[Haskell-cafe] type inference and named fields

[Lennart Augustsson]

It's true that GADTs does just that.  But only if you use them.
And they are not part of Haskell. :)

	-- Lennart

Jacques Carette wrote:
> 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?
> 
> Jacques
> 
> 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
> 
>