RE: Why isn't this Overlapping?
> On Tue Apr 18 10:31:30 UTC 2017, Simon Peyton Jones wrote:
>
> Moreover, as discussed in the user manual section,
> GHC doesnt complain about overlapping instances at the
instance decl,
> but rather where the instances are used.
Thank you Simon, yes I knew that, so I'd written a usage
(just didn't bother putting it in the message ):
foo :: (TypeEq a a' b) => a -> a' -> String
foo _ _ = "blah"
x = foo 'c' "String"
> Thats why there is no overlap complaint here
I didn't get a complaint about `x`, contrary to what I
expected.
On trying again just now:
y = foo 'c' 'd'
GHC _does_ complain of overlap.
I apologise for the distraction.
AntC
>> On 18 April 2017 01:50, Iavor Diatchki wrote
>> these two instances really should be rejected as they
violate the FD of the class:
>> we can derive `TypeEq a a True` using the first instance
and `TypeEq a a False`
>> using the second one. Unfortunately, the check that we
are using
>> to validate FDs when `UndecidableInstances` is on,
>> is not quite correct (relevant tickets are #9210 and
#10675
>> where there are similar examples).
>>> On Sun, Apr 16, 2017 at 12:13 AM, Anthony Clayden wrote:
>>> --ghc 7.10 or 8.0.1
>>>
>>> {-# LANGUAGE DataKinds, KindSignatures, GADTs,
>>> MultiParamTypeClasses,
>>> FunctionalDependencies, FlexibleInstances,
>>> UndecidableInstances, NoOverlappingInstances
#-}
>>> class TypeEq a a' (b :: Bool) | a a' -> b
>>>
>>> instance (b ~ True) => TypeEq a a b
>>> instance (b ~ False) => TypeEq a a' b
>>> Those two instance heads are nearly identical,
>>> surely they overlap?
>> And for a type-level type equality test,
>>> they must be unifiable.
>>> But GHC doesn't complain.
>>>
>>> If I take off the FunDep, then GHC complains.
>>>
>>> AFAICT none of those extensions imply Overlaps,
>>> but to be sure I've put NoOverlapping.
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RE: Why isn't this Overlapping?
Moreover, as discussed in the user manual
section<http://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#overlapping-instances>,
GHC doesn’t complain about overlapping instances at the instance decl, but
rather where the instances are used. That’s why there is no overlap complaint
here
Simon
From: Glasgow-haskell-users [mailto:glasgow-haskell-users-boun...@haskell.org]
On Behalf Of Iavor Diatchki
Sent: 18 April 2017 01:50
To: anthony_clay...@clear.net.nz
Cc: GHC Users Mailing List
Subject: Re: Why isn't this Overlapping?
Hello,
these two instances really should be rejected as they violate the FD of the
class: we can derive `TypeEq a a True` using the first instance and `TypeEq a a
False` using the second one. Unfortunately, the check that we are using to
validate FDs when `UndecidableInstances` is on, is not quite correct (relevant
tickets are #9210 and #10675 where there are similar examples).
-Iavor
On Sun, Apr 16, 2017 at 12:13 AM, Anthony Clayden
mailto:anthony_clay...@clear.net.nz>> wrote:
--ghc 7.10 or 8.0.1
{-# LANGUAGE DataKinds, KindSignatures, GADTs,
MultiParamTypeClasses,
FunctionalDependencies, FlexibleInstances,
UndecidableInstances,
NoOverlappingInstances #-}
class TypeEq a a' (b :: Bool) | a a' -> b
instance (b ~ True) => TypeEq a a b
instance (b ~ False) => TypeEq a a' b
Those two instance heads are nearly identical, surely they
overlap?
And for a type-level type equality test, they must be
unifiable.
But GHC doesn't complain.
If I take off the FunDep, then GHC complains.
AFAICT none of those extensions imply Overlaps,
but to be sure I've put NoOverlapping.
AntC
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Re: Why isn't this Overlapping?
> On Tue Apr 18 00:50:20 UTC 2017, Iavor Diatchki wrote:
>
> these two instances really should be rejected as they
> violate the FD of the class: we can derive `TypeEq a a
> True` using the first instance and `TypeEq a a False`
> using the second one. Unfortunately, the check that we
> are using to validate FDs when `UndecidableInstances` is
> on, is not quite correct (relevant tickets are #9210 and
> #10675 where there are similar examples).
>
Thanks Iavor, it was a propos those tickets I'm asking.
See my comments on #10675 -- we'll have to agree
to disagree on "not quite correct".
(If you want to ban instance selecting on type equality,
you'll make a lot of people grumpy.)
My surprise here is why GHC doesn't complain about overlaps.
(Separately from whether it's doing the right thing.)
Here's another example [GHC 7.10] and here I agree
the FunDep consistency is well broken,
again needs UndecidableInstances:
{-# LANGUAGE MultiParamTypeClasses, GADTs,
FunctionalDependencies,
FlexibleInstances,
UndecidableInstances #-}
class C a b | a -> b where
foo :: a -> b -> String
instance C Int Bool where foo _ _ = "Bool called"
{- So far so good: no bare typevars in instance head;
don't need UndecidableInstances for that -}
instance (b ~ Char) => C Int b where
foo _ _ = "b ~ Char called"
I can't write that instance head `C Int Char`.
GHC complains inconsistent with FunDep [quite correct].
But I can call `foo (5 :: Int) 'c'` ==> "b ~ Char called".
If I call `foo (5 :: Int) undefined` ==> GHC complains of
overlaps.
[I would say fair enough too, except ...]
If I change the instance to
instance {-# OVERLAPPABLE #-} (b ~ Char) => C Int b where
..
Then `foo (5 :: Int) undefined` ==> "Bool called"
So GHC both uses the FunDep to improve the type,
and uses the improvement to select an instance;
and yet seems blind to FunDep inconsistencies in the
instance decls.
AntC
>
> > On Sun, Apr 16, 2017 at 12:13 AM, Anthony Clayden wrote:
> >
> > --ghc 7.10 or 8.0.1
> >
> > {-# LANGUAGE DataKinds, KindSignatures, GADTs,
> > MultiParamTypeClasses,
> > FunctionalDependencies, FlexibleInstances,
> > UndecidableInstances,
> > NoOverlappingInstances #-}
> >
> > class TypeEq a a' (b :: Bool) | a a' -> b
> >
> > instance (b ~ True) => TypeEq a a b
> > instance (b ~ False) => TypeEq a a' b
> >
> > Those two instance heads are nearly identical, surely
> > they overlap?
> > And for a type-level type equality test, they must be
> > unifiable.
> > But GHC doesn't complain.
> >
> > If I take off the FunDep, then GHC complains.
> >
> > AFAICT none of those extensions imply Overlaps,
> > but to be sure I've put NoOverlapping.
> >
> >
> > AntC
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> >
>
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Re: Why isn't this Overlapping?
Hello,
these two instances really should be rejected as they violate the FD of the
class: we can derive `TypeEq a a True` using the first instance and `TypeEq
a a False` using the second one. Unfortunately, the check that we are
using to validate FDs when `UndecidableInstances` is on, is not quite
correct (relevant tickets are #9210 and #10675 where there are similar
examples).
-Iavor
On Sun, Apr 16, 2017 at 12:13 AM, Anthony Clayden <
anthony_clay...@clear.net.nz> wrote:
> --ghc 7.10 or 8.0.1
>
> {-# LANGUAGE DataKinds, KindSignatures, GADTs,
> MultiParamTypeClasses,
> FunctionalDependencies, FlexibleInstances,
> UndecidableInstances,
> NoOverlappingInstances #-}
>
> class TypeEq a a' (b :: Bool) | a a' -> b
>
> instance (b ~ True) => TypeEq a a b
> instance (b ~ False) => TypeEq a a' b
>
> Those two instance heads are nearly identical, surely they
> overlap?
> And for a type-level type equality test, they must be
> unifiable.
> But GHC doesn't complain.
>
> If I take off the FunDep, then GHC complains.
>
> AFAICT none of those extensions imply Overlaps,
> but to be sure I've put NoOverlapping.
>
>
> AntC
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Why isn't this Overlapping?
--ghc 7.10 or 8.0.1
{-# LANGUAGE DataKinds, KindSignatures, GADTs,
MultiParamTypeClasses,
FunctionalDependencies, FlexibleInstances,
UndecidableInstances,
NoOverlappingInstances #-}
class TypeEq a a' (b :: Bool) | a a' -> b
instance (b ~ True) => TypeEq a a b
instance (b ~ False) => TypeEq a a' b
Those two instance heads are nearly identical, surely they
overlap?
And for a type-level type equality test, they must be
unifiable.
But GHC doesn't complain.
If I take off the FunDep, then GHC complains.
AFAICT none of those extensions imply Overlaps,
but to be sure I've put NoOverlapping.
AntC
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