RE: Why isn't this Overlapping?

[Simon Peyton Jones via Glasgow-haskell-users]

Moreover, as discussed in the user manual 
section,
 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?

[Anthony Clayden]

> On Tue Apr 18 10:31:30 UTC 2017, Simon Peyton Jones wrote:
>
> Moreover, as discussed in the user manual section,
> GHC doesn’t 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"

> That’s 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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