[Haskell-cafe] Strange problem with inference

Nikolay Amiantov ab at fmap.me
Thu Sep 10 23:58:14 UTC 2015

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:

    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

* `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!


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