Examples from Haskell Office Hours

gistfile1.hs

{-# LANGUAGE FlexibleInstances #-}

-- | A simple datatype similar to 'Bool'
data A = B | C

a :: A
a = B

-- | A more complicated datatype with values associated with each constructor.
data Foo = Bar Int String | Baz (Bool -> Bool)
  deriving (Show)

-- | Since we don't know how to 'show' '(Bool -> Bool)' values by default, we have to
-- provide an instance so that we can derive 'show' for 'Foo'
instance Show (Bool -> Bool) where
    show _ = "#<procedure (Bool -> Bool)>"

f :: Foo -> String
f (Bar 5 "hello") = "hello"
f (Bar x "hello") = show x
f (Bar x s) = s
f (Baz f) = show (f True)

-- | A list of integers. Note that 'IntList' appears in its own definition;
-- it is a recursive type.
data IntList = Nil | Cons Int IntList
  deriving (Show)

ls = Cons 5 (Cons 3 Nil)

sumIntList :: IntList -> Int
sumIntList Nil = 0
sumIntList (Cons h t) = h + (sumIntList t)