if and else in WebAssembly

Branches are typed: a result-carrying if must fill both arms.

(module
  ;; if is typed: (result i32) says both branches leave one value.
  (func (export "abs") (param $n i32) (result i32)
    (if (result i32)
      (i32.lt_s (local.get $n) (i32.const 0))
      (then (i32.sub (i32.const 0) (local.get $n)))
      (else (local.get $n))))

  ;; Without a result type, if is a statement and else may vanish.
  (func (export "cap_at_ten") (param $n i32) (result i32)
    (if (i32.gt_s (local.get $n) (i32.const 10))
      (then (return (i32.const 10))))
    (local.get $n)))

How it works

1. (if (result i32) ...) promises one value from either branch.
2. (then ...) and (else ...) each leave that value.
3. A statement if may skip else; return exits the whole function.

Keywords and builtins used here

• else
• export
• func
• i32
• if
• module
• param
• result
• return
• then

The run, in numbers

Lines

13

Characters to type

464

Tokens

108

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 100 seconds.

Type this snippet

Step 1 of 3 in Control flow, step 10 of 28 in Language basics.

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