Mojo 1.0 Beta 1 Released — fn Keyword Deprecated, Unified Closures, Type Refinement, Feature-Complete Language Headed to Stable Later 2026

Mojo·medium signal

Modular shipped Mojo 1.0 Beta 1 on May 7, marking the language as feature-complete for 1.0. Breaking changes: the fn keyword is deprecated in favor of def (warning now, error next release). New: unified closures that auto-lift to top-level functions and pass as FFI callbacks, and type refinement from where clauses and comptime if/assert statements. The language compiles Python-syntax code to native performance for AI/ML workloads. New domain at mojolang.org. 85 points on HN May 8.