Tying the knot with fclabels.

tyingtheknot.hs

{-# LANGUAGE
    TypeOperators
  , TemplateHaskell
  #-}
module Tie where

import Prelude hiding ((.), id)
import Control.Category
import Data.Record.Label

-- Tying the knot for two values with two labels indicating the positions to tie.

tie :: a -> b -> (a :-> b) -> (b :-> a) -> (a, b)
tie a b f g = 
  let a' = setL f b' a
      b' = setL g a' b
  in (a', b')

-- Simple test.

data Person = Person
  { _name :: String
  , _job  :: Job
  }

data Job = Job
  { _title :: String
  , _who   :: Person
  }

$(mkLabels [''Person])
$(mkLabels [''Job])

me    :: Person
myJob :: Job

(me, myJob) = tie (Person "Sebas" undefined) (Job "Software Engineer" undefined) job who

test :: String
test = getL (title . job . who . job . who . job . who . job) me