Opened 3 years ago

Closed 3 years ago

#10326 closed bug (wontfix)

ImpredicativeTypes: Unable to use in Functor etc..

Reported by: j80JjBjVNRMajmA Owned by:
Priority: normal Milestone:
Component: Compiler Version: 7.10.1
Keywords: Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


I want to use values of RankNTypes within Functors/Monads/... just like normal values.

The following code shows, that I cannot lift a function ReturnNull -> ReturnNull into a functor the way I would like to (e.g. liftId2). However, using another version that seems equivalent and that is allowed by GHC, liftId3, does not allow me to pipe the result to override. How can I do this?

{-# LANGUAGE ImpredicativeTypes #-}

type ReturnNull = forall m. Monad m => m ()

id2 :: ReturnNull -> ReturnNull
id2 = id

testId2 :: ReturnNull
testId2 = id2 $ return ()

liftId :: Functor f => f ReturnNull -> f ReturnNull
liftId = fmap id

-- rejected
liftId2 :: Functor f => f ReturnNull -> f ReturnNull
liftId2 = fmap id2

liftId3 :: (Monad m,Functor f) => f ReturnNull -> f (m ())
liftId3 = fmap id2

override :: Functor f => f ReturnNull -> f ()
override = fmap $ const ()

testLift :: Functor f => f ReturnNull -> f ()
testLift = override . liftId

testLift3 :: Functor f => f ReturnNull -> f ()
testLift3 = override . liftId3

Change History (1)

comment:1 Changed 3 years ago by goldfire

Resolution: wontfix
Status: newclosed

ImpredicativeTypes is thoroughly broken and is unsupported. In the past, this extension was more properly implemented, but caused more grief than help even then.

If you want this kind of thing, use a newtype instead of a type. It's surely more annoying, but it's better to have an annoying option instead of a totally-broken one.

There are a number of Very Clever People working on integrating impredicativity with GHC-style type inference. (I'm not one of them, however.) In GHC's view, there has not yet been the right solution. We all look forward to when the right solution is found!

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