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diff --git a/pl-pl/haskell-pl.html.markdown b/pl-pl/haskell-pl.html.markdown new file mode 100644 index 00000000..196ad9a6 --- /dev/null +++ b/pl-pl/haskell-pl.html.markdown @@ -0,0 +1,450 @@ +--- +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/). |