diff options
Diffstat (limited to 'haskell.html.markdown')
| -rw-r--r-- | haskell.html.markdown | 348 |
1 files changed, 348 insertions, 0 deletions
diff --git a/haskell.html.markdown b/haskell.html.markdown new file mode 100644 index 00000000..f3baa9a5 --- /dev/null +++ b/haskell.html.markdown @@ -0,0 +1,348 @@ +--- +language: haskell +author: Adit Bhargava +author_url: http://adit.io +--- + +Haskell was designed as a practical, purely functional programming language. It's famous for +it's monads and it's type system, but I keep coming back to it because of it's elegance. Haskell +makes coding a real joy for me. + +```haskell +-- Single line comments start with two dashes. +{- Multiline comments can be enclosed +in a block like this. +-} + +---------------------------------------------------- +-- 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 +"This is a string" !! 0 -- 'T' + + +---------------------------------------------------- +-- Lists and Tuples +---------------------------------------------------- + +-- Every element in a list must have the same type. +-- Two lists that are the same +[1, 2, 3, 4, 5] +[1..5] + +-- 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] + +-- indexing into a list +[0..] !! 5 -- 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 tuple +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 characters! 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 = x + | 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 arrays. Here `x` is the first element +-- in the array, and `xs` is the rest of the array. We can write +-- our own map function: +myMap func [x] = [func x] +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 +-- array as the initial value for the accumulator. +foldl1 (\acc x -> acc + x) [1..5] -- 15 + +---------------------------------------------------- +-- 4. More functions +---------------------------------------------------- + +-- currying: if you don't pass in all the arguments to a function, +-- it gets "curried". 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 (.) function 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 5, and then returns the final value. +foo = (*5) . (+10) + +-- (5 + 10) * 5 = 75 +foo 5 -- 75 + +-- fixing precedence +-- Haskell has another function called `$`. This changes the precedence +-- so that everything to the left of it gets computed first and then applied +-- to everything on the right. You can use `.` and `$` to get rid of a lot +-- of parentheses: + +-- before +(even (fib 7)) -- true + +-- after +even . fib $ 7 -- true + +---------------------------------------------------- +-- 5. Type signatures +---------------------------------------------------- + +-- Haskell has a very strong type system, and everything has a type signature. + +-- 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 it's type above it: +double :: Integer -> Integer +double x = x * 2 + +---------------------------------------------------- +-- 6. Control Flow and If Statements +---------------------------------------------------- + +-- if statements +haskell = if 1 == 1 then "awesome" else "awful" -- haskell = "awesome" + +-- if statements can be on multiple lines too, indentation is important +haskell = if 1 == 1 + then "awesome" + else "awful" + +-- case statements: Here's how you could parse command line arguments +case args of + "help" -> printHelp + "start" -> startProgram + _ -> putStrLn "bad args" + +-- Haskell doesn't have loops because it uses recursion instead. +-- map a function over every element in an array + +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 + +---------------------------------------------------- +-- 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 -> IO String +say Red = putStrLn "You are Red!" +say Blue = putStrLn "You are Blue!" +say Green = putStrLn "You are Green!" + +-- Your data types can have parameters too: + +data Maybe a = Nothing | Just a + +-- These are all of type Maybe +Nothing +Just "hello" +Just 1 + +---------------------------------------------------- +-- 8. Haskell IO +---------------------------------------------------- + +-- While IO can't be explained fully without explaining monads, +-- it is not hard to explain enough to get going. + +-- An `IO a` value is an IO action: you can chain them with do blocks +action :: IO String +action = do + putStrLn "This is a line. Duh" + input <- getLine -- this gets a line and gives it the name "input" + input2 <- getLine + return (input1 ++ "\n" ++ input2) -- This is the result of the whole action + +-- This didn't actually do anything. When a haskell program is executed +-- an IO action called "main" is read and interpreted. + +main = do + putStrLn "Our first program. How exciting!" + result <- action -- our defined action is just like the default ones + putStrLn result + putStrLn "This was all, folks!" + +-- Haskell does IO through a monad because this allows it to be a purely +-- functional language. Our `action` function had a type signature of `IO String`. +-- In general any function that interacts with the outside world (i.e. does IO) +-- gets marked as `IO` in it's 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 with `:t`: + +>:t foo +foo :: Integer +``` + +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 quicksort in Haskell: + +```haskell +qsort [] = [] +qsort (p:xs) = qsort lesser ++ [p] ++ qsort greater + where lesser = filter (< p) xs + greater = filter (>= p) xs +``` + +Haskell is easy to install. Get it [here](http://www.haskell.org/platform/). + +You can find a much gentler introduction from the excellent [Learn you a Haskell](http://learnyouahaskell.com/) + |