Implemented parameters

src/build/expression/List.go

@@ -5,9 +5,20 @@ import (
 )
 
 // List generates a list of expressions from comma separated parameters.
-func List(tokens []token.Token) []*Expression {
+func List(tokens token.List) []*Expression {
 	var list []*Expression
 
+	EachParameter(tokens, func(parameter token.List) error {
+		expression := Parse(parameter)
+		list = append(list, expression)
+		return nil
+	})
+
+	return list
+}
+
+// EachParameter calls the callback function on each parameter in a comma separated list.
+func EachParameter(tokens token.List, call func(token.List) error) error {
 	start := 0
 	groupLevel := 0
 
@@ -25,17 +36,20 @@ func List(tokens []token.Token) []*Expression {
 			}
 
 			parameter := tokens[start:i]
-			expression := Parse(parameter)
-			list = append(list, expression)
+			err := call(parameter)
+
+			if err != nil {
+				return err
+			}
+
 			start = i + 1
 		}
 	}
 
 	if start != len(tokens) {
 		parameter := tokens[start:]
-		expression := Parse(parameter)
-		list = append(list, expression)
+		return call(parameter)
 	}
 
-	return list
+	return nil
 }