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Planet Haskell0 Hot0 comments9 min read
review2 :: Reasonably Polymorphic
A retrospective on an old vim-like text editor's 'repeat' feature implementation, which used a custom monad with MonadState and MonadWriter to cache IO actions but crashed at runtime due to unsafe Dynamic type casting. The failure is traced to the fundamental problem that monadic bind hides branching behind opaque functions, making static analysis impossible. The fix rebuilds the feature using arrows (specifically Kleisli arrows composed with an applicative Static wrapper) which allow static structure analysis while still supporting effectful caching, and further shows that an ArrowChoice instance can derive a Selective functor instance for free, giving users either an arrow-based or applicative-based API for constructing repeatable actions.
Source: https://reasonablypolymorphic.com/blog/reviewing-again/index.html. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.
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