indexing list with Control.Lens requires Monoid constraint

Question

The following code doesn\'t compile:

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens

data MyType = MyType Int
data Outer = Outer { _inners :: [ Inner ] }
         


        

Answer 1

Because ix n can fail (ex: n >= length list) you need a clean way to fail. The clean failure of choice is the mempty element from Monoid. So the question that immediately arises is if your type can't be a Monoid then how would you like this code to fail?

I suggest you use ^? instead of ^., thereby reusing the Monoid named Maybe:

*Main> o ^? inners . ix 2 . val
Nothing
*Main> o ^? inners . ix 0 . val
Just (MyType 1)