Formal proposal for closures

[Caroline (Caroline Frasca)]

Hi @monte,

Apologies for the slow response here! Just wanted to make sure you’ve seen @Steffi’s recent post on the updates to closures:

Mojo Closures 2026

Mojo Closures Before this release, Mojo has had several flavors of closures: Legacy capturing[_] closures — @parameter functions passed only as compile-time type parameters, with implicit captures (and @__copy_capture to opt into copy semantics). Escaping closures — escaping functions emitted as heap-allocated struct instances and passed as runtime values. Unified closures — declared with the unified keyword, with explicit capture lists (e.g. unified {var x, mut y}), and passed as both a type …