Provisional API: This reference documents current compiler behavior. Compatibility is not yet promised.

src/components/pattern-match.x

Match statements.

Design notes

A match lowers to C text around its subject, patterns, and bodies: a block that reads the subject once as a List and switches on the bits of its head. Lowering steps into the nodes the text holds, and the switch is the barrier the cleanup walk knows, so an arm’s break leaves the match and continue reaches the enclosing loop. Each arm tests its pattern, declares its binders, runs its body, and breaks; an arm that fails falls into the next. A static pattern of literals, unique binders, typed captures, nested Lists, and a final * is tested in place by nested if tests on cursors over the subject. Any other pattern captures into a buffer: a macro-valued case through its macro, and the rest through the Match runtime, at a static site when the pattern’s value is the same each time it runs.

Each definition precedes the definitions that call it: the compiler settles whether a linked copy reaches a compile-time operation when the copy binds.