Re: [Haskell-cafe] Strange problem with inference

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) 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 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

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