diff --git a/ph7.go b/ph7.go
index 7336a39..08880f0 100644
--- a/ph7.go
+++ b/ph7.go
@@ -489,6 +489,7 @@ func write_vm_output(buf unsafe.Pointer, bufLen C.uint, userData unsafe.Pointer)
 }
 
 func (vm *VM) SetOutputWriter(w io.Writer) error {
+	// TODO Does this need to reset to the default if it receives nil?
 	vm.outputWriter = w
 	return newError(C.vm_set_output_callback(vm.ptr, vm.pointerKey))
 }
@@ -562,24 +563,22 @@ func (vm *VM) Close() error {
 	return newError(C.ph7_vm_release(vm.ptr))
 }
 
-type ForeignFunction interface {
-	Call(ctx *Context, args []*Value) ResultCode
-}
+type Function func(ctx *Context, args []*Value) ResultCode
 
 //export call_foreign_function
 func call_foreign_function(ctx *C.ph7_context, argc C.int, argv **C.ph7_value) C.int {
 	fkey := C.ph7_context_user_data(ctx)
 	// TODO it looks like ph7 looks for a VFS at the user data pointer. WTF? Should we use it for that?
-	f := pointer.Restore(fkey).(ForeignFunction)
+	f := pointer.Restore(fkey).(Function)
 	args := make([]*Value, 0, argc)
 	argvSlice := unsafe.Slice(argv, argc)
 	for i := range argvSlice {
 		args = append(args, (*Value)(argvSlice[i]))
 	}
-	return C.int(f.Call((*Context)(ctx), args))
+	return C.int(f((*Context)(ctx), args))
 }
 
-func (vm *VM) CreateFunction(name string, f ForeignFunction) error {
+func (vm *VM) CreateFunction(name string, f Function) error {
 	cname := C.CString(name)
 	defer C.free(unsafe.Pointer(cname))
 
diff --git a/ph7_test.go b/ph7_test.go
index d574a51..09edf2b 100644
--- a/ph7_test.go
+++ b/ph7_test.go
@@ -139,23 +139,23 @@ func TestOutputWriter(t *testing.T) {
 // TODO Test failing writer, including vm.OutputWriteError
 // TODO Test setting writer twice
 
-type CtxFunc func(*ph7.Context) error
-
-func (r CtxFunc) Call(ctx *ph7.Context, _ []*ph7.Value) ph7.ResultCode {
-	err := r(ctx)
-	if err != nil {
-		return r(ctx).(ph7.Error).Code
+func CtxFunc(f func(*ph7.Context) error) ph7.Function {
+	return func(ctx *ph7.Context, _ []*ph7.Value) ph7.ResultCode {
+		err := f(ctx)
+		if err != nil {
+			return f(ctx).(ph7.Error).Code
+		}
+		return ph7.ResultCodeOK
 	}
-	return ph7.ResultCodeOK
 }
 
-func testResultCode(t *testing.T, expectedOutput string, f CtxFunc) {
+func testResultCode(t *testing.T, expectedOutput string, f func(*ph7.Context) error) {
 	t.Helper()
 	const script = "echo doStuff();"
 	vm, close := setupVM(t, script)
 	defer close()
 
-	mustSucceed(t, vm.CreateFunction("doStuff", f))
+	mustSucceed(t, vm.CreateFunction("doStuff", CtxFunc(f)))
 	expectOutput(t, vm, expectedOutput)
 }