30 years on, Lean turns Edinburgh's 'proofs beside code' idea into reality

Sanity · x · 2026-11-06

The author is learning Lean, a programming language that doubles as a proof assistant. He recalls Edinburgh University lecturers (where ML was born) describing a language where you could include proofs alongside code, with the compiler ignoring them.

Thirty years later, Lean can actually verify those proofs — the old vision has become reality.

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