Tutorial.hs

module Maybe where

import Prelude hiding (Maybe (..))

-- Take a look at Hackage: Data.Maybe
-- https://hackage.haskell.org/package/base-4.8.1.0/docs/Data-Maybe.html

data Maybe a = Nothing
             | Just a
             deriving (Eq, Show, Ord, Read)

instance Functor Maybe where
  fmap f Nothing = Nothing
  fmap f (Just x) = Just (f x)

instance Applicative Maybe where
  pure  = Just
  Nothing <*> _ = Nothing
  Just f <*> Nothing = Nothing
  Just f <*> (Just x) = Just (f x)

instance Monad Maybe where
  return x = Just x
  Nothing >>= _ = Nothing
  (Just x) >>= f = f x

module Test where

import Prelude hiding (sum, map)

sum' ::  Num a => [a] -> a
sum' [] = 0
sum' xs = head xs + sum' (tail xs)


sum'' ::  Num a => [a] -> a
sum'' [] = 0
sum'' (x:xs) = x + sum'' xs

sum''' ::  Num a => [a] -> a
sum''' xs = foldl (+) 0 xs


sum ::  (Foldable t, Num a) => t a -> a
sum = foldl (+) 0

map f [] = []
map f (x:xs) = f x : map f xs

-- data [] a = []
--           | a : [a]

data Tree a = Empty
            | Tree a (Tree a) (Tree a)
            deriving (Eq, Show, Read)