....Because trying something new is fun!

What is Haskell?
Functional - Given the arguments, always returns the same output ie no global or local state.
Lazy
Statically typed
Learn you a Haskell
Learn You a Haskell for Great Good the goal of the reading group is to get through this book.

Week 1 is about

Getting Started
Lists (and list comprehension)
Getting started
REPL

Running from Powershell GIT in windows

string1 = "hello"
string2 = "world"
greeting = string1 ++ string2
doubleMe x = x + x
-- 6
b = doubleMe 3
-- 16.6 as + works on ints and floating-point numbers
c = doubleMe 8.3
-- * is an infix function which takes 2 numbers and multiplies (as opposed to a prefix)
a = 49 * 100
-- 10/3 gives 1 as a remainder
s = mod 10 3
-- backtick infix style gives 1 as a remainder
t = 10 ` mod ` 3
-- Cons operator
-- [3,5,7,11]
favNums = 3 : 5 : 7 : 11 : []

Loading hello.hs and calling functions

Ctrl D - exit from GHCI in Powershell
Ctrl L - cls
Euler 1
Project Euler is where I start when learning a new language to flex my new ‘toolbelt’..

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
Find the sum of all the multiples of 3 or 5 below 1000.

-- 1. Head of list
a = head [ 1 , 2 , 3 ]
-- 2,3. Tail of the list
b = tail [ 1 , 2 , 3 ]

-- List comprehension (output function is before the pipe)
-- x 'such that', x goes to [1..999]
-- filtering (weeding out lists by predicates)
h = sum [ x | x <- [ 1 .. 999 ], x ` mod ` 3 == 0 || x ` mod ` 5 == 0 ]

running the function h to get the answer. Reloading changes :r in file euler.hs

Euler 2
-- Each new term in the Fibonacci sequence is generated by adding the previous two terms.
-- By starting with 1 and 2, the first 10 terms will be:
-- 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
-- By considering the terms in the Fibonacci sequence whose values do not exceed four million,
-- find the sum of the even-valued terms.
-- [7,9,11,13,15]
sumOfLists = zipWith ( + ) [ 1 , 2 , 3 , 4 , 5 ] [ 6 , 7 , 8 , 9 , 10 ]
-- zipWith adds 2 lists together
fibs = 1 : 1 : zipWith ( + ) fibs ( tail fibs )
-- [1,1,2,3,5,8,13,21,34,55,89,144,233,377]
q = takeWhile ( < 4000000 ) fibs
--4613732
o = sum [ x | x <- takeWhile ( < 4000000 ) fibs , even x ]

Euler 3
The prime factors of 13195 are 5, 7, 13 and 29.
What is the largest prime factor of the number 600851475143 ?

-- Euler 3
-- The prime factors of 13195 are 5, 7, 13 and 29.
-- What is the largest prime factor of the number 600851475143 ?
-- number is prime if only divisible by 1 and itself
-- http://stackoverflow.com/a/5811176/26086
-- eg if checking isPrime 12
-- 3 * 4 are factors
-- 2 * 6 are factors..
-- one of the factors has to be less than the sqrt
-- and if there is a factor, then it isn't a prime
-- only need check up to the sqrt of number to see if it can be divided with remainder of 0
-- getting the square root as an int
-- x 35 = 5
-- x 144 = 12
x num = floor ( sqrt ( fromIntegral num ))
-- If result set is empty isPrime = True
isPrime num = [ x | x <- [ 2 .. floor ( sqrt ( fromIntegral num ))], num ` mod ` x == 0 ] == []
-- The factors of a number
facs num = [ x | x <- [ 3 .. num ], num ` mod ` x == 0 ]
-- [5,7,13,29,35,65,91,145,203,377,455,1015,1885,2639,13195]
q = facs 13195
-- Prime factors
facs' num = [ x | x <- [ 3 .. num ], num ` mod ` x == 0 , isPrime x ]
-- [5,7,13,29]
q' = facs' 13195
-- Problem is that it doesn't stop after giving the correct answer
-- as its checking all the way up to 600851475143 to see if its a factor
-- [71,839,1471,6857]
r = facs' 600851475143
-- only need to check up to sqrt of 600851475143 to see if it's a factor and a prime
facs'' num = [ x | x <- [ 3 .. floor ( sqrt ( fromIntegral num ))], num ` mod ` x == 0 , isPrime x ]
s = facs'' 600851475143

Euler 4
-- Euler 4
-- A palindromic number reads the same both ways.
-- The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
-- Find the largest palindrome made from the product of two 3-digit numbers.
-- [4,3,2,1]
u = reverse [ 1 , 2 , 3 , 4 ]
-- int to string
v = show 345
-- string to int
w = read "345" :: Int
-- can reverse the string (as it is a list of chars really)
u' = reverse "1234"
z = reverse v
-- 9009
t = maximum [ x * y | x <- [ 10 .. 99 ], y <- [ 10 .. 99 ], isPalindrome ( x * y )]
-- int to string, reverse int to string ie compare 2 strings
isPalindrome num = show num == reverse ( show num )
-- 906609
t' = maximum [ x * y | x <- [ 100 .. 999 ], y <- [ 100 .. 999 ], isPalindrome ( x * y )]

_ and Tuples
-- implementing own version of length (of a list)
-- _ means we don't care what we draw from the list.
-- replace every element of a list with 1, then sum
length' xs = sum [ 1 | _ <- xs ]
-- 4
o = length' [ 1 , 2 , 3 , 4 ]
-- Tuple can contain several types
-- returns first component of the Tuple
-- 8
p = fst ( 8 , 11 )
-- "Wow"
q = fst ( "Wow" , False )
-- Which right triangle that has integers for all sides and all sides equal to or
-- smaller than 10 has a perimeter of 24?
-- a^2 + b^2 = c^2
-- eg 3,4,5 triangle.. perimeter = 12
-- 9 + 16 = 25
-- generate numbers for all sides returning a Tuple
-- [(6,8,10),(8,6,10)]
r = [( a , b , c ) | a <- [ 1 .. 10 ], b <- [ 1 .. 10 ], c <- [ 1 .. 10 ], a ^ 2 + b ^ 2 == c ^ 2 , a + b + c == 24 ]

Summary of Book so far
Have been able to solve most of the first 9 Euler problems with just list comprehensions.

-- List comprehension (output function is before the pipe)
-- filtering (weeding out lists by predicates)
a = [ x * 2 | x <- [ 1 .. 10 ], odd x , x > 3 ]

There are a ton of interesting functions in the language!

BrightonFunctionalReadingGroup
Meetup

Highlights from this meeting were

no let needed in REPL now
brackets around (+) mean passing + as an argument (actually an infix function to add 2 numbers)
-- zipWith f [a,b,c] [x,y,z]
-- [7,9,11,13,15]
sumOfLists = zipWith ( + ) [ 1 , 2 , 3 , 4 , 5 ] [ 6 , 7 , 8 , 9 , 10 ]

’ is for single characters, “ is for strings (list of chars!)
aa = 'x'
ab = "this is a string"
ac = length ab

-- x such that, x is drawn from 1 to 10
-- filter by odd x's and x>3
a = [ x | x <- [ 1 .. 10 ], odd x , x > 3 ]

Noel Markham’s exercises link

-- Question 1
-- last element in a list
-- 4
ad = last [ 1 , 2 , 3 , 4 ]
-- returns everything except the last element
-- [1,2,3]
ae = init [ 1 , 2 , 3 , 4 ]
-- Find the penultimate element in list l
penultimate l = last ( init l )
-- Question 2
-- determine if list l is a palindrome
-- palindrome is the same forward as reversed
-- l is a list of characters (a string)
isPalindrome l = l == reverse l
-- True
w = isPalindrome "abba"
-- Question 3
-- Duplicate the elements in list xs, for example "duplicate [1,2,3]"
-- would give the list [1,1,2,2,3,3]
-- Hint: The "concat [l]" function flattens a list of lists into a single list.
-- (You can see the function definition by typing ":t concat" into the interpreter.
-- Perhaps try this with other variables and functions)
-- For example: concat [[1,2,3],[3,4,5]] returns [1,2,3,3,4,5]
-- concat the list of lists into a single list
-- [1,1,2,2,3,3]
ag = concat [[ 1 , 1 ],[ 2 , 2 ],[ 3 , 3 ]]
-- returns a list of lists
-- [[1,1],[2,2],[3,3]]
ai = [[ x , x ] | x <- [ 1 .. 3 ]]
duplicate xs = concat [[ x , x ] | x <- xs ]
--[1,1,2,2,3,3]
ah = duplicate [ 1 , 2 , 3 ]
-- Question 4
-- Imitate the functionality of zip
-- The function "min x y" returns the lower of values x and y
-- For example "ziplike [1,2,3] ['a', 'b', 'c', 'd']" returns [(1,'a'), (2, 'b'), (3, 'c')]
-- ziplike xs ys = undefined
-- use min to get the number of elements
--ab = min ((length [1,2,3]) length(['a', 'b', 'c', 'd']))
-- returns a list of Tuples with the last 'd' missed off as nothing to match
-- [(1,'a'),(2,'b'),(3,'c')]
aj = zip [ 1 , 2 , 3 ] [ 'a' , 'b' , 'c' , 'd' ]
-- find min length of both lists
xs = [ 1 , 2 , 3 ]
ys = [ 'a' , 'b' , 'c' , 'd' ]
-- 2 (3-1)
ak = min ( length xs ) ( length ys ) - 1
-- x's at element i, y's at element i, such that i is drawn from 0 to the min length of both lists
ziplike xs ys = [( xs !! i , ys !! i ) | i <- [ 0 .. ( min ( length xs ) ( length ys ) - 1 )]]
-- DG 1 - Factorials
-- the product of an integer and all the integers below it; e.g. factorial four ( 4! ) is equal to 24.
-- 4*3*2*1 = 24
-- recursion
fac n =
if n < 2
-- base case of the recursion.. stop at 1
then 1
else n * fac ( n - 1 )
an = fac 4
-- simplicity
fac' n = product [ 1 .. n ]
ao = fac' 4
-- works
ap = product [ 1 .. 4 ]
aq = product [ 4 , 3 .. 1 ]
-- pattern match?
-- same logic as recursion
fact 1 = 1
fact n = n * fact ( n - 1 )
ar = fact 4
-- fold right (higher order fn.. coming to that in the book, so leave for now)
fact2 n = foldr ( * ) 1 [ 1 .. n ]
as = fact2 4
-- DG 2 - Fibonacci
-- Each new term in the Fibonacci sequence is generated by adding the previous two terms.
-- By starting with 1 and 2, the first 10 terms will be:
-- 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
-- write a function to get first 10 fibs
--[1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
--at = fibs 10
-- keep generating fibs. No base case for recursion
fibs a b =
--if a > 89
-- base case of the recursion.. stop at max (eg 89)..return empty list
-- then []
-- cons operator add a to start of list
--else a : fibs b (a+b)
a : fibs b ( a + b )
-- finding the 10,000th fib
at = last ( take 10000 ( fibs 1 2 ))
au = ( fibs 1 2 ) !! 10000
fibs' n = ( fibs 1 2 ) !! n
-- nice way to test fibs', by throwing a range at it
av = [ fibs' x | x <- [ 1 .. 30 ]]
-- lazy
fibs'' = 1 : 1 : [ x + y | ( x , y ) <- zip fibs'' ( tail fibs'' )]
-- zipWith adds 2 lists together
fibs3 = 1 : 1 : zipWith ( + ) fibs3 ( tail fibs3 )

Evolution of a Haskell coder