Lesson 19 · While statements - Build a Programming Language

While statements

Loops are how a program repeats. Today you parse a while statement - a condition and a body block - reusing the block parser you just built for if.

The goal

Parse a while loop into a statement holding a condition and a body block.

Start here - the target

TO DO

Scenario: Parsing a while loop

Giventhe source while (x < 10) { x }

Whenthe parser parses the program and prints it

Thenthe printed form is while (x < 10) x

Andthe statement records a condition that prints as (x < 10) and a body block

Background

A while statement is structurally an if without the else: the keyword, a parenthesized condition, and a body block. Add a WHILE case to the statement dispatch, parse the condition inside its parentheses, and reuse parseBlock for the body - no new container needed.

Unlike if, a loop is a statement, not an expression: it exists to repeat work, not to produce a value. That is a design choice you are baking into the tree now and will honor in the evaluator, where the loop’s job is to run its body over and over while the condition holds.

Make it work

type WhileStatement struct { Condition Expression; Body *BlockStatement }
func (ws *WhileStatement) String() string {
  // the condition already prints its own parens when it is an infix
  // expression, so don't add another pair here (matches how if prints)
  return "while " + ws.Condition.String() + " " + ws.Body.String()
}
// add a WHILE case to parseStatement, mirroring if's condition+block

CheckpointDONE

While loops parse into a statement with a condition and a body block. Commit.