[Haskell-cafe] Pattern match checking for GADTs

[Oliver Charles]

I have some code that I could provide, but I don't seem to see George's
email address in the CC (only Tom's). Perhaps GMail ate it for some reason
- but either way, I don't know where to send my code :)

-- Ollie

On Thu, Feb 19, 2015 at 5:37 PM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

>  Friends
>
> George Karachalas, Tom Schrijvers, Dimitrios Vytiniotis, and I are working
> on finally cracking the problem of accurately reporting pattern-match
> overlap and redundancy warnings, in the presence of GADTs.   You know the
> problem; consider
>
> vzip :: Vect n a -> Vect n b -> Vect n (a,b)
>
> vzip VN        VN        = VN
>
> vzip (VC x xs) (VC y ys) = VC (x,y) (vzip xs ys)
>
>
>
> data Vect :: Nat -> * -> * where
>
>   VN :: Vect Zero a
>
>   VC :: a -> Vect n a -> Vect (Succ n) a
>
> Are there any missing equations in vzip?  No!  But GHC complains anyway.
> We have lots of Trac tickets about this; e.g.
> https://ghc.haskell.org/trac/ghc/ticket/3927.
>
> We now have a draft paper (wait a week) and a prototype implementation,
> that fixes the problem. But we need your help. We’d like to try our
> prototype on real code, not just toy examples.
>
> *So, could you send George a pointer to any packages you have, or know of,
> that*
>
> ·         *use GADTS (or other fancy type features) and *
>
> ·         *would benefit from accurate pattern-match overlap/redundancy
> warnings?*
>
> Specifically:
>
> ·         Where you have had to add a catch-all
>
> f _ _ = error “impossible”
>
> to silence GHC from saying “missing patterns”
>
> ·         Or where you have added {-# OPTIONS_GHC
> –fno-warn-missing-patterns #-} to silence the warnings.
>
> ·         Or something else like that.
>
> George’s email is in cc.
>
> Time is short – the ICFP deadline is on 27 Feb.  So sooner is better than
> later for us.  But later is better than never.
>
> Thank you!
>
> Simon
>
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>
>
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