The function type constructor actually has type
(->) :: forall (r1::RuntimeRep) (r2::RuntimeRep). TYPE r1 -> TYPE r2 -> TYPE LiftedRep
But when it appears applied to two argument, thus ((->) r1 r2 t1 t2), we print it infix (t1 -> t2).
So (-> 'GHC.Types.LiftedRep 'GHC.Types.LiftedRep thing thing)
is the same as (thing -> thing).
I cannot explain why it is being printed in that non-infix way, but your email doesn’t give enough context to know.
I hope this helps a bit. Where did you look for information? I’d like to add pointers that would have been helpful to you.
From: Ranjit Jhala <[hidden email]>
Sent: 24 April 2019 04:56
To: [hidden email]; Simon Peyton Jones <[hidden email]>
Subject: Puzzled by GHC.Types.LiftedRep
Consider the following program:
import Control.Arrow (second)
import qualified Control.Monad.Trans.Reader as R
myLocal :: (r -> r) -> R.ReaderT r m a -> R.ReaderT r m a
myLocal = undefined
foo :: (thing -> thing) -> R.ReaderT (d, thing) m a -> R.ReaderT (d, thing) m a
foo arg = myLocal (second arg)
When compiled with GHC 8.6.4, I get the odd behavior that
at the call `second arg`, the type of `arg` is just:
thing -> thing
but the *input* type of `second` (after applying the
various type and dictionary parameters) is something
that looks like
while the *input* type of `second` (after the various type and dictionary applications etc.) is the mysterious (to me):
(-> 'GHC.Types.LiftedRep 'GHC.Types.LiftedRep thing thing)
Can someone explain or point me to some documentation or
paper that describes what the above means? Specifically,
how/why does GHC "equate" the two things above in order
to consider the core well-formed?
Many thanks in advance!