diff --git a/config/lang/check_types.go b/config/lang/check_types.go
index 882c64ca3..0165eadfc 100644
--- a/config/lang/check_types.go
+++ b/config/lang/check_types.go
@@ -24,9 +24,19 @@ type TypeCheck struct {
 	// value is the function to call (which must be registered in the Scope).
 	Implicit map[ast.Type]map[ast.Type]string
 
-	stack []ast.Type
-	err   error
-	lock  sync.Mutex
+	// Stack of types. This shouldn't be used directly except by implementations
+	// of TypeCheckNode.
+	Stack []ast.Type
+
+	err  error
+	lock sync.Mutex
+}
+
+// TypeCheckNode is the interface that must be implemented by any
+// ast.Node that wants to support type-checking. If the type checker
+// encounters a node that doesn't implement this, it will error.
+type TypeCheckNode interface {
+	TypeCheck(*TypeCheck) (ast.Node, error)
 }
 
 func (v *TypeCheck) Visit(root ast.Node) error {
@@ -42,49 +52,69 @@ func (v *TypeCheck) visit(raw ast.Node) ast.Node {
 		return raw
 	}
 
+	var result ast.Node
+	var err error
 	switch n := raw.(type) {
 	case *ast.Call:
-		v.visitCall(n)
+		tc := &typeCheckCall{n}
+		result, err = tc.TypeCheck(v)
 	case *ast.Concat:
-		v.visitConcat(n)
+		tc := &typeCheckConcat{n}
+		result, err = tc.TypeCheck(v)
 	case *ast.LiteralNode:
-		v.visitLiteral(n)
+		tc := &typeCheckLiteral{n}
+		result, err = tc.TypeCheck(v)
 	case *ast.VariableAccess:
-		v.visitVariableAccess(n)
+		tc := &typeCheckVariableAccess{n}
+		result, err = tc.TypeCheck(v)
 	default:
-		v.createErr(n, fmt.Sprintf("unknown node: %#v", raw))
+		tc, ok := raw.(TypeCheckNode)
+		if !ok {
+			err = fmt.Errorf("unknown node: %#v", raw)
+			break
+		}
+
+		result, err = tc.TypeCheck(v)
 	}
 
-	return raw
+	if err != nil {
+		pos := raw.Pos()
+		v.err = fmt.Errorf("At column %d, line %d: %s",
+			pos.Column, pos.Line, err)
+	}
+
+	return result
 }
 
-func (v *TypeCheck) visitCall(n *ast.Call) {
+type typeCheckCall struct {
+	n *ast.Call
+}
+
+func (tc *typeCheckCall) TypeCheck(v *TypeCheck) (ast.Node, error) {
 	// Look up the function in the map
-	function, ok := v.Scope.LookupFunc(n.Func)
+	function, ok := v.Scope.LookupFunc(tc.n.Func)
 	if !ok {
-		v.createErr(n, fmt.Sprintf("unknown function called: %s", n.Func))
-		return
+		return nil, fmt.Errorf("unknown function called: %s", tc.n.Func)
 	}
 
 	// The arguments are on the stack in reverse order, so pop them off.
-	args := make([]ast.Type, len(n.Args))
-	for i, _ := range n.Args {
-		args[len(n.Args)-1-i] = v.stackPop()
+	args := make([]ast.Type, len(tc.n.Args))
+	for i, _ := range tc.n.Args {
+		args[len(tc.n.Args)-1-i] = v.StackPop()
 	}
 
 	// Verify the args
 	for i, expected := range function.ArgTypes {
 		if args[i] != expected {
-			cn := v.implicitConversion(args[i], expected, n.Args[i])
+			cn := v.ImplicitConversion(args[i], expected, tc.n.Args[i])
 			if cn != nil {
-				n.Args[i] = cn
+				tc.n.Args[i] = cn
 				continue
 			}
 
-			v.createErr(n, fmt.Sprintf(
+			return nil, fmt.Errorf(
 				"%s: argument %d should be %s, got %s",
-				n.Func, i+1, expected, args[i]))
-			return
+				tc.n.Func, i+1, expected, args[i])
 		}
 	}
 
@@ -94,75 +124,86 @@ func (v *TypeCheck) visitCall(n *ast.Call) {
 		for i, t := range args {
 			if t != function.VariadicType {
 				realI := i + len(function.ArgTypes)
-				cn := v.implicitConversion(
-					t, function.VariadicType, n.Args[realI])
+				cn := v.ImplicitConversion(
+					t, function.VariadicType, tc.n.Args[realI])
 				if cn != nil {
-					n.Args[realI] = cn
+					tc.n.Args[realI] = cn
 					continue
 				}
 
-				v.createErr(n, fmt.Sprintf(
+				return nil, fmt.Errorf(
 					"%s: argument %d should be %s, got %s",
-					n.Func, realI,
-					function.VariadicType, t))
-				return
+					tc.n.Func, realI,
+					function.VariadicType, t)
 			}
 		}
 	}
 
 	// Return type
-	v.stackPush(function.ReturnType)
+	v.StackPush(function.ReturnType)
+
+	return tc.n, nil
 }
 
-func (v *TypeCheck) visitConcat(n *ast.Concat) {
+type typeCheckConcat struct {
+	n *ast.Concat
+}
+
+func (tc *typeCheckConcat) TypeCheck(v *TypeCheck) (ast.Node, error) {
+	n := tc.n
 	types := make([]ast.Type, len(n.Exprs))
 	for i, _ := range n.Exprs {
-		types[len(n.Exprs)-1-i] = v.stackPop()
+		types[len(n.Exprs)-1-i] = v.StackPop()
 	}
 
 	// All concat args must be strings, so validate that
 	for i, t := range types {
 		if t != ast.TypeString {
-			cn := v.implicitConversion(t, ast.TypeString, n.Exprs[i])
+			cn := v.ImplicitConversion(t, ast.TypeString, n.Exprs[i])
 			if cn != nil {
 				n.Exprs[i] = cn
 				continue
 			}
 
-			v.createErr(n, fmt.Sprintf(
-				"argument %d must be a string", i+1))
-			return
+			return nil, fmt.Errorf(
+				"argument %d must be a string", i+1)
 		}
 	}
 
 	// This always results in type string
-	v.stackPush(ast.TypeString)
+	v.StackPush(ast.TypeString)
+
+	return n, nil
 }
 
-func (v *TypeCheck) visitLiteral(n *ast.LiteralNode) {
-	v.stackPush(n.Typex)
+type typeCheckLiteral struct {
+	n *ast.LiteralNode
 }
 
-func (v *TypeCheck) visitVariableAccess(n *ast.VariableAccess) {
+func (tc *typeCheckLiteral) TypeCheck(v *TypeCheck) (ast.Node, error) {
+	v.StackPush(tc.n.Typex)
+	return tc.n, nil
+}
+
+type typeCheckVariableAccess struct {
+	n *ast.VariableAccess
+}
+
+func (tc *typeCheckVariableAccess) TypeCheck(v *TypeCheck) (ast.Node, error) {
 	// Look up the variable in the map
-	variable, ok := v.Scope.LookupVar(n.Name)
+	variable, ok := v.Scope.LookupVar(tc.n.Name)
 	if !ok {
-		v.createErr(n, fmt.Sprintf(
-			"unknown variable accessed: %s", n.Name))
-		return
+		return nil, fmt.Errorf(
+			"unknown variable accessed: %s", tc.n.Name)
 	}
 
 	// Add the type to the stack
-	v.stackPush(variable.Type)
+	v.StackPush(variable.Type)
+
+	return tc.n, nil
 }
 
-func (v *TypeCheck) createErr(n ast.Node, str string) {
-	pos := n.Pos()
-	v.err = fmt.Errorf("At column %d, line %d: %s",
-		pos.Column, pos.Line, str)
-}
-
-func (v *TypeCheck) implicitConversion(
+func (v *TypeCheck) ImplicitConversion(
 	actual ast.Type, expected ast.Type, n ast.Node) ast.Node {
 	if v.Implicit == nil {
 		return nil
@@ -186,16 +227,16 @@ func (v *TypeCheck) implicitConversion(
 }
 
 func (v *TypeCheck) reset() {
-	v.stack = nil
+	v.Stack = nil
 	v.err = nil
 }
 
-func (v *TypeCheck) stackPush(t ast.Type) {
-	v.stack = append(v.stack, t)
+func (v *TypeCheck) StackPush(t ast.Type) {
+	v.Stack = append(v.Stack, t)
 }
 
-func (v *TypeCheck) stackPop() ast.Type {
+func (v *TypeCheck) StackPop() ast.Type {
 	var x ast.Type
-	x, v.stack = v.stack[len(v.stack)-1], v.stack[:len(v.stack)-1]
+	x, v.Stack = v.Stack[len(v.Stack)-1], v.Stack[:len(v.Stack)-1]
 	return x
 }