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+---
+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/).