lang: funcs: structs: Add Call method for const

lang/funcs/structs/const.go

@@ -94,12 +94,25 @@ func (obj *ConstFunc) Init(init *interfaces.Init) error {
 
 // Stream returns the single value that this const has, and then closes.
 func (obj *ConstFunc) Stream(ctx context.Context) error {
+	value, err := obj.Call(ctx, nil)
+	if err != nil {
+		return err
+	}
 	select {
-	case obj.init.Output <- obj.Value:
+	case obj.init.Output <- value:
 		// pass
 	case <-ctx.Done():
 		return nil
 	}
 	close(obj.init.Output) // signal that we're done sending
 	return nil
 }
+
+// Call this function with the input args and return the value if it is possible
+// to do so at this time.
+func (obj *ConstFunc) Call(ctx context.Context, args []types.Value) (types.Value, error) {
+	if obj.Value == nil {
+		return nil, fmt.Errorf("no value available from const")
+	}
+	return obj.Value, nil
+}