Biology extra credit

README.txt

To add more phyla, put a '|' and then the Phylum's name in Phyla.hs.
To create a Key, you use 'Either', followed by a question, then two things that can be either a terminating Phylum or another Key (created in the same way).
To create a terminating Phylum, enter "Answer <Phylum name>"
Every time you add another level of the key you need parentheses.

Biology.hs

module Main (main) where

import Data.Char (toLower)
import Prelude hiding (Either)
import Phyla

data Key a = Answer a | Either Question (Key a) (Key a)
instance (Show a) => Show (Key a) where show (Answer x) = show x 

type Question = String

phyloKey :: Key Phylum
phyloKey = Either "Does it have an irregular body form?" (Answer Porifera) (
    Either "Does it have radial symmetry?" (
        Either "Is its body leathery?" (Answer Echinodermata) (Answer Cnidaria_Or_Ctenophora)
    ) (
        Either "Does it have present gills, a spine, or appendages in pairs of two?"  (Answer Chordata) (
            Either "Does it have jointed appendages or an exoskeleton?" (Answer Arthropoda) (
                Either "Is its body flat, soft, or wormlike?" (Answer Platyhelminthes) (
                    Either "Is its body wormlike with conspicuous segments?" (Answer Annelida) (
                        Either "Is its body long, slender, smooth, and wormlike?" (Answer Nematoda) (Answer Mollusca)
                    )
                )
            )
        )
    )
 )
 
getToTheBottomOf :: Key a -> IO a
getToTheBottomOf (Answer x) = return x
getToTheBottomOf (Either q t1 t2) = do
    print q
    getLine >>= parse
  where parse str = case (map toLower . take 1) str of
          "y"       -> getToTheBottomOf t1
          "n"       -> getToTheBottomOf t2
          otherwise -> do
            putStrLn "Sorry, Yes or No answers only."
            getToTheBottomOf (Either q t1 t2) 

main = getToTheBottomOf phyloKey

Phyla.hs

module Phyla where

data Phylum =   Porifera
              | Echinodermata
              | Cnidaria_Or_Ctenophora
              | Chordata
              | Arthropoda
              | Platyhelminthes
              | Annelida
              | Nematoda
              | Mollusca
    deriving Show