Lean 4 fork for HoTT-compatible kernel extensions (Path types, transport, HITs). Maintained against upstream leanprover/lean4.
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In order to guarantee termination of type class synthesis when resolving coercions, a good rule of thumb is that the instances should not introduce metavariables (during TC synthesis). For common coercion type classes this means: - `Coe T S`, `CoeTail T S`: the variables of `T` must be a subset of those of `S` - `CoeHead T S`: the variables of `S` must be a subset of those of `T` If these rules are not followed, we can easily get nontermination. In this case: `CoeTC Foo Syntax` is reduced to `CoeTC Foo (TSyntax ?m_1)` using the (dangerous) `Coe (TSyntax k) Syntax` instance, to which we can then apply the otherwise fine `Coe (TSyntax [k]) (TSyntax (k'::ks))` coercion infinitely often. |
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This is the repository for Lean 4, which is currently being released as milestone releases towards a first stable release. Lean 3 is still the latest stable release.
About
- Quickstart
- Walkthrough installation video
- Quick tour video
- Homepage
- Theorem Proving Tutorial
- Functional Programming in Lean first chapter is available!
- Manual
- Release notes starting at v4.0.0-m3
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- FAQ
Installation
See Setting Up Lean.
Contributing
Please read our Contribution Guidelines first.
Building from Source
See Building Lean.