Reference Types

WAT syntax: (ref null? <heap-type>)

Heap Types

Heap types classify reference types.

• Abstract:
  any, eq, array, struct, func, extern

• Unboxed scalars: i31

• Bottom: none, noextern, nofunc

• User-defined (indexed):
  Arrays, Structs, Subtypes

Already existing heap types

• extern and func – via reference-types,

• (ref null? $t) – via function-references

Type Hierarchy

(from Chromium's Wasm GC proposal:
Spec for Prototype Implementation v.6)

Shorthands

• funcref == (ref null func)
• externref == (ref null extern)
• anyref == (ref null any)
• nullref == (ref null none)
• nullexternref == (ref null noextern)
• nullfuncref == (ref null nofunc)
• eqref == (ref null eq)
• structref == (ref null struct)
• arrayref == (ref null array)
• i31ref == (ref null i31)

Type Definitions

• Subtype – a single type, defined as a subtype of other types.

• Recursive type group – a group of mutually recursive types that can refer to each other.

New Instructions

The Wasm GC proposal introduces approximately 30 new instructions, including:

• i31.*: For creating and manipulating "unboxed scalars" (31-bit integer values).

• array.* and struct.*: For creating and manipulating arrays and structs.

• ref.*: For casting and testing values of reference types.

• br_on_*: For conditional branching depending on values of reference types.

• extern.*: For external reference conversion.