[Haskell-cafe] Strange problem with inference

[adam vogt]

I've tested with ghc-7.8.4, and the test1/test2 definitions are accepted if
you use -XNoMonomorphismRestriction.

For test2 to work without annotating a result type, you could use a
superclass constraint to show that the type family has an inverse. In
ghc-7.10 it doesn't work (see https://ghc.haskell.org/trac/ghc/ticket/10009)

https://gist.github.com/aavogt/7a10024f0199dc2e8478 shows both features.

On Thu, Sep 10, 2015 at 10:17 PM, Sumit Sahrawat, Maths & Computing, IIT
(BHU) <sumit.sahrawat.apm13 at iitbhu.ac.in> wrote:

> One possible fix (tested on GHC-7.10.1 with lens-4.12.3):
>
> test2 :: (Test a, t ~ TestT a) => t -> a
> test2 = view (from myiso)
>
> This might have something to do with type families not being injective,
> but I'm not completely sure.
>
> I also agree that it might be possible to trigger this without lens, will
> try to find an example and post if I succeed.
>
> On 11 September 2015 at 05:28, Nikolay Amiantov <ab at fmap.me> wrote:
>
>> Hi Cafe,
>>
>> I've been playing around with lens and stumbled upon strange GHC
>> behaviour. Consider this source (using lens package and GHC 7.10.2):
>>
>> {-# LANGUAGE TypeFamilies #-}
>>
>> import Control.Lens
>>
>> class Test a where
>>   type TestT a
>>   myiso :: Iso' a (TestT a)
>>
>> test1 :: Test a => a -> TestT a
>> test1 = view myiso
>>
>> test2 :: Test a => TestT a -> a
>> test2 = view (from myiso)
>>
>> GHC would emit this error:
>>
>> /tmp/test.hs:13:9:
>>     Could not deduce (Control.Monad.Reader.Class.MonadReader
>>                         (TestT a) ((->) (TestT a)))
>>       arising from a use of ‘view’
>>     from the context (Test a)
>>       bound by the type signature for test2 :: Test a => TestT a -> a
>>       at /tmp/test.hs:12:10-31
>>     In the expression: view (from myiso)
>>     In an equation for ‘test2’: test2 = view (from myiso)
>> Failed, modules loaded: none.
>>
>> However, `MonadReader r ((->) r)` is defined for any and all `r`!
>> Furthermore, `test1` has no problem with this and `view` there uses this
>> instance too. The only difference that I see is the presence of a type
>> family:
>>
>> * `test1` needs `MonadReader a ((->) a)`
>> * `test2` needs `MonadReader (TestT a) ((->) (TestT a))`
>>
>> , but I don't understand how can this result in a different behavior.
>> Notice that this likely may be reproduced somehow without lens -- I've
>> spent some time trying to minify this example further but alas to no
>> avail.
>>
>> Thanks in advance!
>>
>> --
>> Nikolay.
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>>
>
>
>
> --
> Regards
>
> Sumit Sahrawat
>
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>
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