[Haskell-cafe] Impredicative types and Lens?

[Artyom Kazak]

Here’s a small example, which, when compiled, gives an error. Why?

    {-# LANGUAGE FlexibleInstances, ImpredicativeTypes,
                 TemplateHaskell #-}

    import Control.Lens

    class Item a where
      name :: a -> String

    instance Item (String, Int) where
      name = fst

    type ItemFilter = Item a => a -> Bool

    data ItemBox = ItemBox { _itemFilter :: ItemFilter }
    makeLenses ''ItemBox

The error is

    Couldn't match type `a0 -> Bool'
                  with `forall a. Item a => a -> Bool'
    Expected type: ItemFilter
      Actual type: a0 -> Bool
    In the expression: b_aaZE
    In the first argument of `iso', namely
      `\ (ItemBox b_aaZE) -> b_aaZE'
    In the expression: iso (\ (ItemBox b_aaZE) -> b_aaZE) ItemBox

I’m using GHC 7.6.2, if it’s important.

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