ConfigurationBuilder_ComonadExample.hs

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TypeSynonymInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverlappingInstances #-}

import Control.Comonad
import Language.Haskell.Codo

type Option = String
data Config = MakeConfig [Option] deriving (Show)

type Builder = (->) [Option]
type Setter a = Builder a -> a

instance Comonad Builder where
  extract bld = bld []
  extend setter builder = \opts2 -> setter (\opts1 -> builder (opts1 ++ opts2))

defaultConfig :: Builder Config
defaultConfig opts = MakeConfig (["-Wall"] ++ opts)

profile :: Setter Config
profile bld = bld ["-prof", "-auto-all"]

goFaster :: Setter Config
goFaster bld = bld ["-O2"]

(#) :: a -> (a -> b) -> b
x # f = f x

config :: Config
config = defaultConfig #
  [codo| config0 => config1 <- profile config0
                    config2 <- goFaster config1
                    extract config2 |]
-- >>> config
-- MakeConfig ["-Wall","-prof, -auto-all","-O2"]

{-
Quotation from "http://www.haskellforall.com/2013/02/you-could-have-invented-comonads.html"

config :: Config
config = defaultConfig # method
    this # profile   -- no apostrophes, these are setters
    this # goFaster
-}