mirror of
https://github.com/adambard/learnxinyminutes-docs.git
synced 2024-12-23 15:24:09 +03:00
346 lines
9.5 KiB
Markdown
346 lines
9.5 KiB
Markdown
---
|
||
language: haskell
|
||
author: Adit Bhargava
|
||
author_url: http://adit.io
|
||
filename: learnhaskell.hs
|
||
---
|
||
|
||
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
|
||
|
||
-- 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/)
|
||
|