func: Wasm type definition

Syntax

;; Declare a named function type
(type $bin_op (func (param i32) (param i32) (result i32)))

;; A function inheriting that signature
(func $add (type $bin_op) (param $a i32) (param $b i32) (result i32)
  local.get $a
  local.get $b
  i32.add)

;; A call_indirect site using the same type
(call_indirect (type $bin_op))

Description

A function type maps a sequence of parameter value types to a sequence of result value types. Either list may be empty.

Function types appear in three places:

• Function declarations: Every func has a type. You can either inline param and result clauses, or reference a named type with (type $name). When both are present, they must agree. Inlining a function type on a declaration is equivalent to writing a separate function type and referencing it.
• Indirect calls: call_indirect takes a function type and dispatches through a table of funcref values. The engine checks at runtime that the indexed function's type matches the declared type and traps otherwise.
• Imports: Function imports declare a type that must be provided for a function.

Function types use structural equality: two function types are identical if and only if their parameter and result type sequences match element-for-element. The identity of the type index or the module it came from does not matter. A call_indirect will therefore succeed against a callee whose type was declared independently in another module, as long as the parameter and result sequences are the same.

Examples

Sharing a function type across two functions and a dispatcher

The following module declares a single $bin_op type and uses it for two functions and a call_indirect site:

(module
  (type $bin_op (func (param i32) (param i32) (result i32)))

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

  (func $mul (type $bin_op) (param $a i32) (param $b i32) (result i32)
    local.get $a
    local.get $b
    i32.mul)

  (table 2 funcref)
  (elem (i32.const 0) $add $mul)

  (func (export "dispatch") (param $op i32) (param $a i32) (param $b i32) (result i32)
    local.get $a
    local.get $b
    local.get $op
    call_indirect (type $bin_op)))

Calling dispatch(0, 3, 4) invokes $add and returns 7; calling dispatch(1, 3, 4) invokes $mul and returns 12. Passing an index that points to a function with a different signature traps.

Specifications

See also

• funcref value type
• table definition