This looks very similar to https://ghc.haskell.org/trac/ghc/ticket/11319, but might be worth including as a separate example there. Note that it does compile if you swap the order of the case alternatives. Regards, Reid Barton On Fri, Mar 4, 2016 at 8:43 AM, Kosyrev Serge <_deepfire@feelingofgreen.ru> wrote:
Good day!
I realise that ImpredicativeTypes is a problematic extension, but I have found something that looks like an outright bug -- no polymorphism involved:
,---- | {-# LANGUAGE ImpredicativeTypes #-} | | module Foo where | | foo :: IO (Maybe Int) | foo = do | pure $ case undefined :: Maybe String of | Nothing | -> Nothing | Just _ | -> (undefined :: Maybe Int) `----
produces the following errors:
,---- | foo.hs:7:3: error: | • Couldn't match type ‘forall a. Maybe a’ with ‘Maybe Int’ | Expected type: IO (Maybe Int) | Actual type: IO (forall a. Maybe a) | • In a stmt of a 'do' block: | pure | $ case undefined :: Maybe String of { | Nothing -> Nothing | Just _ -> (undefined :: Maybe Int) } | In the expression: | do { pure | $ case undefined :: Maybe String of { | Nothing -> Nothing | Just _ -> (undefined :: Maybe Int) } } | In an equation for ‘foo’: | foo | = do { pure | $ case undefined :: Maybe String of { | Nothing -> Nothing | Just _ -> (undefined :: Maybe Int) } } | | foo.hs:11:19: error: | • Couldn't match type ‘a’ with ‘Int’ | ‘a’ is a rigid type variable bound by | a type expected by the context: | forall a. Maybe a | at foo.hs:11:19 | Expected type: forall a. Maybe a | Actual type: Maybe Int | • In the expression: (undefined :: Maybe Int) | In a case alternative: Just _ -> (undefined :: Maybe Int) | In the second argument of ‘($)’, namely | ‘case undefined :: Maybe String of { | Nothing -> Nothing | Just _ -> (undefined :: Maybe Int) }’ `----
-- с уважениeм / respectfully, Косырев Сергей _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs