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