{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE TypeFamilies               #-}
module Main where

import           Control.Monad.Trans.Writer
import           Data.String                (IsString (..))

newtype ListBuilder e a = ListBuilder { runListBuilder :: Writer [e] a }
  deriving (Functor, Applicative, Monad)

instance (IsString e, a ~ ()) => IsString (ListBuilder e a) where
  fromString s = ListBuilder $ tell [fromString s]

el :: e -> ListBuilder e ()
el x = ListBuilder $ tell [x]

buildInfo :: Config -> String
buildInfo config = unlines $ execWriter $ runListBuilder $ do
  "************** INFORMATION **************"
  "Welcome to the Hydraulic Press Channel."
  "Here is your configuration:"
  ""
  el $ "    depth:       " ++ show (depth config)
  el $ "    width:       " ++ show (width config)
  ""
  "*****************************************"

data Config = Config { depth :: Int, width :: Int }

main :: IO ()
main = putStrLn $ buildInfo defaultConfig

defaultConfig :: Config
defaultConfig = Config { depth = 5, width = 10 }