[Haskell-cafe] Strange problem with inference

[Sumit Sahrawat, Maths & Computing, IIT (BHU)]

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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