pattern-fly.purs

module Main where

import Prelude
import Control.Monad.Eff.Console (log)


-- give a name to an extensible record with fields common to all ducks

type Duck a = { name :: String
              , color :: String | a }


-- function that works on all ducks

display :: forall a. Duck a -> String
display duck = duck.name <> " looks " <> duck.color <> "."


-- flight stuff

-- give a name to a thing that has a fly function that takes a duck
type Fly a = ( fly :: forall b. Duck b -> String | a )


-- shortcut to call fly
fly :: forall a. Duck (Fly a) -> String
fly duck = duck.fly duck

-- ways to fly

woosh :: forall a. Duck a -> String
woosh {name} = name <> " flies, woosh!"

wee :: Duck _ -> String
wee {name, color} = name <> " looks " <> color <> " flies, wee!"


-- different stuff to say

quack name = name <> " quacks!"
squeak name = name <> " squeaks!"


-- ducks

mallard :: Duck ( fly :: forall a. Duck a -> String, say :: String )
mallard = 
  let name = "Mally"
  in { name, color: "blue", fly: woosh, say: quack name } 

redhead :: Duck (Fly ( say :: String ))
redhead = 
  let name = "Red" 
  in { name, color: "red", fly: wee, say: quack name }

rubber :: Duck _
rubber = 
  let name = "Rube"
  in { name, color: "yellow", say: squeak name }


-- tests

main = do
  log (fly mallard) --> Mally flies, woosh!
  
  let mallory = mallard { name = "Mallory" }
  log (fly mallory) --> Mallory flies, woosh!
  

  log (fly redhead) --> Red looks red and flies, wee!
  
  -- log rubber.fly 
  -- -- will not compile: rubber can't fly
  -- -- in OO, there is no error but nothing happens
  
  log mallard.say --: Mally quacks!
  
  log rubber.say --> Rob squeaks!

  log (display mallard) --> Mally looks red.

  
-- you can extend anything without touching anything else
-- add a way to fly or say
-- add functions common to all ducks
-- add ghost duck with new opacity attribute