Add rewriteOf and transformOf.

[quasicomputational]

These are taken verbatim from Control.Lens.Plated in lens, commit
206661eb3d502c0430ded9c95b78b5972961dc5c.

I've left the haddocks as they are, mostly because I wasn't sure what to do with the dangling references.

[neongreen]

It looks like transformOf and rewriteOf are used very little: https://codesearch.aelve.com/haskell/search?query=\b(transformOf|rewriteOf)\b&insensitive=off&space=off&precise=off&sources=on.

What is your usecase? (Scripting? Library? Application? Switching an existing codebase to lens?)

[quasicomputational]

I've found that transformOf is pretty nice for manipulating ASTs, and I'd like to use it in dhall-to-cabal.

[neongreen]

Alright then! Will merge and release a new version soon after.

[quasicomputational]

Thanks!

[neongreen]

Do you have examples for rewriteOf and transformOf that I could put in the docs? I guess I could do an example with JSON, but it would be better if there were base-only examples as well.

[quasicomputational]

Here's a semi-contrived example of using rewriteOf's compositionality for optimisation and separation of concerns:

{-# LANGUAGE LambdaCase #-}

module TinyLang where

import Control.Applicative ( (<|>) )
import Control.Lens ( Traversal', rewriteOf )

data Expr
  = Var String
  | Lit Int
  | Add Expr Expr
  deriving
    ( Eq )

subExprs :: Traversal' Expr Expr
subExprs f = \case
  Var name -> pure ( Var name )
  Lit n -> pure ( Lit n )
  Add a b -> Add <$> f a <*> f b

constantFold :: Expr -> Maybe Expr
constantFold ( Add ( Lit a ) ( Lit b ) ) =
  Just ( Lit ( a + b ) )
constantFold _ =
  Nothing

zeroAdditionIdentity :: Expr -> Maybe Expr
zeroAdditionIdentity ( Add ( Lit 0 ) a ) =
  Just a
zeroAdditionIdentity ( Add a ( Lit 0 ) ) =
  Just a
zeroAdditionIdentity _ =
  Nothing

optimise :: Expr -> Expr
optimise = rewriteOf
  subExprs
  ( \ expr -> constantFold expr <|> zeroAdditionIdentity expr )

example =
  optimise ( Add ( Lit 2 ) ( Add ( Lit (-1) ) ( Lit (-1) ) ) )  == Lit 0

[neongreen]

Released in microlens-0.4.11.