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[GHC] #7019: Wrong error message when using forall in constraints

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[GHC] #7019: Wrong error message when using forall in constraints

#7019: Wrong error message when using forall in constraints
------------------------------+---------------------------------------------
Reporter: sjoerd_visscher | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.4.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
------------------------------+---------------------------------------------
I tried this in GHC 7.4.1:

{{{
{-# LANGUAGE ConstraintKinds, RankNTypes, UndecidableInstances #-}

newtype Free c a = Free { runFree :: forall r. c r => (a -> r) -> r }

type C c = forall a. c (Free c a)
instance C c => Monad (Free c) where
return a = Free ($ a)
Free f >>= g = f g
}}}

Which gives this error:

{{{
Could not deduce (c (Free c b)) arising from a use of `f'
from the context (C c)
}}}

So apparently the forall is silently dropped.

Without UndecidableInstances the error is:

{{{
Illegal irreducible constraint forall a. c (Free c a)
in superclass/instance head context (Use -XUndecidableInstances to
permit this)
}}}

giving the impression that it is allowed.

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7019>
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Re: [GHC] #7019: Wrong error message when using forall in constraints

GHC

Re: [GHC] #7019: Wrong error message when using forall in constraints

In reply to this post by GHC

#7019: Wrong error message when using forall in constraints
---------------------------------+------------------------------------------
Reporter: sjoerd_visscher | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
---------------------------------+------------------------------------------

Comment(by sjoerd_visscher):

Apparently it is allowed to use forall in constraints (I just saw that
[http://stackoverflow.com/questions/11258536/how-to-put-constraints-on-
the-associated-data here]). I used the type synonym as a workaround, but
that's not needed, so here's a simplified case:

{{{
{-# LANGUAGE ConstraintKinds, RankNTypes, UndecidableInstances #-}

import Data.Monoid

newtype Free a = Free { runFree :: forall r. Monoid r => (a -> r) -> r }

instance (forall a. Monoid (Free a)) => Monad (Free) where
return a = Free ($ a)
(>>=) = runFree
}}}

gives the error

{{{
Could not deduce (Monoid (Free b)) arising from a use of `runFree'
from the context (forall a. Monoid (Free a))
}}}

I replaced the constaint variable with Monoid, to check that that's not
the problem. So this new version of course has a simple workaround, but
that workaround is not available in the constraint variable version.

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7019#comment:2>
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The Glasgow Haskell Compiler

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Re: [GHC] #7019: Wrong error message when using forall in constraints

In reply to this post by GHC

#7019: Wrong error message when using forall in constraints
---------------------------------+------------------------------------------
Reporter: sjoerd_visscher | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
---------------------------------+------------------------------------------

Comment(by simonpj):

GHC does not support polymorphic constraints to the left of an arrow. The
!StackOverflow thread you point to contains this program:
{{{
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE UndecidableInstances #-}

module T7019a where

class Context c where
func1 :: c -> String

class (forall b. Context (Associated a b)) -- URK!
=> Class a where
data Associated a :: * -> *

newtype ClassTest = ClassTest { runClassTest :: String }

instance (forall b. Context (Associated ClassTest b)) -- URK!
=> Class ClassTest where
data Associated ClassTest b = ClassTestAssoc b (b -> ClassTest)

instance Context (Associated ClassTest b) where
func1 (ClassTestAssoc b strFunc) = runClassTest $ strFunc b

main = putStr . func1 $ ClassTestAssoc 37 (ClassTest . show)
}}}
I'm astonished that this program typechecks. It certainly shouldn't. We
have not begun to think about constraints like `(forall b. (Associated
ClassTest b))` I'm afraid.

Actually, your Monoid example makes a lot more sense, but can't you just
have a top-level instance
{{{
instance Monoid (Free a) where ...(not sure)....
}}}
in which case you are fine.

Wanting foralls on the left of an arrow is not stupid. See #5927, #2893,
#2456. But it's not going to happen soon, because I don't know how to
specify or implement it. (Which is not to say that someone else may not
be able to crack it.)

Meanwhile, these programs should be rejected, and I'll add the tests so
that they are.

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7019#comment:3>
GHC <http://www.haskell.org/ghc/>
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Re: [GHC] #7019: Wrong error message when using forall in constraints

In reply to this post by GHC

#7019: Wrong error message when using forall in constraints
--------------------------------------------------+-------------------------
Reporter: sjoerd_visscher | Owner: simonpj
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
Resolution: fixed | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Difficulty: Unknown
Testcase: typecheck/should_fail/T7019, T7019a | Blockedby:
Blocking: | Related:
--------------------------------------------------+-------------------------
Changes (by simonpj):

* status: new => closed
* testcase: => typecheck/should_fail/T7019, T7019a
* resolution: => fixed

Comment:

Done, and tests added.

--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7019#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler

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