| language | contributors | |||
|---|---|---|---|---|
haskell |
|
Haskell was designed as a practical, purely functional programming language. It's famous for its monads and its type system, but I keep coming back to it because of its elegance. Haskell makes coding a real joy for me.
-- Single line comments start with two dashes.
{- Multiline comments can be enclosed
en 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 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 = 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 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
----------------------------------------------------
-- 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 its 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 applies 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
-- 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-
foldr (\x y -> 2*x + y) 4 [1,2,3] -- 16
-- This is now the same as
(2 * 3 + (2 * 2 + (2 * 1 + 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, the function `main` is
-- called. It must return a value of type `IO ()`. 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 "input"
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
-- store 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 with `:t`:
>:t foo
foo :: Integer
-- 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 quicksort in Haskell:
qsort [] = []
qsort (p:xs) = qsort lesser ++ [p] ++ qsort greater
where lesser = filter (< p) xs
greater = filter (>= p) xsHaskell is easy to install. Get it here.
You can find a much gentler introduction from the excellent Learn you a Haskell or Real World Haskell.