ppap.hs

module PPAP where

-- Objects are either a unit of something (Thing) or some combination (Union)
data Object = Thing String
            | Union Object Object

instance Show Object where
  show (Thing name) = name
  show (Union a b)  = show a ++ "-" ++ show b

-- Some simple constructors

-- unions merging the thing on the right with the left after flipping the thing on the right
-- this is the same as combining left to right, pivoting on the left, and "reading" top to bottom
union :: Object -> Object -> Object
union a b = Union (flip b) a
  where flip :: Object -> Object
        flip (Thing t)   = Thing t
        flip (Union a b) = Union b a

thing = Thing


-- simple method for describing objects
describe :: Object -> IO ()
describe (Thing s)      = putStrLn $ "I have a " ++ s ++ "..."
describe ab@(Union a b) = putStrLn $ show ab ++ "..."


-- simple description of the act of combining
combine :: Object -> Object -> IO Object
combine a b = do
    let ab = union a b

    describe a
    describe b
    putStrLn "*Unh*"
    putStrLn $ show ab ++ "!"
    putStrLn ""

    return ab


-- sample program which combines a bunch of objects
main = do
    let pen       = thing "Pen"
    let apple     = thing "Apple"
    let pineapple = thing "Pineapple"

    putStrLn "PPAP"
    putStrLn ""

    ap <- combine pen apple
    pp <- combine pen pineapple

    combine ap pp