Lesson 18 · If expressions - Build a Programming Language

If expressions

Conditionals are how a program chooses. Today you parse if/else - which needs a block statement to hold each branch's body, so you build that too and use if as its first home.

The goal

Parse an if/else into an expression holding a condition and consequence and alternative blocks.

Start here - the target

TO DO

Scenario: Parsing an if/else expression

Giventhe source if (x < y) { x } else { y }

Whenthe parser parses the program and prints it

Thenthe printed form is if (x < y) x else y

Andparsing if (x < y) { x } (no else) prints if (x < y) x

Background

An if needs somewhere to put the statements in each branch, so first build a block statement: a { ... } holding a list of statements, parsed with the same loop as the whole program but stopping at the closing brace. This is the container you will reuse for function bodies and loop bodies too.

With blocks in hand, an if expression is a condition (in parentheses), a consequence block, and an optional else alternative block - store nil for the alternative when there is no else. It is an expression, not a statement, because in this language if produces a value; that will matter when you evaluate it. The printed form drops the braces for readability, so if (x < y) x else y still shows the structure clearly.

Make it work

type BlockStatement struct { Statements []Statement }
type IfExpression struct {
  Condition Expression
  Consequence *BlockStatement
  Alternative *BlockStatement   // nil when there is no else
}
// parseBlock: consume '{', parse statements until '}'

CheckpointDONE

If/else parses into an expression with a condition and one or two blocks. Commit.