---
language: Haskell
lang: pl-pl
contributors:
    - ["Remigiusz Suwalski", "https://github.com/remigiusz-suwalski"]
---

Haskell został zaprojektowany jako praktyczy, czysto funkcyjny język 
programowania. Jest znany przede wszystkim ze względu na jego monady oraz system
typów, ale ja lubię do niego wracać przez jego elegancję. Sprawił on, że 
programowanie jest prawdziwą przyjemnością.

```haskell
-- Komentarze jednolinijkowe zaczynają się od dwóch myślników
{- Komentarze wielolinijkowe należy 
zamykać w bloki klamrami.
-}

----------------------------------------------------
-- 1. Primitive Datatypes and Operators
----------------------------------------------------

-- You have numbers
3 -- 3

-- Math is what you would expect
1 + 1 -- 2
8 - 1 -- 7
10 * 2 -- 20
35 / 5 -- 7.0

-- Division is not integer division by default
35 / 4 -- 8.75

-- integer division
35 `div` 4 -- 8

-- Boolean values are primitives
True
False

-- Boolean operations
not True -- False
not False -- True
1 == 1 -- True
1 /= 1 -- False
1 < 10 -- True

-- In the above examples, `not` is a function that takes one value.
-- Haskell doesn't need parentheses for function calls...all the arguments
-- are just listed after the function. So the general pattern is:
-- func arg1 arg2 arg3...
-- See the section on functions for information on how to write your own.

-- Strings and characters
"This is a string."
'a' -- character
'You cant use single quotes for strings.' -- error!

-- Strings can be concatenated
"Hello " ++ "world!" -- "Hello world!"

-- A string is a list of characters
['H', 'e', 'l', 'l', 'o'] -- "Hello"
"This is a string" !! 0 -- 'T'


----------------------------------------------------
-- Lists and Tuples
----------------------------------------------------

-- Every element in a list must have the same type.
-- These two lists are the same:
[1, 2, 3, 4, 5]
[1..5]

-- Ranges are versatile.
['A'..'F'] -- "ABCDEF"

-- You can create a step in a range.
[0,2..10] -- [0, 2, 4, 6, 8, 10]
[5..1] -- This doesn't work because Haskell defaults to incrementing.
[5,4..1] -- [5, 4, 3, 2, 1]

-- indexing into a list
[1..10] !! 3 -- 4

-- You can also have infinite lists in Haskell!
[1..] -- a list of all the natural numbers

-- Infinite lists work because Haskell has "lazy evaluation". This means
-- that Haskell only evaluates things when it needs to. So you can ask for
-- the 1000th element of your list and Haskell will give it to you:

[1..] !! 999 -- 1000

-- And now Haskell has evaluated elements 1 - 1000 of this list...but the
-- rest of the elements of this "infinite" list don't exist yet! Haskell won't
-- actually evaluate them until it needs to.

-- joining two lists
[1..5] ++ [6..10]

-- adding to the head of a list
0:[1..5] -- [0, 1, 2, 3, 4, 5]

-- more list operations
head [1..5] -- 1
tail [1..5] -- [2, 3, 4, 5]
init [1..5] -- [1, 2, 3, 4]
last [1..5] -- 5

-- list comprehensions
[x*2 | x <- [1..5]] -- [2, 4, 6, 8, 10]

-- with a conditional
[x*2 | x <- [1..5], x*2 > 4] -- [6, 8, 10]

-- Every element in a tuple can be a different type, but a tuple has a
-- fixed length.
-- A tuple:
("haskell", 1)

-- accessing elements of a pair (i.e. a tuple of length 2)
fst ("haskell", 1) -- "haskell"
snd ("haskell", 1) -- 1

----------------------------------------------------
-- 3. Functions
----------------------------------------------------
-- A simple function that takes two variables
add a b = a + b

-- Note that if you are using ghci (the Haskell interpreter)
-- You'll need to use `let`, i.e.
-- let add a b = a + b

-- Using the function
add 1 2 -- 3

-- You can also put the function name between the two arguments
-- with backticks:
1 `add` 2 -- 3

-- You can also define functions that have no letters! This lets
-- you define your own operators! Here's an operator that does
-- integer division
(//) a b = a `div` b
35 // 4 -- 8

-- Guards: an easy way to do branching in functions
fib x
  | x < 2 = 1
  | otherwise = fib (x - 1) + fib (x - 2)

-- Pattern matching is similar. Here we have given three different
-- definitions for fib. Haskell will automatically call the first
-- function that matches the pattern of the value.
fib 1 = 1
fib 2 = 2
fib x = fib (x - 1) + fib (x - 2)

-- Pattern matching on tuples:
foo (x, y) = (x + 1, y + 2)

-- Pattern matching on lists. Here `x` is the first element
-- in the list, and `xs` is the rest of the list. We can write
-- our own map function:
myMap func [] = []
myMap func (x:xs) = func x:(myMap func xs)

-- Anonymous functions are created with a backslash followed by
-- all the arguments.
myMap (\x -> x + 2) [1..5] -- [3, 4, 5, 6, 7]

-- using fold (called `inject` in some languages) with an anonymous
-- function. foldl1 means fold left, and use the first value in the
-- list as the initial value for the accumulator.
foldl1 (\acc x -> acc + x) [1..5] -- 15

----------------------------------------------------
-- 4. More functions
----------------------------------------------------

-- partial application: if you don't pass in all the arguments to a function,
-- it gets "partially applied". That means it returns a function that takes the
-- rest of the arguments.

add a b = a + b
foo = add 10 -- foo is now a function that takes a number and adds 10 to it
foo 5 -- 15

-- Another way to write the same thing
foo = (10+)
foo 5 -- 15

-- function composition
-- the operator `.` chains functions together.
-- For example, here foo is a function that takes a value. It adds 10 to it,
-- multiplies the result of that by 4, and then returns the final value.
foo = (4*) . (10+)

-- 4*(10 + 5) = 60
foo 5 -- 60

-- fixing precedence
-- Haskell has another operator called `$`. This operator applies a function 
-- to a given parameter. In contrast to standard function application, which 
-- has highest possible priority of 10 and is left-associative, the `$` operator 
-- has priority of 0 and is right-associative. Such a low priority means that
-- the expression on its right is applied as the parameter to the function on its left.

-- before
even (fib 7) -- false

-- equivalently
even $ fib 7 -- false

-- composing functions
even . fib $ 7 -- false


----------------------------------------------------
-- 5. Type signatures
----------------------------------------------------

-- Haskell has a very strong type system, and every valid expression has a type. 

-- Some basic types:
5 :: Integer
"hello" :: String
True :: Bool

-- Functions have types too.
-- `not` takes a boolean and returns a boolean:
-- not :: Bool -> Bool

-- Here's a function that takes two arguments:
-- add :: Integer -> Integer -> Integer

-- When you define a value, it's good practice to write its type above it:
double :: Integer -> Integer
double x = x * 2

----------------------------------------------------
-- 6. Control Flow and If Expressions
----------------------------------------------------

-- if expressions
haskell = if 1 == 1 then "awesome" else "awful" -- haskell = "awesome"

-- if expressions can be on multiple lines too, indentation is important
haskell = if 1 == 1
            then "awesome"
            else "awful"

-- case expressions: Here's how you could parse command line arguments
case args of
  "help" -> printHelp
  "start" -> startProgram
  _ -> putStrLn "bad args"

-- Haskell doesn't have loops; it uses recursion instead.
-- map applies a function over every element in a list

map (*2) [1..5] -- [2, 4, 6, 8, 10]

-- you can make a for function using map
for array func = map func array

-- and then use it
for [0..5] $ \i -> show i

-- we could've written that like this too:
for [0..5] show

-- You can use foldl or foldr to reduce a list
-- foldl <fn> <initial value> <list>
foldl (\x y -> 2*x + y) 4 [1,2,3] -- 43

-- This is the same as
(2 * (2 * (2 * 4 + 1) + 2) + 3)

-- foldl is left-handed, foldr is right-handed
foldr (\x y -> 2*x + y) 4 [1,2,3] -- 16

-- This is now the same as
(2 * 1 + (2 * 2 + (2 * 3 + 4)))

----------------------------------------------------
-- 7. Data Types
----------------------------------------------------

-- Here's how you make your own data type in Haskell

data Color = Red | Blue | Green

-- Now you can use it in a function:


say :: Color -> String
say Red = "You are Red!"
say Blue = "You are Blue!"
say Green =  "You are Green!"

-- Your data types can have parameters too:

data Maybe a = Nothing | Just a

-- These are all of type Maybe
Just "hello"    -- of type `Maybe String`
Just 1          -- of type `Maybe Int`
Nothing         -- of type `Maybe a` for any `a`

----------------------------------------------------
-- 8. Haskell IO
----------------------------------------------------

-- While IO can't be explained fully without explaining monads,
-- it is not hard to explain enough to get going.

-- When a Haskell program is executed, `main` is
-- called. It must return a value of type `IO a` for some type `a`. For example:

main :: IO ()
main = putStrLn $ "Hello, sky! " ++ (say Blue)
-- putStrLn has type String -> IO ()

-- It is easiest to do IO if you can implement your program as
-- a function from String to String. The function
--    interact :: (String -> String) -> IO ()
-- inputs some text, runs a function on it, and prints out the
-- output.

countLines :: String -> String
countLines = show . length . lines

main' = interact countLines

-- You can think of a value of type `IO ()` as representing a
-- sequence of actions for the computer to do, much like a
-- computer program written in an imperative language. We can use
-- the `do` notation to chain actions together. For example:

sayHello :: IO ()
sayHello = do
   putStrLn "What is your name?"
   name <- getLine -- this gets a line and gives it the name "name"
   putStrLn $ "Hello, " ++ name

-- Exercise: write your own version of `interact` that only reads
--           one line of input.

-- The code in `sayHello` will never be executed, however. The only
-- action that ever gets executed is the value of `main`.
-- To run `sayHello` comment out the above definition of `main`
-- and replace it with:
--   main = sayHello

-- Let's understand better how the function `getLine` we just
-- used works. Its type is:
--    getLine :: IO String
-- You can think of a value of type `IO a` as representing a
-- computer program that will generate a value of type `a`
-- when executed (in addition to anything else it does). We can
-- name and reuse this value using `<-`. We can also
-- make our own action of type `IO String`:

action :: IO String
action = do
   putStrLn "This is a line. Duh"
   input1 <- getLine
   input2 <- getLine
   -- The type of the `do` statement is that of its last line.
   -- `return` is not a keyword, but merely a function
   return (input1 ++ "\n" ++ input2) -- return :: String -> IO String

-- We can use this just like we used `getLine`:

main'' = do
    putStrLn "I will echo two lines!"
    result <- action
    putStrLn result
    putStrLn "This was all, folks!"

-- The type `IO` is an example of a "monad". The way Haskell uses a monad to
-- do IO allows it to be a purely functional language. Any function that
-- interacts with the outside world (i.e. does IO) gets marked as `IO` in its
-- type signature. This lets us reason about what functions are "pure" (don't
-- interact with the outside world or modify state) and what functions aren't.

-- This is a powerful feature, because it's easy to run pure functions
-- concurrently; so, concurrency in Haskell is very easy.


----------------------------------------------------
-- 9. The Haskell REPL
----------------------------------------------------

-- Start the repl by typing `ghci`.
-- Now you can type in Haskell code. Any new values
-- need to be created with `let`:

let foo = 5

-- You can see the type of any value or expression with `:t`:

> :t foo
foo :: Integer

-- Operators, such as `+`, `:` and `$`, are functions.
-- Their type can be inspected by putting the operator in parentheses:

> :t (:)
(:) :: a -> [a] -> [a]

-- You can get additional information on any `name` using `:i`:

> :i (+)
class Num a where
  (+) :: a -> a -> a
  ...
    -- Defined in ‘GHC.Num’
infixl 6 +

-- You can also run any action of type `IO ()`

> sayHello
What is your name?
Friend!
Hello, Friend!

```

There's a lot more to Haskell, including typeclasses and monads. These are the
big ideas that make Haskell such fun to code in. I'll leave you with one final
Haskell example: an implementation of a quicksort variant in Haskell:

```haskell
qsort [] = []
qsort (p:xs) = qsort lesser ++ [p] ++ qsort greater
    where lesser  = filter (< p) xs
          greater = filter (>= p) xs
```

There are two popular ways to install Haskell: The traditional [Cabal-based installation](http://www.haskell.org/platform/), and the newer [Stack-based process](https://www.stackage.org/install).

You can find a much gentler introduction from the excellent
[Learn you a Haskell](http://learnyouahaskell.com/) or
[Real World Haskell](http://book.realworldhaskell.org/).