learn-you-a-haskell-chapter-2

You A Haskell Chapter 2

Starting Out

Ready, Set, go!

To start coding in haskell, type in ghci.

ghci

2 + 15
-- 17
49 _ 100
-- 4900
1892 - 1472
-- 420
5 / 2
-- 2.5

We can use parentheses too:

(50 _ 100) - 4999
-- 1
50 _ 100 - 4999
-- 1
50 _ (100 - 4999)
-- -244950

Boolean Algebra:

True && False
-- False
True && True
-- True
False || True
-- True
not False
-- True
not (True && True)
-- False

Equality Testing:

5 == 5
-- True
1 == 0
-- False
5 /= 5
-- False
5 /= 4
-- True
"hello" == "hello"
-- True

We can’t add two different types though:

5 + "llama"
-- No instance for (Num [Char])
-- arising from a use of `+' at <interactive>:1:0-9
-- Possible fix: add an instance declaration for (Num [Char])
-- In the expression: 5 + "llama"
-- In the definition of `it': it = 5 + "llama"

Succ and Pred return the next and previous item.

succ 8
-- 9
-- succ returns the next of an item.
pred 8
-- 7
-- pred returns the predicate, or previous of an item.
min 9 10
-- 9
min 3.4 3.2
-- 3.2
max 100 101
-- 101
succ 9 + max 5 4 + 1
-- 16
(succ 9) + max (5 4) + 1
-- 16

Check if a number is even or odd:

even 4
-- True
odd 5
-- True

Baby’s First Functions

doubleMe x = x + x
 
doubleUs x y = doubleMe x + doubleMe y
 
doubleSmallNumber x = if x > 10 then x else x * 2

conanO'Brien = "It's a me, Conan O'Brien!"
 
lostNumbers = [4,8,15,16,23,42]
 
[1,2,3,4] ++ [9,10,11,12]
-- [1,2,3,4,9,10,11,12]

use ++ to add two lists together. strings are just lists of chars. Remember that ++ takes O(n) time

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

whereas :, the cons operator takes O(1)

'A': " SMALL CAT"
-- "A SMALL CAT"
5:[1,2,3,4,5]
-- [5,1,2,3,4,5]

Access by index is done with !!.

"Steve Buscemi" !! 6
-- 'B'
[9.4,33.2,96.2,11.2,23.25] !! 1
-- 33.2
 
b = [1,2,3,4],[5,3,3,3],[1,2,2,3,4],[1,2,3]]
-- [1,2,3,4],[5,3,3,3],[1,2,2,3,4],[1,2,3]]
b ++ [1,1,1,1]]
-- [1,2,3,4],[5,3,3,3],[1,2,2,3,4],[1,2,3],[1,1,1,1]]
[6,6,6]:b
-- [6,6,6],[1,2,3,4],[5,3,3,3],[1,2,2,3,4],[1,2,3]]
b !! 2
-- [1,2,2,3,4]

Lists can be compared if they have elements that meet the typeclass Ord Lists are compared from the head..tail

[3,2,1] > [2,1,0]
-- True
[3,2,1] > [2,10,100]
-- True
[3,4,2] > [3,4]
-- True
[3,4,2] > [2,4]
-- True
[3,4,2] == [3,4,2]
-- True

Head takes a list and returns its head, or first element

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

tail takes a list and returns its head, which is everything but the head

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

last takes a list and returns its last element

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

init takes a list and returns everything except its last element

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

using any of the above blows up on an empty list

head []
-- *** Exception: Prelude.head: empty list

length takes a list and returns its length

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

null checks if a list is empty. If it is, it returns True, else, False

null [1,2,3]
-- False
null []
-- True

reverse reverses a list

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

take takes from the beginning of the list

take 3 [5,4,3,2,1]
-- [5,4,3]
take 1 [3,9,3]
-- [3]
take 5 [1,2]
-- [1,2]
take 0 [6,6,6]
-- []

drop works in a similar way, but it drops elements then takes the remaining elements.

drop 3 [8,4,2,1,5,6]
-- [1,5,6]
drop 0 [1,2,3,4]
-- [1,2,3,4]
drop 100 [1,2,3,4]
-- []

maximum and minimum returns the max and minimum of a list

minimum [8,4,2,1,5,6]
-- 1
maximum [1,9,2,3,4]
-- 9

sum takes a list of numbers and returns their sum

sum [1,2,3,4]
-- 10

product takes a list of numbers and returns their product

product [1,2,3,4]
-- 24

elem takes a thing and a list of things and tells us if that thing is an element of the list.

4 `elem` [3,4,5,6]
-- True
10 `elem` [3,4,5,6]
-- False

Texas Ranges

ranges are defined for elements that have a succ

[1..20]
-- [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
['a'..'z']
-- "abcdefghijklmnopqrstuvwxyz"
['K'..'Z']
-- "KLMNOPQRSTUVWXYZ"

steps can be specified as the first element before the comma

[2,4..20]
-- [2,4,6,8,10,12,14,16,18,20]
[3,6..20]
-- [3,6,9,12,15,18]

ranges in reverse have to be specified with the first element

[20,19..1]
-- [20,19,18,17,16,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1]

be careful about floats in haskell.

[0.1, 0.3 .. 1]
-- [0.1,0.3,0.5,0.7,0.8999999999999999,1.0999999999999999]

ranges can be lazy, just don’t end the right hand side

x = [13,26..]
-- x
take 3 x
-- [13,26,39]

cycle takes a list and cycles it into an infinite list

take 10 (cycle [1,2,3])
-- [1,2,3,1,2,3,1,2,3,1]
take 12 (cycle
-- "LOL LOL LOL "

repeat takes an element and produces an infinite list of that element

take 10 (repeat 5)
-- [5,5,5,5,5,5,5,5,5,5]

(replicate times item) returns the same element x times

replicate 3 10
-- [10, 10, 10]

List comprehensions

[x*2 | x <- [1..10]]
-- [2,4,6,8,10,12,14,16,18,20]

with an if block, only take items that are greater than 12.

I’m a list comprehension

[x*2 | x <- [1..10], x*2 >= 12]
-- [12,14,16,18,20]

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

list comprehensions can be used for outer joins as well

let nouns = ["hobo","frog","pope"]
let adjectives = ["lazy","grouchy","scheming"]
[adjective ++ " " ++ noun | adjective <- adjectives, noun <- nouns]
-- ["lazy hobo","lazy frog","lazy pope","grouchy hobo","grouchy frog", "grouchy pope","scheming hobo","scheming frog","scheming pope"]

we can define our own length by replacing everything with 1 and summing it up.

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

remove nonuppercase characters

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

Tuples

tuples are like non-homogenous lists

tup = (1,
 
fst tup
-- 8
snd tup
-- "hi"

zip produces a list of pairs

zip [1,2,3,4,5] [5,5,5,5,5]
-- [(1,5),(2,5),(3,5),(4,5),(5,5)]
 
zip [1 .. 5] ["one", "two", "three", "four", "five"]
-- [(1,]
 
let rightTriangles = [ (a,b,c) | c <- [1..10], b <- [1..c], a <- [1..b], a^2 + b^2 == c^2]
-- [(3,4,5), (6,8,10)]

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