If a name does not resolve to anything then it resolves to itself

vm/function.go

@@ -1,6 +1,7 @@
 package vm
 
 import (
+	"errors"
 	"fmt"
 
 	"code.betamike.com/mediocregopher/ginger/gg"
@@ -79,8 +80,6 @@ func FunctionFromGraph(g *gg.Graph, scope Scope) (Function, error) {
 			return edgeFn(Identity(val)), nil
 		}
 
-		name := *val.Name
-
 		if val.Equal(valNameIn) {
 			return edgeFn(FunctionFunc(func(inArg Value) Value {
 				return inArg
@@ -89,15 +88,17 @@ func FunctionFromGraph(g *gg.Graph, scope Scope) (Function, error) {
 
 		edgesIn := g.ValueIns(val.Value)
 
+		name := *val.Name
+
 		if l := len(edgesIn); l == 0 {
-
-			val, err := scope.Resolve(name)
-
-			if err != nil {
+			resolvedVal, err := scope.Resolve(name)
+			if errors.Is(err, ErrNameNotDefined) {
+				return edgeFn(Identity(val)), nil
+			} else if err != nil {
 				return nil, fmt.Errorf("resolving name %q from the outer scope: %w", name, err)
 			}
 
-			return edgeFn(Identity(val)), nil
+			return edgeFn(Identity(resolvedVal)), nil
 
 		} else if l != 1 {
 			return nil, fmt.Errorf("resolved name %q to %d input edges, rather than one", name, l)
@@ -123,7 +124,6 @@ func FunctionFromGraph(g *gg.Graph, scope Scope) (Function, error) {
 				return valToEdgeFn(Value{Value: ggVal})
 			},
 			func(ggEdgeVal gg.OptionalValue, inEdgeFns []edgeFn) (edgeFn, error) {
-
 				if ggEdgeVal.Equal(valNameIf.Value) {
 
 					if len(inEdgeFns) != 3 {

vm/scope.go

@@ -1,13 +1,18 @@
 package vm
 
 import (
-	"fmt"
+	"errors"
 )
 
+// ErrNameNotDefined is returned from Scope.Resolve when a name could not be
+// resolved within a Scope.
+var ErrNameNotDefined = errors.New("not defined")
+
 // Scope encapsulates a set of name->Value mappings.
 type Scope interface {
 
-	// Resolve accepts a name and returns an Value.
+	// Resolve accepts a name and returns an Value, or returns
+	// ErrNameNotDefined.
 	Resolve(string) (Value, error)
 }
 
@@ -23,7 +28,7 @@ func (m ScopeMap) Resolve(name string) (Value, error) {
 	v, ok := m[name]
 
 	if !ok {
-		return Value{}, fmt.Errorf("%q not defined", name)
+		return Value{}, ErrNameNotDefined
 	}
 
 	return v, nil

vm/vm_test.go

@@ -32,10 +32,16 @@ func TestVM(t *testing.T) {
 			in:     Value{Value: gg.Number(1)},
 			expErr: "name !foo cannot start with a '!'",
 		},
+		{
+			src: `{foo = bar; !out = foo;}`,
+			in:  Value{},
+			exp: Value{Value: gg.Name("bar")},
+		},
 	}
 
 	for i, test := range tests {
 		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			t.Log(test.src)
 			val, err := EvaluateSource(
 				bytes.NewBufferString(test.src), test.in, GlobalScope,
 			)