Haskell Identity Lens

Question

I'm wondering if there is an identity lens in Haskell. A lens identity such that if I had a type data MyType = MyType { _myField :: Int }, then I can do myType ^. identity . myField .~ 2 . There seemed to be one in lens-1.1.1, but I can't find one in lens-4.19.2.

I'm no Lens expert, but I don't really see the use of this - and if you do need it, given the definition of the `Lens` type, is it not just `id :: Lens s t s t`? Apologies if I've overlooked something obvious. — Robin Zigmond, Jun 23 '20 at 08:57

score 10 · Accepted Answer · answered Jun 23 '20 at 09:05

One of the nice things about lens-style lenses is that they're really just functions. So just as you can use the function composition operator . on lenses, you can also the identity function id as a lens, and it acts indeed as an identity-lens in the sense that it “focuses” on the entire data structure.

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens

data MyType = MyType { _myField :: Int }
makeLenses ''MyType

main :: IO ()
main = print $ MyType 37 ^. id . myField

The implementation of `traverseOf` is also `id`. – danidiaz Jun 23 '20 at 09:15 — danidiaz, Jun 23 '20 at 09:15

Haskell Identity Lens

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