Add MonadWriter instance #534
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mrkkrp
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Jul 7, 2023
| @@ -100,6 +103,7 @@ instance (Ord t) => Monoid (Hints t) where | |||
| -- | |||
| -- See also: 'Consumption', 'Result'. | |||
| data Reply e s a = Reply (State s e) Consumption (Result s e a) | |||
| deriving (Functor) | |||
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Functor instances can be automatically derived.
| hoistP :: | ||
| (Stream s, Monad m) => | ||
| (m (Reply e s a) -> m (Reply e s b)) -> | ||
| ParsecT e s m a -> |
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Here, I opted for a less polymorphic version of the signature, since the extra flexibility is not actually needed, so it could confuse future readers (e.g. "do we actually perform a natural tranformation in these cases?").
| instance (Stream s, MonadWriter w m) => MonadWriter w (ParsecT e s m) where | ||
| tell w = lift (tell w) | ||
| listen = hoistP (fmap (\(repl, w) -> fmap (,w) repl) . listen) | ||
| pass = hoistP $ \m -> pass $ do |
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While I do appreciate the clever point-free implementation using traversals and Endo, I think it is not the most readable one. I went ahead and rewrote it in a more verbose way.
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A general function
hoistPcan be used to implement monad transformer class methods of the formMonadFoo m => m (f a) -> m (g a)for examplelocal,listenandpass. Usage ofhoistPbecomes more ergonomic whenResultandReplyhaveTraversableinstances.