Types

WAT’s type system is intentionally small. Most modules only need a handful of core types.

Value types

Number types:

• i32 and i64: 32-bit and 64-bit integers. Signed/unsigned use the same type — the operator determines interpretation.
• f32 and f64: 32-bit and 64-bit IEEE 754 floats.

Reference types:

• funcref: reference to a function.
• externref: opaque reference to a host value (e.g. a JS object).

Function types

Functions declare parameter and result types. You can name parameters for readability.

(module
  (func $add (param $a i32) (param $b i32) (result i32)
    (i32.add (local.get $a) (local.get $b)))

  (func $mix (param $x i64) (param $y f32) (result f64)
    (f64.add
      (f64.convert_i64_s (local.get $x))
      (f64.promote_f32 (local.get $y))))
)

• Multiple results are supported: (result i32 i64) pushes two values.
• Use local to declare function-local variables: (local $acc f64).

Blocks and block types

Control blocks (block, loop, if) can declare result types, which affect stack typing.

(module
  (func (param $n i32) (result i32)
    (if (result i32) (i32.eqz (local.get $n))
      (then (i32.const 42))
      (else (i32.const 7))))
)

Both branches of an if with a result must produce the same typed value(s).

Tables, memories, and globals

• Tables: funcref or externref elements with limits, e.g. (table 1 funcref) or (table 1 10 funcref).
• Memories: linear byte arrays sized in 64 KiB pages, e.g. (memory 1) or (memory 1 4).
• Globals: typed, optionally mutable values, e.g. (global (mut i32) (i32.const 0)).

(module
  (table $t 1 funcref)
  (memory $mem 1)
  (global $counter (mut i32) (i32.const 0))
)

Instruction Reference

• Type Names — all value type keywords
• Module Structure — func, param, result, local, global, table, memory