[Haskell-cafe] incomplete-uni-patterns

David Feuer david.feuer at gmail.com
Thu Jan 26 17:46:05 UTC 2023

There are occasional situations where the data structure is right but the
limitations of Haskell's type system prevent us from proving totality. I've
only ever written a couple I'm proud of:

1. Data.Biapplicative.traverseBiaWith
2. Data.Sequence.zipWith and Data.Sequence.chunksOf

traverseBiaWith seems to need an "impossible" case to achieve the right
semantics for lazy biapplicatives and infinite structures. Code shared by
zipWith and chunksOf uses one (relying on the tree annotation invariant) to
compute the result incrementally and (therefore) more efficiently.

On Thu, Jan 26, 2023, 8:34 AM Dan Dart <haskellcafe at dandart.co.uk> wrote:

> > {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}
> I feel like that's just avoiding facing the problem. It makes sense
> though, if you know 100% that there really, really is no other case.
> But as previous repliers have said, that's probably not a case for
> partials. If it's both known and partial, you're probably using the
> wrong datastructure for it. There's a reason many Prelude replacements
> don't include partial functions, and there's generally a better
> solution for it in them. And even if you're using base Prelude (which
> I am 99% of the time), things are only Maybe if they come from
> somewhere where they possibly could be.
> So, taking (!?) as an example, if you've statically set a list to
> contain a value and want it out, and want to avoid using partial
> functions like (!!) then perhaps static lists were not a good use case
> for you, is what I think people are getting at.
> > what if args is wrong?
> Another solution I use a lot is to match on lists, e.g.:
>     doSomethingWithListOfParameters :: [Text] -> IO ()
>     doSomethingWithListOfParameters xs = \case
>         [] -> someActionUsingNoParameters
>         [a] -> someActionUsingOneParameter
>         [a, b] -> someActionUsingTwoParameters
>         _ -> fail "don't know that one, sorry"
> I have adapted a lot of my code to use these uni patterns rather than
> avoiding them, as most of the time when this happens, it's not an
> impossible case for someone to pick the function up somewhere else and
> call it in the "wrong" way, and I handle it.
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