Overview of MATCH

MATCH observes a union or variant and selects an arm by option or type.

When every non-VOID variant alternative should compile the same source region and rely on overload resolution, use VISIT instead.

Union matching

MATCH failure AS payload
{
  CASE ok
  {
    ASSERT(FALSE);
  }
  CASE error AS code
  {
    ASSERT(code == 5);
  }
}

CASE name selects a union option. AS name on the header binds the payload for every payload-bearing arm; an arm-local AS name replaces that binding for the arm.

Variant matching

MATCH number AS payload
{
  TYPE I32 WHERE TRUE
  {
    payload++;
  }
  TYPE I32 OTHERWISE
  {
    payload := 0;
  }
  TYPE VOID
  {
    ASSERT(FALSE);
  }
  DEFAULT FAIL;
}

TYPE T selects a variant alternative. WHERE condition guards an arm. OTHERWISE is the final guarded arm for the same selector. DEFAULT handles unmatched or valueless states; DEFAULT FAIL; makes that path an explicit failure.

Shadow binding

When the subject is one bare name, SHADOW reuses it as the payload binding:

MATCH value SHADOW
{
  TYPE I32 { value++; }
  TYPE VOID { }
}

The arm binding is local and does not change the fusion object's type outside the arm.

An unguarded match must cover the declaration's options or types unless a DEFAULT arm handles what remains. A DEFAULT also handles a valueless state; without one, reaching that state fails. Union arms use CASE; variant arms use TYPE.

Reference

See the MATCH Reference for the complete language rules, constraints, and technical edge cases.