2013-06-29 04:01:58 +04:00
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---
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2014-10-12 23:24:52 +04:00
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language: Haskell
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2013-07-04 09:59:13 +04:00
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contributors:
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- ["Adit Bhargava", "http://adit.io"]
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2013-06-29 04:01:58 +04:00
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---
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2013-12-02 16:09:58 +04:00
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Haskell was designed as a practical, purely functional programming
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language. It's famous for its monads and its type system, but I keep coming back
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to it because of its elegance. Haskell makes coding a real joy for me.
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2013-06-29 04:01:58 +04:00
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```haskell
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-- Single line comments start with two dashes.
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{- Multiline comments can be enclosed
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2013-10-23 23:19:55 +04:00
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in a block like this.
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2013-06-29 04:01:58 +04:00
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-}
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----------------------------------------------------
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-- 1. Primitive Datatypes and Operators
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----------------------------------------------------
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-- You have numbers
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3 -- 3
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-- Math is what you would expect
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1 + 1 -- 2
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8 - 1 -- 7
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10 * 2 -- 20
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35 / 5 -- 7.0
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-- Division is not integer division by default
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35 / 4 -- 8.75
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-- integer division
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35 `div` 4 -- 8
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-- Boolean values are primitives
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True
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False
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-- Boolean operations
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not True -- False
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not False -- True
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1 == 1 -- True
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1 /= 1 -- False
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1 < 10 -- True
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2013-06-30 21:03:42 +04:00
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-- In the above examples, `not` is a function that takes one value.
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-- Haskell doesn't need parentheses for function calls...all the arguments
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-- are just listed after the function. So the general pattern is:
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-- func arg1 arg2 arg3...
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-- See the section on functions for information on how to write your own.
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2013-06-29 04:01:58 +04:00
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-- Strings and characters
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"This is a string."
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'a' -- character
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'You cant use single quotes for strings.' -- error!
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2013-06-30 21:03:42 +04:00
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-- Strings can be concatenated
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2013-06-29 04:09:34 +04:00
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"Hello " ++ "world!" -- "Hello world!"
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2013-06-29 04:01:58 +04:00
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2013-06-30 21:03:42 +04:00
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-- A string is a list of characters
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2015-02-02 00:39:52 +03:00
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['H', 'e', 'l', 'l', 'o'] -- "Hello"
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2013-06-29 04:09:34 +04:00
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"This is a string" !! 0 -- 'T'
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2013-06-29 04:01:58 +04:00
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----------------------------------------------------
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-- Lists and Tuples
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----------------------------------------------------
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-- Every element in a list must have the same type.
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2015-02-02 00:39:52 +03:00
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-- These two lists are the same:
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2013-06-29 04:01:58 +04:00
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[1, 2, 3, 4, 5]
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[1..5]
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2015-02-01 03:40:55 +03:00
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-- Ranges are versatile.
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['A'..'F'] -- "ABCDEF"
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-- You can create a step in a range.
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[0,2..10] -- [0, 2, 4, 6, 8, 10]
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[5..1] -- This doesn't work because Haskell defaults to incrementing.
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[5,4..1] -- [5, 4, 3, 2, 1]
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2015-03-01 22:16:29 +03:00
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-- indexing into a list
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[0..] !! 5 -- 5
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2013-06-29 04:01:58 +04:00
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-- You can also have infinite lists in Haskell!
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[1..] -- a list of all the natural numbers
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2013-06-30 21:03:42 +04:00
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-- Infinite lists work because Haskell has "lazy evaluation". This means
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-- that Haskell only evaluates things when it needs to. So you can ask for
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-- the 1000th element of your list and Haskell will give it to you:
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[1..] !! 999 -- 1000
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-- And now Haskell has evaluated elements 1 - 1000 of this list...but the
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-- rest of the elements of this "infinite" list don't exist yet! Haskell won't
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-- actually evaluate them until it needs to.
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2013-08-12 19:17:37 +04:00
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-- joining two lists
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2013-06-29 04:01:58 +04:00
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[1..5] ++ [6..10]
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-- adding to the head of a list
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0:[1..5] -- [0, 1, 2, 3, 4, 5]
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-- more list operations
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head [1..5] -- 1
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tail [1..5] -- [2, 3, 4, 5]
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init [1..5] -- [1, 2, 3, 4]
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last [1..5] -- 5
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-- list comprehensions
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[x*2 | x <- [1..5]] -- [2, 4, 6, 8, 10]
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-- with a conditional
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2013-06-29 04:09:34 +04:00
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[x*2 | x <- [1..5], x*2 > 4] -- [6, 8, 10]
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2013-06-29 04:01:58 +04:00
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2013-06-29 07:53:43 +04:00
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-- Every element in a tuple can be a different type, but a tuple has a
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-- fixed length.
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2013-06-29 04:01:58 +04:00
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-- A tuple:
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("haskell", 1)
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2014-11-21 20:28:38 +03:00
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-- accessing elements of a pair (i.e. a tuple of length 2)
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2013-06-29 04:01:58 +04:00
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fst ("haskell", 1) -- "haskell"
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snd ("haskell", 1) -- 1
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----------------------------------------------------
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-- 3. Functions
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----------------------------------------------------
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-- A simple function that takes two variables
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add a b = a + b
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2013-06-30 01:17:52 +04:00
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-- Note that if you are using ghci (the Haskell interpreter)
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-- You'll need to use `let`, i.e.
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-- let add a b = a + b
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2013-06-29 04:01:58 +04:00
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-- Using the function
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add 1 2 -- 3
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2013-06-29 07:53:43 +04:00
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-- You can also put the function name between the two arguments
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-- with backticks:
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2013-06-29 04:01:58 +04:00
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1 `add` 2 -- 3
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2013-08-08 23:05:01 +04:00
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-- You can also define functions that have no letters! This lets
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2013-06-29 07:53:43 +04:00
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-- you define your own operators! Here's an operator that does
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-- integer division
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2013-06-29 04:01:58 +04:00
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(//) a b = a `div` b
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35 // 4 -- 8
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-- Guards: an easy way to do branching in functions
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fib x
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2015-03-16 23:07:19 +03:00
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| x < 2 = 1
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2013-06-29 04:01:58 +04:00
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| otherwise = fib (x - 1) + fib (x - 2)
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2013-06-29 04:09:34 +04:00
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-- Pattern matching is similar. Here we have given three different
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-- definitions for fib. Haskell will automatically call the first
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2015-03-16 23:07:19 +03:00
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-- function that matches the pattern of the value.
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2013-06-29 04:01:58 +04:00
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fib 1 = 1
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fib 2 = 2
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fib x = fib (x - 1) + fib (x - 2)
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-- Pattern matching on tuples:
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foo (x, y) = (x + 1, y + 2)
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2013-07-01 19:33:25 +04:00
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-- Pattern matching on lists. Here `x` is the first element
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-- in the list, and `xs` is the rest of the list. We can write
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2013-06-29 04:09:34 +04:00
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-- our own map function:
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2013-07-01 19:33:25 +04:00
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myMap func [] = []
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2013-06-30 21:03:42 +04:00
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myMap func (x:xs) = func x:(myMap func xs)
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2013-06-29 04:01:58 +04:00
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2013-06-29 07:53:43 +04:00
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-- Anonymous functions are created with a backslash followed by
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-- all the arguments.
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2013-06-30 21:03:42 +04:00
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myMap (\x -> x + 2) [1..5] -- [3, 4, 5, 6, 7]
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2013-06-29 04:01:58 +04:00
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2013-06-29 07:53:43 +04:00
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-- using fold (called `inject` in some languages) with an anonymous
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-- function. foldl1 means fold left, and use the first value in the
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2013-07-01 19:33:25 +04:00
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-- list as the initial value for the accumulator.
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2013-06-29 04:01:58 +04:00
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foldl1 (\acc x -> acc + x) [1..5] -- 15
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----------------------------------------------------
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2013-06-29 04:09:34 +04:00
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-- 4. More functions
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2013-06-29 04:01:58 +04:00
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----------------------------------------------------
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2014-10-22 02:30:30 +04:00
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-- partial application: if you don't pass in all the arguments to a function,
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2015-03-16 23:07:19 +03:00
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-- it gets "partially applied". That means it returns a function that takes the
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2013-06-29 04:09:34 +04:00
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-- rest of the arguments.
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2013-06-29 04:01:58 +04:00
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add a b = a + b
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foo = add 10 -- foo is now a function that takes a number and adds 10 to it
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foo 5 -- 15
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-- Another way to write the same thing
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foo = (+10)
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foo 5 -- 15
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-- function composition
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2013-06-29 04:09:34 +04:00
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-- the (.) function chains functions together.
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-- For example, here foo is a function that takes a value. It adds 10 to it,
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-- multiplies the result of that by 5, and then returns the final value.
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2013-06-29 04:01:58 +04:00
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foo = (*5) . (+10)
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-- (5 + 10) * 5 = 75
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foo 5 -- 75
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-- fixing precedence
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2015-03-25 19:08:32 +03:00
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-- Haskell has another operator called `$`. This operator applies a function
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-- to a given parameter. In contrast to standard function application, which
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-- has highest possible priority of 10 and is left-associative, the `$` operator
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2015-03-27 16:25:33 +03:00
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-- has priority of 0 and is right-associative. Such a low priority means that
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-- the expression on its right is applied as the parameter to the function on its left.
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2013-06-29 04:01:58 +04:00
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-- before
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2015-03-16 22:24:21 +03:00
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(even (fib 7)) -- false
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2013-06-29 04:01:58 +04:00
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-- after
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2015-03-16 22:24:21 +03:00
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even . fib $ 7 -- false
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2013-06-29 04:01:58 +04:00
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2014-11-21 20:28:38 +03:00
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-- equivalently
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2015-03-16 22:24:21 +03:00
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even $ fib 7 -- false
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2014-11-21 20:28:38 +03:00
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2013-06-29 04:01:58 +04:00
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----------------------------------------------------
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-- 5. Type signatures
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----------------------------------------------------
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2013-06-29 04:09:34 +04:00
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-- Haskell has a very strong type system, and everything has a type signature.
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2013-06-29 04:01:58 +04:00
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2013-06-29 04:09:34 +04:00
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-- Some basic types:
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2013-06-29 04:01:58 +04:00
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5 :: Integer
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"hello" :: String
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True :: Bool
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2013-06-29 04:09:34 +04:00
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-- Functions have types too.
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-- `not` takes a boolean and returns a boolean:
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2013-06-30 21:03:42 +04:00
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-- not :: Bool -> Bool
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2013-06-29 04:01:58 +04:00
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2013-06-29 04:09:34 +04:00
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-- Here's a function that takes two arguments:
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2013-06-30 21:03:42 +04:00
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-- add :: Integer -> Integer -> Integer
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2013-07-01 19:50:25 +04:00
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-- When you define a value, it's good practice to write its type above it:
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2013-06-30 21:03:42 +04:00
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double :: Integer -> Integer
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double x = x * 2
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2013-06-29 04:01:58 +04:00
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----------------------------------------------------
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2014-11-21 20:28:38 +03:00
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-- 6. Control Flow and If Expressions
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2013-06-29 04:01:58 +04:00
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----------------------------------------------------
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2014-11-21 20:28:38 +03:00
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-- if expressions
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2013-06-29 04:01:58 +04:00
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haskell = if 1 == 1 then "awesome" else "awful" -- haskell = "awesome"
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2014-11-21 20:28:38 +03:00
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-- if expressions can be on multiple lines too, indentation is important
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2013-06-29 04:01:58 +04:00
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haskell = if 1 == 1
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then "awesome"
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else "awful"
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2014-11-21 20:28:38 +03:00
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-- case expressions: Here's how you could parse command line arguments
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2013-06-29 04:01:58 +04:00
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case args of
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"help" -> printHelp
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"start" -> startProgram
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_ -> putStrLn "bad args"
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2014-11-21 20:28:38 +03:00
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-- Haskell doesn't have loops; it uses recursion instead.
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2013-07-02 22:05:22 +04:00
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-- map applies a function over every element in an array
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2013-06-29 04:01:58 +04:00
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map (*2) [1..5] -- [2, 4, 6, 8, 10]
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-- you can make a for function using map
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for array func = map func array
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-- and then use it
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2013-06-29 04:09:34 +04:00
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for [0..5] $ \i -> show i
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2013-06-29 04:01:58 +04:00
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2013-06-29 04:09:34 +04:00
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-- we could've written that like this too:
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for [0..5] show
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2013-06-29 04:01:58 +04:00
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2013-07-02 22:00:30 +04:00
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-- You can use foldl or foldr to reduce a list
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-- foldl <fn> <initial value> <list>
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foldl (\x y -> 2*x + y) 4 [1,2,3] -- 43
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-- This is the same as
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(2 * (2 * (2 * 4 + 1) + 2) + 3)
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-- foldl is left-handed, foldr is right-
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foldr (\x y -> 2*x + y) 4 [1,2,3] -- 16
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-- This is now the same as
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2015-04-27 11:16:16 +03:00
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(2 * 1 + (2 * 2 + (2 * 3 + 4)))
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2013-07-02 22:00:30 +04:00
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2013-06-29 04:01:58 +04:00
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----------------------------------------------------
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-- 7. Data Types
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----------------------------------------------------
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-- Here's how you make your own data type in Haskell
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data Color = Red | Blue | Green
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2013-06-29 04:09:34 +04:00
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-- Now you can use it in a function:
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2013-06-29 04:01:58 +04:00
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2013-07-04 09:52:48 +04:00
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say :: Color -> String
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say Red = "You are Red!"
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say Blue = "You are Blue!"
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say Green = "You are Green!"
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2013-06-29 04:01:58 +04:00
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-- Your data types can have parameters too:
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data Maybe a = Nothing | Just a
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-- These are all of type Maybe
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2013-07-04 10:12:53 +04:00
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Just "hello" -- of type `Maybe String`
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Just 1 -- of type `Maybe Int`
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Nothing -- of type `Maybe a` for any `a`
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2013-06-29 04:01:58 +04:00
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----------------------------------------------------
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2013-06-30 02:10:47 +04:00
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-- 8. Haskell IO
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----------------------------------------------------
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2013-06-30 21:03:42 +04:00
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-- While IO can't be explained fully without explaining monads,
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-- it is not hard to explain enough to get going.
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2013-06-30 02:10:47 +04:00
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2014-02-04 03:09:52 +04:00
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-- When a Haskell program is executed, `main` is
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2013-07-04 09:52:48 +04:00
|
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-- called. It must return a value of type `IO ()`. For example:
|
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main :: IO ()
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2015-03-16 23:07:19 +03:00
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main = putStrLn $ "Hello, sky! " ++ (say Blue)
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2013-07-04 09:52:48 +04:00
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-- putStrLn has type String -> IO ()
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|
2015-03-16 23:07:19 +03:00
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-- It is easiest to do IO if you can implement your program as
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-- a function from String to String. The function
|
2013-07-04 09:52:48 +04:00
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-- interact :: (String -> String) -> IO ()
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2015-03-16 23:07:19 +03:00
|
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-- inputs some text, runs a function on it, and prints out the
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2013-07-04 09:52:48 +04:00
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-- output.
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countLines :: String -> String
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countLines = show . length . lines
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main' = interact countLines
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-- You can think of a value of type `IO ()` as representing a
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-- sequence of actions for the computer to do, much like a
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-- computer program written in an imperative language. We can use
|
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-- the `do` notation to chain actions together. For example:
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sayHello :: IO ()
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2015-03-16 23:07:19 +03:00
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sayHello = do
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2013-07-04 09:52:48 +04:00
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putStrLn "What is your name?"
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2013-09-20 09:25:36 +04:00
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name <- getLine -- this gets a line and gives it the name "name"
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2013-07-04 09:52:48 +04:00
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putStrLn $ "Hello, " ++ name
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2015-03-16 23:07:19 +03:00
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2013-07-04 09:52:48 +04:00
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-- Exercise: write your own version of `interact` that only reads
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-- one line of input.
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2015-03-16 23:07:19 +03:00
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2013-07-04 09:52:48 +04:00
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-- The code in `sayHello` will never be executed, however. The only
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2015-03-16 23:07:19 +03:00
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-- action that ever gets executed is the value of `main`.
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-- To run `sayHello` comment out the above definition of `main`
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2013-07-04 09:52:48 +04:00
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-- and replace it with:
|
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|
-- main = sayHello
|
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|
2015-03-16 23:07:19 +03:00
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|
-- Let's understand better how the function `getLine` we just
|
2013-07-04 09:52:48 +04:00
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|
-- used works. Its type is:
|
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-- getLine :: IO String
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2013-07-04 10:12:53 +04:00
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-- You can think of a value of type `IO a` as representing a
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2015-03-16 23:07:19 +03:00
|
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|
-- computer program that will generate a value of type `a`
|
2013-07-04 09:52:48 +04:00
|
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-- when executed (in addition to anything else it does). We can
|
2015-03-16 23:07:19 +03:00
|
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|
-- store and reuse this value using `<-`. We can also
|
2013-07-04 09:52:48 +04:00
|
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|
-- make our own action of type `IO String`:
|
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|
2013-06-30 21:03:42 +04:00
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action :: IO String
|
2013-06-30 02:10:47 +04:00
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|
action = do
|
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|
putStrLn "This is a line. Duh"
|
2015-03-16 23:07:19 +03:00
|
|
|
input1 <- getLine
|
2013-06-30 02:10:47 +04:00
|
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|
input2 <- getLine
|
2013-07-04 09:52:48 +04:00
|
|
|
-- The type of the `do` statement is that of its last line.
|
2015-03-16 23:07:19 +03:00
|
|
|
-- `return` is not a keyword, but merely a function
|
2013-07-04 09:52:48 +04:00
|
|
|
return (input1 ++ "\n" ++ input2) -- return :: String -> IO String
|
2013-06-30 02:10:47 +04:00
|
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|
|
2013-07-04 09:52:48 +04:00
|
|
|
-- We can use this just like we used `getLine`:
|
2013-06-30 02:10:47 +04:00
|
|
|
|
2013-07-04 09:52:48 +04:00
|
|
|
main'' = do
|
|
|
|
putStrLn "I will echo two lines!"
|
2015-03-16 23:07:19 +03:00
|
|
|
result <- action
|
2013-06-30 02:10:47 +04:00
|
|
|
putStrLn result
|
|
|
|
putStrLn "This was all, folks!"
|
2013-06-30 21:03:42 +04:00
|
|
|
|
2013-07-04 10:01:47 +04:00
|
|
|
-- 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.
|
2013-06-30 02:10:47 +04:00
|
|
|
|
|
|
|
|
|
|
|
----------------------------------------------------
|
|
|
|
-- 9. The Haskell REPL
|
2013-06-29 04:01:58 +04:00
|
|
|
----------------------------------------------------
|
|
|
|
|
|
|
|
-- 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
|
2013-07-04 09:52:48 +04:00
|
|
|
|
|
|
|
-- You can also run any action of type `IO ()`
|
|
|
|
|
|
|
|
> sayHello
|
|
|
|
What is your name?
|
|
|
|
Friend!
|
|
|
|
Hello, Friend!
|
|
|
|
|
2013-06-29 04:01:58 +04:00
|
|
|
```
|
|
|
|
|
2013-12-02 16:09:58 +04:00
|
|
|
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:
|
2013-06-29 04:01:58 +04:00
|
|
|
|
|
|
|
```haskell
|
|
|
|
qsort [] = []
|
|
|
|
qsort (p:xs) = qsort lesser ++ [p] ++ qsort greater
|
|
|
|
where lesser = filter (< p) xs
|
|
|
|
greater = filter (>= p) xs
|
|
|
|
```
|
2013-06-29 04:09:34 +04:00
|
|
|
|
|
|
|
Haskell is easy to install. Get it [here](http://www.haskell.org/platform/).
|
2013-06-29 07:53:43 +04:00
|
|
|
|
2013-07-09 18:42:37 +04:00
|
|
|
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/).
|