[Haskell-cafe] Applying typed hole proposal leads to compilation failure
Vlatko Basic
vlatko.basic at gmail.com
Fri Aug 24 16:06:30 UTC 2018
Hello Cafe,
I encountered a strange issue regarding typed hole. If I ask GHC (8.4.3) for a
typed hole proposal, it says 'use c', but if I use 'c', it complains that it
can't unify 'c' and 'c1'.
Why does GHC think those two are different and how to tell to GHC they are the
same?
Best regards,
vlatko
*Typed hole proposal:*
• Found type wildcard ‘_c’ standing for ‘c’
Where: ‘c’ is a rigid type variable bound by
the type signature for:
f1 :: forall (m :: * -> *) c. MonadIO m => c -> m Bool
at Test.hs:15:1-32
• In the type signature:
run :: MS _c Int a -> (Either String a, Int)
*Error when typed hole used:*
• Couldn't match type ‘c1’ with ‘c’
‘c1’ is a rigid type variable bound by
the type signature for:
run :: forall c1 a. MS c1 Int a -> (Either String a, Int)
at Test.hs:21:5-47
‘c’ is a rigid type variable bound by
the type signature for:
f1 :: forall (m :: * -> *) c. MonadIO m => c -> m Bool
at Test.hs:15:1-32
Expected type: MS c1 Int a -> (Either String a, Int)
Actual type: MS c Int a -> (Either String a, Int)
*
*
*Minimal runnable example:*
{-# LANGUAGE ScopedTypeVariables, PartialTypeSignatures #-}
module Test where
import Prelude
import Control.Monad.IO.Class (MonadIO)
import Control.Monad.Trans.Except (ExceptT, runExceptT)
import Control.Monad.Trans.Reader (Reader, runReader)
import Control.Monad.Trans.State (StateT, runStateT)
type MS c s = ExceptT String (StateT s (Reader c))
runMS :: c -> s -> MS c s a -> (Either String a, s)
runMS c s f = runReader (runStateT (runExceptT f) s) c
f1 :: (MonadIO m) => c -> m ()
f1 c = do
let _x1 = run f2
let _x2 = run f3
return ()
where
run :: MS *_c*Int a -> (Either String a, Int) --- failure or success, 'c'
or '_'
run = runMS c 0
f2 :: MS c s Bool
f2 = pure False
f3 :: MS c s [Int]
f3 = pure []
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