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diff --git a/haskell.html.markdown b/haskell.html.markdown new file mode 100644 index 00000000..5ffc63f3 --- /dev/null +++ b/haskell.html.markdown @@ -0,0 +1,290 @@ +--- +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 + +-- Strings and characters +"This is a string." +'a' -- character +'You cant use single quotes for strings.' -- error! + +-- Strings can be added too! +"Hello " ++ "world!" #=> "Hello world!" + +-- A string can be treated like 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 + +-- 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 -- 4 + +-- 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 + +-- 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) + +-- You can do the same thing with pattern matching. +fib 1 = 1 +fib 2 = 2 +fib x = fib (x - 1) + fib (x - 2) + +-- So we have given three different definitions for fib. +-- Haskell will automatically call the first function that matches +-- the pattern of the value. + +-- 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: +map func [x] = [func x] +map func (x:xs) = func x:(map func xs) + +-- Anonymous functions are created with a backslash followed by all the arguments. +map (\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. Some fancy things you can do with 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 (double 7)) -- true + +-- after +even . double $ 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 + +---------------------------------------------------- +-- 6. Control Flow +---------------------------------------------------- + +-- 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 in Haskell + +case args of + "help" -> printHelp + "start" -> startProgram + _ -> putStrLn "bad args" + + +-- loops: recursion +-- 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 -> print i + +---------------------------------------------------- +-- 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. 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 +``` |