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 apattern J a = Just apattern N :: Maybe apattern 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

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

Reply via email to