Suppressing False Incomplete Pattern Matching Warnings for Polymorphic Pattern Synonyms

[Richard Eisenberg]

I'm afraid I don't understand what your new syntax means. And, while I know it doesn't work today, what's wrong (in theory) with

{-# COMPLETE LL #-}

No types! (That's a rare thing for me to extol...)

I feel I must be missing something here.

Thanks,
Richard

> On Oct 25, 2018, at 12:24 PM, Sylvain Henry <sylvain at haskus.fr> wrote:
> 
> > In the case where all the patterns are polymorphic, a user must
> > provide a type signature but we accept the definition regardless of
> > the type signature they provide. 
> Currently we can specify the type *constructor* in a COMPLETE pragma:
> 
> pattern J :: a -> Maybe a
> pattern J a = Just a
> 
> pattern N :: Maybe a
> pattern N = Nothing
> 
> {-# COMPLETE N, J :: Maybe #-}
> 
> Instead if we could specify the type with its free vars, we could refer to them in conlike signatures:
> 
> {-# COMPLETE N, [ J :: a -> Maybe a ] :: Maybe a #-}
> The COMPLETE pragma for LL could be:
> 
> {-# COMPLETE [ LL :: HasSrcSpan a => SrcSpan -> SrcSpanLess a -> a ] :: a #-}
> 
> I'm borrowing the list comprehension syntax on purpose because it would allow to define a set of conlikes from a type-list (see my request [1]):
> 
> {-# COMPLETE [ V :: (c :< cs) => c -> Variant cs
>              | c <- cs
>              ] :: Variant cs
>   #-}
> 
> >    To make things more formal, when the pattern-match checker
> > requests a set of constructors for some data type constructor T, the
> > checker returns:
> >
> >    * The original set of data constructors for T
> >    * Any COMPLETE sets of type T
> >
> > Note the use of the phrase *type constructor*. The return type of all
> > constructor-like things in a COMPLETE set must all be headed by the
> > same type constructor T. Since `LL`'s return type is simply a type
> > variable `a`, this simply doesn't work with the design of COMPLETE
> > sets.
> 
> Could we use a mechanism similar to instance resolution (with FlexibleInstances) for the checker to return matching COMPLETE sets instead?
> 
> 
> --Sylvain
> 
> 
> [1] https://mail.haskell.org/pipermail/ghc-devs/2018-July/016053.html <https://mail.haskell.org/pipermail/ghc-devs/2018-July/016053.html>
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