[Haskell-cafe] type inference and named fields

Jacques Carette carette at mcmaster.ca
Thu Jun 23 14:16:50 EDT 2005

I was under the impression that, in ghc 6.4 at least, GADTs did just that: use information gained by matching on the 
type constructor to refine types.  I sort-of expected that the extension to pattern matching would follow.

Or is that a nice paper waiting to be written?


Lennart Augustsson <lennart at augustsson.net> wrote:
> A somewhat similar problem exists even without fields:
> foo :: Either a b -> Either () b
> foo (Left _) = Left ()
> foo x@(Right _) = x
> Since Haskell type checking doesn't use the information gained
> by pattern matching to refine types we just have to accept that
> some perfectly safe programs don't type check.
> 	-- Lennart

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