haskell - Starting out

Haskell is a pure functional programming languge, We will later come back to things like pure and functional. In this post, we will first check on the surface, how it looks like, how it works, and etc... 

 

this tutorial refers to the official twiki site at http://www.haskell.org/haskellwiki/Tutorials at http://haskell.org; while the examples are most take from site "Learn You a Haskell for Great Good!(LYAH)"

 

the example have examples on the following.

 

• function definition
• function application
• functions (sum, mininum, maxinum, product, elem
• list
• list functions head, tail, init, last, null, reverse, take, drop, repeat, replicate
• list comprehension - filtering, multiple filtering, multiple lists
• zip
• tuple
• tuple function - fst, snd

doubleMe x = x + x
doubleUs x y = doubleMe x + doubleMe y
doubleSmallNumber x = if x > 100 then x else x * 2
-- else part is mandatory is Haskell
doubleSmallNumber' x = (if x > 100 then x else x * 2) + 1
--  function cannot begin with uppercase letter
conanO'Brien = "It's a-me, Conan O'Brian!"

-- intro to list
let lostNumbers = [4, 8, 15, 16, 23, 42]

-- list concat
-- [1,2,3,4] ++ [9,10,11,12]
--"hello" ++ "  "+ "world"
--['w', 'o'] ++ ['', 't']

-- putting something at the beginning of one list 
-- 'A' : " SMALL CAT"
-- 5:[1,2,3,4,5]

-- get one element out 
"Steve Buscemi" !! 6 
[9.4,33.2,96.2,11.2,23.25] !! 1 

-- list can be compared if the stuff they contain can be compared.
[3,2,1] > [2, 1, 0]
[3,4,2] == [3,4,2]   

-- takes a list and returns its head 
head [5,4,3,2,1]
-- takes a list and returnsits tail
tail [5,4,3,2,1]
last [5,4,3,2,1]
init [5,4,3,2,1]  

-- exception on empty list
head []


-- check null on a list
null [1, 2, 3]

-- reverse
everse [5,4,3,2,1]

-- take
take 3 [5,4,3,2,1] 

-- drop
drop 3 [8,4,2,1,5,6]

-- maximum 
 maximum [1,9,2,3,4]  

-- mininum 
minimum [8,4,2,1,5,6]

-- sum
sum [5,2,1,6,3,2,5,7]  
-- product
product [6,2,1,2]  

-- elem : tells if a thing is element of the list
4 `elem` [3,4,5,6]
10 `elem` [3,4,5,6]



-- ranges

[1..20
['a'..'z']

-- range with steps 
-- it reads, 2, 4, ... until 20
[2, 4..20]

-- cannot do 
--[20..1]


-- cycle 
-- take finite numbers of elements from a cycled list
take 10 (cycle [1,2,3])
take 12 (cycle "LOL  ")


-- repeat
take 10 (repeat 5)
-- replicate 
replicate 3 10 


-- List comprehension
[x*2 | x <- [1..10]]

-- with condition
[x * 2 | x <- [1..10], x * 2 >= 12]


-- 
[x*2 | x <-[50..100], x `mod` 7 == 3]


-- to a function
boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]   
boomBangs [7..13]

-- chain predicate 
[ x | x <- [10..20], x /= 13, x /= 15, x /= 19]

-- from multiple llsts
[ x*y | x <- [2,5,10], y <- [8,10,11]] 

-- with filter
[ x*y | x <- [2,5,10], y <- [8,10,11], x*y > 50]  

-- length` with wildcards
length' xs = sum [1 | _ <- xs ]

-- list comprehension on list 
removeNonUppercase st = [ c | c <- st, c `elem` ['A'..'Z']]   


-- multiple list comprehension 
let xxs = [[1,3,5,2,3,1,2,4,5],[1,2,3,4,5,6,7,8,9],[1,2,4,2,1,6,3,1,3,2,3,6]] 
[ [ x | x <- xs, even x ] | xs <- xxs]  

-- tuples
--
-- not allowed
-- [(1,2),(8,11,5),(4,5)]

-- fst
fst (8, 11)
fst ("Wow", False)  

-- snd
snd (8, 11)
snd ("Wow", False)

-- zip
zip[1,2,3,4,5] [5,5,5,5,5]

-- zip : loger get cut 
zip [5,3,2,6,2,7,2,5,4,6,6] ["im","a","turtle"]  

-- zip : finite with infinite
zip [1..] ["apple", "orange", "cherry", "mango"] 

-- a mind numbing techiniques

let triangles = [ (a,b,c) | c <- [1..10], b <- [1..10], a <- [1..10] ]  

let rightTriangles = [ (a,b,c) | c <- [1..10], b <- [1..c], a <- [1..b], a^2 + b^2 == c^2]   

let rightTriangles' = [ (a,b,c) | c <- [1..10], b <- [1..c], a <- [1..b], a^2 + b^2 == c^2, a+b+c == 24]