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?
vlatko
Typed hole proposal:
Minimal runnable example: {-# LANGUAGE ScopedTypeVariables,
PartialTypeSignatures #-} _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
> • 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 Emphasis on "rigid". It's not telling you to introduce a new type variable and put that there. It's telling you that the type you need to put there is an existing type variable's type. When you write 'run :: MS c Int a -> (Either String a, Int)' you implicitly mean 'run :: forall c.' which is exactly introducing a new type variable. > • 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) This is the 'c' you bound with the implicit 'forall'. The compiler is asked to verify that 'run' indeed works 'forall c1', so during typechecking of the function body the 'c1' variable is also rigid. > ‘c’ is a rigid type variable bound by > the type signature for: > f1 :: forall (m :: * -> *) c. MonadIO m => c -> m Bool This is the 'c' from the typed hole suggestion up above, still rigid. A part of the typechecking algorithm is that two rigid type variables cannot be equated. The solution *actually* proposed by GHC in the wildcard suggestion is to use the 'c' variable from 'f1's type for which you need to make it scoped with an explicit 'forall': f1 :: forall c. (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) run = runMS c 0 f2 :: MS c s Bool f2 = pure False f3 :: MS c s [Int] f3 = pure [] _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
Note that this also requires ScopedTypeVariables; the Haskell standard specifies that type variables are only in scope within the type signature, not the accompanying binding. Which is also why the explicit "forall" is required, to tell it to use the modified rules here, which otherwise could cause other code that expects standard Haskell behavior to fail to compile if it happens to reuse type variables from the signature. On Fri, Aug 24, 2018 at 5:37 PM mniip <[hidden email]> wrote: > • Found type wildcard ‘_c’ standing for ‘c’ brandon s allbery kf8nh _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
On Fri, Aug 24, 2018 at 06:25:34PM -0400, Brandon Allbery wrote:
> Note that this also requires ScopedTypeVariables; the Haskell standard > specifies that type variables are only in scope within the type signature, > not the accompanying binding. Which is also why the explicit "forall" is > required, to tell it to use the modified rules here, which otherwise could > cause other code that expects standard Haskell behavior to fail to compile > if it happens to reuse type variables from the signature. I had thought of mentioning this but their original snippet included the extension so I thought they already knew. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
In reply to this post by mniip
Hi mniip, Let me first apologise for my very late response. I went for a visit to the analog world, and stayed much longer than planned. :-)
I have ScopedTypeVariables enabled as a default extension in .cabal file, but have never encountered such an error, to have to manually specify forall just for making scoped types to work. I'm using local signatures quite often, but still not quite clear as to how/where the original code differs, for example, from this one (which compiles fine):
mkTransUnitValTag :: (HasGlobals s) =>
InNode -> MS c s TransUnitValT ...
Is the main diff that 'run' is having monad
stack as input and is running it, while 'mkSegTag' is run in
it (so forall does not have to be specified manually)? f1 :: forall m c. (MonadIO m) => c
-> m () -- original code where run :: MS c Int a -> (Either String a, Int) Thanks for pointing me to read the whole error/warning. Everything is actually written there, but seems I have developed some kind of forall blindness. :-(
On 24/08/2018 23:36, mniip wrote:
• 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:15:1-32Emphasis on "rigid". It's not telling you to introduce a new type variable and put that there. It's telling you that the type you need to put there is an existing type variable's type. When you write 'run :: MS c Int a -> (Either String a, Int)' you implicitly mean 'run :: forall c.' which is exactly introducing a new type variable.• 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)This is the 'c' you bound with the implicit 'forall'. The compiler is asked to verify that 'run' indeed works 'forall c1', so during typechecking of the function body the 'c1' variable is also rigid.‘c’ is a rigid type variable bound by the type signature for: f1 :: forall (m :: * -> *) c. MonadIO m => c -> m BoolThis is the 'c' from the typed hole suggestion up above, still rigid. A part of the typechecking algorithm is that two rigid type variables cannot be equated. The solution *actually* proposed by GHC in the wildcard suggestion is to use the 'c' variable from 'f1's type for which you need to make it scoped with an explicit 'forall': f1 :: forall c. (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) run = runMS c 0 f2 :: MS c s Bool f2 = pure False f3 :: MS c s [Int] f3 = pure [] _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. |
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