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 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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

-- Regards Sumit Sahrawat

Thank you all for responses! There are always more things to learn ^_^. I'll also take a look at the paper to understand better what's going on. On 09/11/2015 02:58 AM, Nikolay Amiantov 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.