Re: Suppressing False Incomplete Pattern Matching Warnings for Polymorphic Pattern Synonyms

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 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 _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs