Lean 4 fork for HoTT-compatible kernel extensions (Path types, transport, HITs). Maintained against upstream leanprover/lean4.
049b7ebee2
This PR adds `whileM`, a counterpart to `Lean.Loop.forIn` that admits a one-step unfolding lemma `whileM_eq` (impossible to prove for the original `partial def`). `Lean.Loop.forIn` now expands to `whileM`, so `repeat`/`while` keep working without source changes, and the `Spec.whileM`/`Spec.forIn_loop` `@[spec]` theorems let `mvcgen` discharge their bodies given a Nat variant and an `α ⊕ β` invariant. `whileM.impl` is still a `partial def`, but returns a `Subtype (whileM.Pred f a)` whose property pins the value to an `Acc.recOn` term whenever an `Acc` and a `MonadAttach` witness exist; `whileM_eq` extracts that property. A `@[implemented_by]` `whileM.erased` keeps the runtime a tail call after specialization and would be unnecessary if the compiler were able eta-expand through the trivial `Subtype` structure. Supporting infrastructure: `Internal.Ensures`/`MayReturn`/`ErasesTo`/`IsAttach` and `WPAdequate` for `Id`/`ReaderT`/`StateT`/`ExceptT`/`OptionT`. The resulting `while` loops take more work to optimize, hence a modest increase in build time instructions. |
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