ba0e755adc
This PR adds the function `Std.Iter.first?` and proves the specification lemma `Std.Iter.first?_eq_match_step` if the iterator is productive. The monadic variant on `Std.IterM` is also provided. We use this new function to fix the default implementation for `startsWith` and `dropPrefix` on `String` patterns, which used to fail if the searcher returned a `skip` at the beginning. None of the patterns we ship out of the box were affected by this, but user-defined patterns were vulnerable. --------- Co-authored-by: Paul Reichert <6992158+datokrat@users.noreply.github.com> |
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| LawfulMonadLiftFunction.lean | ||