[Haskell-cafe] Impredicative types and Lens?

[Edward Kmett]

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 <yom at artyom.me> 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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