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snoyberg / mono-traversable

Type classes for mapping, folding, and traversing monomorphic containers
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Provide a pattern version of `uncons` in `classy-prelude` #241

Open vigress8 opened 2 months ago

vigress8 commented 2 months ago

I'm thinking of something like:

{-# language ViewPatterns #-}

pattern x ::: xs <- (uncons -> Just (x, xs)) where
  (:::) = cons

I find myself wanting sequence-polymorphic pattern matching often, personally. It's trivial to define manually but having it in classy-prelude would make things simpler. Curious what anyone else thinks

BebeSparkelSparkel commented 2 months ago

I see many instances of case uncons xs of Just (x, xs) -> ..., so it seems like a useful addition.

However, I do not see any patterns already defined in mono-traversable. @gregwebs do you have any thoughts on adding pattern exports ghc has had them since 7.8.1 so it seems like a well established feature that does not need CPP for version checking.

@vigress8 can you comment on how to have completeness for the pattern? It seems like this will generate many warnings for incomplete patterns.

gregwebs commented 2 months ago

hmm, why would this package be the one that defines patterns to export? It doesn't seem related to mono-traversable.

I think that a custom prelude package would define this.

vigress8 commented 2 months ago

Sorry I wasn't specific, by "this package" I meant classy-prelude (which is in this repository). Edited now

vigress8 commented 2 months ago

It doesn't seem related to mono-traversable

It's related because the pattern is meant to be generic over IsSequence.

gregwebs commented 2 months ago

sure, makes sense for classy-prelude.

vigress8 commented 2 months ago

@vigress8 can you comment on how to have completeness for the pattern? It seems like this will generate many warnings for incomplete patterns.

To me the simplest way to achieve completeness seems to be

pattern Empty <- (uncons -> Nothing)
{-# complete Empty, (:::) #-}
BebeSparkelSparkel commented 2 months ago

Is the name clash acceptable https://hoogle.haskell.org/?hoogle=%3A%3A%3A

gregwebs commented 2 months ago

Probably better to just use a name and not have a symbol? At least to start?

vigress8 commented 2 months ago

The specific symbol isn't critically important to me, I just chose it for example's sake

BebeSparkelSparkel commented 2 days ago

Ok, let's name the pattern Cons. @vigress8 can you crate a PR?