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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