Lesson 17 · Return statements - Build a Programming Language

Return statements

Functions need a way to hand back a value, and that is the return statement. Today you parse it - the same dispatch-and-parse shape as let, with just an expression to carry.

The goal

Parse a return statement into a node holding the returned expression.

Start here - the target

TO DO

Scenario: Parsing a return statement

Giventhe source return 10;

Whenthe parser parses the program and prints it

Thenthe printed form is return 10;

Andparsing return 2 * 5; prints return (2 * 5);

Background

A return statement is the keyword return followed by an expression and a semicolon. It fits the exact shape you built for let: add a RETURN case to the statement dispatch, consume the keyword, and parse the value expression that follows.

There is nothing to bind on the left this time - just the returned value - so the node is even simpler than a let. The printed form return (2 * 5); confirms the value expression is parsed with full precedence like everywhere else. You are not running returns yet; you are building the tree the evaluator will later use to jump out of a function.

Make it work

type ReturnStatement struct { ReturnValue Expression }
func (rs *ReturnStatement) String() string {
  return "return " + rs.ReturnValue.String() + ";"
}
// add a RETURN case to parseStatement's dispatch

CheckpointDONE

Return statements parse into a node holding the returned expression. Commit.