lens-syntax

This library provides an alternative syntax for composing lenses and other optics. Here is a comparison between the standard, pointfree syntax and our pointful syntax based on list comprehensions:

-- Some nested data to manipulate using Setters
input :: [Maybe (Int, String)]
input
  = [Just (1, "abc"), Nothing, Just (2, "def")]

-- |
-- >>> pointfreeSyntax
-- [Just (1,"ABC"),Nothing,Just (2,"DEF")]
pointfreeSyntax :: [Maybe (Int, String)]
pointfreeSyntax
  = input & each . _Just . _2 . each %~ toUpper

-- |
-- >>> pointfulSyntax
-- [Just (1,"ABC"),Nothing,Just (2,"DEF")]
pointfulSyntax :: [Maybe (Int, String)]
pointfulSyntax
  = $(Syntax.over [| [ toUpper c
                     | Just (_, s) <- each input
                     , c <- each s
                     ] |])

When composing functions, Haskell gives us the choice between using a pointfree style or a pointful style. Sometimes one style is clearly more readable than the other, and sometimes both styles are about as readable and so which one to choose becomes a question of personal preference.

When composing lenses, the lens library is only giving us one choice, the pointfree style. I certainly do not claim that this library's syntax is more readable in all cases, and in fact at first the pointful syntax is bound to always look less readable due to unfamiliarity. I am hoping that over time, we can acquire a taste for when each style is appropriate.

See more examples here.