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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>
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