leanprover/lean4

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Lean 4 programming language and theorem prover

Stars: 9.2k
Forks: 990
Open beginner issues: 0
Indexed issues: 1k
Avg merge: 1d 17h
Merged PRs (30d): 175

Dominant language: Lean
License: Apache-2.0
Last GitHub push: Sep 19, 2026
Latest indexed: Sep 20, 2026
Contributing guide: Contributing guide
Code of conduct: No code of conduct
Beginner labels: help wanted

Beginner friendly Everything

Open All

14 beginner-friendly issues open

`simp [...]` with a left arrow (`←`) on a simproc should error Open

bug

Difficulty 2/5 1-3 hours Newbie friendliness 84/100

leanprover/lean4#15197 · Opened Sep 17, 2026
MULTI_THREAD=OFF build fails: std::adopt_lock_t used in thread.h without including <mutex> Open

Difficulty 2/5 1-3 hours Newbie friendliness 86/100

leanprover/lean4#15172 · 1 comment · Opened Sep 15, 2026
Delaborator omits an instance binder's name when it is used only in a later binder's type Open

P-medium

Difficulty 2/5 1-3 hours Newbie friendliness 78/100

leanprover/lean4#15037 · Opened Sep 4, 2026
`Nat.lt` docstring Open

P-low

Difficulty 1/5 Under an hour Newbie friendliness 85/100

leanprover/lean4#15014 · Opened Sep 4, 2026
Emscripten libuv stub definitions do not match their declarations Open

P-medium

Difficulty 2/5 1-3 hours Newbie friendliness 84/100

leanprover/lean4#14973 · 1 comment · Opened Aug 30, 2026
deriving ToJson/FromJson generates ill-typed code for self-recursion in a non-first mutual member Open

Difficulty 2/5 1-3 hours Newbie friendliness 85/100

leanprover/lean4#14958 · Opened Aug 28, 2026
simp still panics with appArg!/appFn! in the non-indexed rewriting path Open

Difficulty 2/5 1-3 hours Newbie friendliness 82/100

leanprover/lean4#14930 · 1 comment · Opened Aug 25, 2026
`Lean.Meta.Sym.Simp.toHave` reverses dependencies when reconstructing `have` telescopes Open

bug P-medium

Difficulty 2/5 1-3 hours Newbie friendliness 78/100

leanprover/lean4#14804 · Opened Aug 17, 2026
`lambdaMetaTelescope` doesn't preserve binder names Open

bug P-low

Difficulty 2/5 1-3 hours Newbie friendliness 78/100

leanprover/lean4#14264 · 1 reaction · Opened Jul 3, 2026
RFC: expand reducibility warning about class-typed `def`s Open

P-medium RFC

Difficulty 2/5 1-3 hours Newbie friendliness 70/100

leanprover/lean4#13351 · Opened Apr 9, 2026
bug: error message of `Lean.Json.setObjVal!` lacks string formatting Open

bug P-low

Difficulty 1/5 Under an hour Newbie friendliness 62/100

leanprover/lean4#7307 · Opened Mar 3, 2025
Vector.getElem_take range proof not resolved by unification Open

bug P-medium

Difficulty 2/5 1-3 hours Newbie friendliness 76/100

leanprover/lean4#7212 · 1 comment · 1 reaction · Opened Feb 24, 2025
`--threads=0` leads to an assertion violation Open

bug P-low

Difficulty 2/5 1-3 hours Newbie friendliness 75/100

leanprover/lean4#4927 · 1 comment · Opened Aug 6, 2024
ToString instance for Xml does not correctly escape Open

bug P-low

Difficulty 2/5 1-3 hours Newbie friendliness 75/100

leanprover/lean4#4411 · 3 comments · 1 reaction · Opened Jun 9, 2024

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