Re: [Haskell-cafe] Impredicative types and Lens?

8 Sep 2013

      You can't write that lens by hand, so it isn't surprising that the template
haskell can't generate it either. =)

ImpredicativeTypes don't work all that well.

-Edward

On Sun, Sep 8, 2013 at 9:49 AM, Artyom Kazak  wrote:
...
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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Re: [Haskell-cafe] Impredicative types and Lens?

Edward Kmett