Lesson 16 · Let statements - Build a Programming Language

Let statements

So far a program is just a bare expression. Today you parse your first real statement - a let binding - which means teaching the parser to dispatch on the leading token and choose between statement kinds.

The goal

Parse a let binding into a statement holding the name and the value expression.

Start here - the target

TO DO

Scenario: Parsing a let binding

Giventhe source let x = 5;

Whenthe parser parses the program and prints it

Thenthe printed form is let x = 5;

Andthe statement records the name x and a value expression that prints as 5

Background

Until now every “statement” was really just an expression in disguise. A let statement is different: it has fixed structure - the keyword let, a name, an =, a value expression, and a semicolon. To handle it, parseStatement grows a dispatch: look at the leading token, and if it is let, parse a let statement; otherwise fall back to an expression statement.

That dispatch is the pattern every statement kind will follow, so set it up cleanly now. Parse the name into an identifier, require the =, then reuse parseExpression for the value - the binding’s value can be any expression you have already built, from 5 to 1 + 2 * 3. Return statements come next, and they slot into the very same dispatch.

Make it work

type LetStatement struct { Name *Identifier; Value Expression }
func (ls *LetStatement) String() string {
  return "let " + ls.Name.String() + " = " + ls.Value.String() + ";"
}
// parseStatement dispatches on the first token:
//   LET -> parseLet, else -> expression statement

CheckpointDONE

Let bindings parse into a statement holding a name and a value. Commit.