Lennart Augustsson
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.
Well, not perfectly safe. We already had this discussion (about []), but to say it again: the value bound by any pattern has some particular type. x above doesn't have type (Either a b) for arbitrary a and b, but rather (Either a b) for some specific a and b. Unless that specific a happens to be (), x doesn't have the correct type to be the rhs of foo. It would require a radical change to GHC (moving away from System F as a basis for the intermediate language Core) to implement a language in which this wasn't true. Here is (approximately) the Core translation of foo above:
foo :: forall a b. Either a b -> Either () b foo = /\ a b -> \ e -> case e of Left _ -> Left @() @b () Right _ -> e
You call this as (foo @a @b e), where e has type (Either a b). So, the call (foo @a @b (Right @a @b x)) reduces (ignoring types) to (Right @a @b x). This reduces a term of type (Either () b) to one of type (Either a b). That's not type safe. Jon Cast