
 type Node interface {
-	// Accept is called to dispatch to the visitors.
-	Accept(Visitor)
+	// Accept is called to dispatch to the visitors. It must return the
+	// resulting Node (which might be different in an AST transform).
+	Accept(Visitor) Node
 
 	// Pos returns the position of this node in some source.
 	Pos() Pos
@@ -24,11 +25,18 @@ func (p Pos) String() string {
 
 // Visitors are just implementations of this function.
 //
+// The function must return the Node to replace this node with. "nil" is
+// _not_ a valid return value. If there is no replacement, the original node
+// should be returned. We build this replacement directly into the visitor
+// pattern since AST transformations are a common and useful tool and
+// building it into the AST itself makes it required for future Node
+// implementations and very easy to do.
+//
 // Note that this isn't a true implementation of the visitor pattern, which
 // generally requires proper type dispatch on the function. However,
 // implementing this basic visitor pattern style is still very useful even
 // if you have to type switch.
-type Visitor func(Node)
+type Visitor func(Node) Node
 
 //go:generate stringer -type=Type
 

config/lang/ast/call.go

@@ -12,12 +12,12 @@ type Call struct {
 	Posx Pos
 }
 
-func (n *Call) Accept(v Visitor) {
-	for _, a := range n.Args {
-		a.Accept(v)
+func (n *Call) Accept(v Visitor) Node {
+	for i, a := range n.Args {
+		n.Args[i] = a.Accept(v)
 	}
 
-	v(n)
+	return v(n)
 }
 
 func (n *Call) Pos() Pos {

config/lang/ast/concat.go

@@ -12,12 +12,12 @@ type Concat struct {
 	Posx  Pos
 }
 
-func (n *Concat) Accept(v Visitor) {
-	for _, n := range n.Exprs {
-		n.Accept(v)
+func (n *Concat) Accept(v Visitor) Node {
+	for i, expr := range n.Exprs {
+		n.Exprs[i] = expr.Accept(v)
 	}
 
-	v(n)
+	return v(n)
 }
 
 func (n *Concat) Pos() Pos {

config/lang/ast/literal.go

@@ -12,8 +12,8 @@ type LiteralNode struct {
 	Posx  Pos
 }
 
-func (n *LiteralNode) Accept(v Visitor) {
-	v(n)
+func (n *LiteralNode) Accept(v Visitor) Node {
+	return v(n)
 }
 
 func (n *LiteralNode) Pos() Pos {

config/lang/ast/variable_access.go

@@ -10,8 +10,8 @@ type VariableAccess struct {
 	Posx Pos
 }
 
-func (n *VariableAccess) Accept(v Visitor) {
-	v(n)
+func (n *VariableAccess) Accept(v Visitor) Node {
+	return v(n)
 }
 
 func (n *VariableAccess) Pos() Pos {

config/lang/check_identifier.go

@@ -25,9 +25,9 @@ func (c *IdentifierCheck) Visit(root ast.Node) error {
 	return c.err
 }
 
-func (c *IdentifierCheck) visit(raw ast.Node) {
+func (c *IdentifierCheck) visit(raw ast.Node) ast.Node {
 	if c.err != nil {
-		return
+		return raw
 	}
 
 	switch n := raw.(type) {
@@ -42,6 +42,9 @@ func (c *IdentifierCheck) visit(raw ast.Node) {
 	default:
 		c.createErr(n, fmt.Sprintf("unknown node: %#v", raw))
 	}
+
+	// We never do replacement with this visitor
+	return raw
 }
 
 func (c *IdentifierCheck) visitCall(n *ast.Call) {

config/lang/check_types.go

@@ -37,9 +37,9 @@ func (v *TypeCheck) Visit(root ast.Node) error {
 	return v.err
 }
 
-func (v *TypeCheck) visit(raw ast.Node) {
+func (v *TypeCheck) visit(raw ast.Node) ast.Node {
 	if v.err != nil {
-		return
+		return raw
 	}
 
 	switch n := raw.(type) {
@@ -54,6 +54,8 @@ func (v *TypeCheck) visit(raw ast.Node) {
 	default:
 		v.createErr(n, fmt.Sprintf("unknown node: %#v", raw))
 	}
+
+	return raw
 }
 
 func (v *TypeCheck) visitCall(n *ast.Call) {

config/lang/engine.go

@@ -105,9 +105,9 @@ func (v *executeVisitor) Visit(root ast.Node) (interface{}, ast.Type, error) {
 	return result.Value, result.Type, resultErr
 }
 
-func (v *executeVisitor) visit(raw ast.Node) {
+func (v *executeVisitor) visit(raw ast.Node) ast.Node {
 	if v.err != nil {
-		return
+		return raw
 	}
 
 	switch n := raw.(type) {
@@ -122,6 +122,8 @@ func (v *executeVisitor) visit(raw ast.Node) {
 	default:
 		v.err = fmt.Errorf("unknown node: %#v", raw)
 	}
+
+	return raw
 }
 
 func (v *executeVisitor) visitCall(n *ast.Call) {

config/lang/types_test.go

@@ -64,11 +64,11 @@ func TestLookupType(t *testing.T) {
 
 type customUntyped struct{}
 
-func (n customUntyped) Accept(ast.Visitor) {}
+func (n customUntyped) Accept(ast.Visitor) ast.Node { return n }
 func (n customUntyped) Pos() (v ast.Pos)            { return }
 
 type customTyped struct{}
 
-func (n customTyped) Accept(ast.Visitor)            {}
+func (n customTyped) Accept(ast.Visitor) ast.Node   { return n }
 func (n customTyped) Pos() (v ast.Pos)              { return }
 func (n customTyped) Type(*Scope) (ast.Type, error) { return ast.TypeString, nil }