CODE_HASKELL.md

Code Basics

A list of basics topics working with Haskell.

Commands:

• ghci - run the package
• :l - load file
• :r - reload package
• :q - close interpreter

Hello world

• helloWorld.hs:

module HelloWorld where

-- Monad
main :: IO ()
main = putStrLn "Hello World"

• Run program:

ghci helloWorld.hs
main

Sum a range of numbers

• myLib.hs:

module MyLib (sumNumbers) where

-- Monad
sumNumbers :: IO ()
sumNumbers = print (sum [1..10])

• Run program:

ghci myLib.hs
sumNumbers

Functions

hello name = "Hello, " ++ name
hello "Juan"

Types

-- 2 params and return data
f :: Int -> Int -> Int
f x y = x*y+x+y
f 2 3 -- 11

Lists

list = ["A", "B", "C"]
head list -- "A"
tail list -- ["B", "C"]

Anonymous functions

-- a function without a name (Lambda abstraction)
f = \x y -> x*y+x+y
f 2 3 -- 11

Higher-order functions

[2*x | x <- [0..10]]
-- [0, 2, 4, etc]
            
map :: (elm -> res) -> [elm] -> [res]
map (\x -> x*2+1) [1..10]

-- Free point style (Event delegation)
mul2 = \x -> x * 2
map mul2 [1..5]

I/O

• io.hs:

greeting() = do
  name <- getLine
  eventName <- getLine
  putStrLn ("Hola" ++ name)
  putStrLn ("Espero " ++ eventName ++ " te guste")

• Run program:

stack ghci
:l io.hs
greeting()

Recursion

• recursion.hs:

-- State machines?
myFilter :: (a -> Bool) -> [a] -> [a]
myFilter fn [] = []
myFilter fn (head:tail)
  | fn head = head : myFilter fn tail
  | otherwise = myFilter fn tail

• Run program:

stack ghci
:l recursion.hs
myFilter (\x -> x > 3) [1, 2, 3, 4] # greater than 3
myFilter (>3) [1, 2, 3, 4]

Loops

• loops.hs:

-- map
f x = x*(x+1)
lst = map f [1..10]

-- using prefix functions
div = (/)

-- reduce
resultLeft = foldl div 1 [1..10]

-- reduceRight
resultRight = foldr div 1 [1..10]

main = do
  print lst
  print resultLeft
  print resultRight

• Run program:

stack runhaskell loops.hs

Currying

currying :: Int -> Int -> Int -> Int
currying x y z = x*y+z
currying 2 3 4 -- 10

-- manual example (not necessary in Haskell, currying works automatically)
currying'   = \x y z -> x*y+z
currying''  = \x -> (\y z -> x*y+z)
currying''' = \x -> (\y -> (\z -> x*y+z))

Credits

• JS vs Haskell una comparación impura by @jessecogollo
• Haskell
• Speller example
• Hanged example
• Functional Programming in Haskell - Supercharge Your Coding.