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@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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