2013-06-29 00:01:58 +00:00
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---
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2014-10-12 19:24:52 +00:00
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language: Haskell
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2017-08-25 08:01:38 +00:00
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filename: learnhaskell.hs
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2013-07-04 05:59:13 +00:00
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contributors:
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- ["Adit Bhargava", "http://adit.io"]
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2013-06-29 00:01:58 +00:00
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---
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2013-12-02 12:09:58 +00:00
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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 00:01:58 +00:00
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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 19:19:55 +00:00
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in a block like this.
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2013-06-29 00:01:58 +00:00
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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 17:03:42 +00:00
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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 00:01:58 +00:00
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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 17:03:42 +00:00
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-- Strings can be concatenated
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2013-06-29 00:09:34 +00:00
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"Hello " ++ "world!" -- "Hello world!"
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2013-06-29 00:01:58 +00:00
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2013-06-30 17:03:42 +00:00
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-- A string is a list of characters
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2015-02-01 21:39:52 +00:00
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['H', 'e', 'l', 'l', 'o'] -- "Hello"
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2013-06-29 00:09:34 +00:00
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"This is a string" !! 0 -- 'T'
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2013-06-29 00:01:58 +00:00
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----------------------------------------------------
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2017-04-01 20:19:58 +00:00
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-- 2. Lists and Tuples
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2013-06-29 00:01:58 +00:00
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----------------------------------------------------
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-- Every element in a list must have the same type.
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2017-02-09 15:26:11 +00:00
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-- These two lists are equal:
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2013-06-29 00:01:58 +00:00
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[1, 2, 3, 4, 5]
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[1..5]
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2015-02-01 00:40:55 +00:00
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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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2017-02-09 15:26:11 +00:00
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[5..1] -- [] (Haskell defaults to incrementing)
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2015-02-01 00:40:55 +00:00
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[5,4..1] -- [5, 4, 3, 2, 1]
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2015-03-01 19:16:29 +00:00
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-- indexing into a list
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2017-02-09 15:26:11 +00:00
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[1..10] !! 3 -- 4 (zero-based indexing)
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2015-03-01 19:16:29 +00:00
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2013-06-29 00:01:58 +00:00
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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 17:03:42 +00:00
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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 15:17:37 +00:00
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-- joining two lists
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2013-06-29 00:01:58 +00:00
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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 00:09:34 +00:00
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[x*2 | x <- [1..5], x*2 > 4] -- [6, 8, 10]
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2013-06-29 00:01:58 +00:00
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2013-06-29 03:53:43 +00:00
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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 00:01:58 +00:00
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-- A tuple:
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("haskell", 1)
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2014-11-21 17:28:38 +00:00
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-- accessing elements of a pair (i.e. a tuple of length 2)
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2013-06-29 00:01:58 +00:00
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fst ("haskell", 1) -- "haskell"
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snd ("haskell", 1) -- 1
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2018-07-10 22:12:23 +00:00
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-- pair element accessing does not work on n-tuples (i.e. triple, quadruple, etc)
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2018-07-10 22:34:42 +00:00
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snd ("snd", "can't touch this", "da na na na") -- error! see function below
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2018-07-10 22:12:23 +00:00
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2013-06-29 00:01:58 +00:00
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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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2013-06-29 21:17:52 +00:00
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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 00:01:58 +00:00
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-- Using the function
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add 1 2 -- 3
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2013-06-29 03:53:43 +00:00
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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 00:01:58 +00:00
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1 `add` 2 -- 3
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2013-08-08 19:05:01 +00:00
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-- You can also define functions that have no letters! This lets
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2013-06-29 03:53:43 +00:00
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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 00:01:58 +00:00
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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 20:07:19 +00:00
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| x < 2 = 1
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2013-06-29 00:01:58 +00:00
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| otherwise = fib (x - 1) + fib (x - 2)
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2013-06-29 00:09:34 +00:00
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-- Pattern matching is similar. Here we have given three different
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2017-02-09 15:27:29 +00:00
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-- equations that define fib. Haskell will automatically use the first
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2017-02-09 15:26:11 +00:00
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-- equation whose left hand side pattern matches the value.
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2013-06-29 00:01:58 +00:00
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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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2018-07-10 22:15:26 +00:00
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-- Pattern matching on tuples
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2018-07-10 22:34:42 +00:00
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sndOfTriple (_, y, _) = y -- use a wild card (_) to bypass naming unused value
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2013-06-29 00:01:58 +00:00
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2013-07-01 15:33:25 +00:00
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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 00:09:34 +00:00
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-- our own map function:
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2013-07-01 15:33:25 +00:00
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myMap func [] = []
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2013-06-30 17:03:42 +00:00
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myMap func (x:xs) = func x:(myMap func xs)
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2013-06-29 00:01:58 +00:00
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2013-06-29 03:53:43 +00:00
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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 17:03:42 +00:00
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myMap (\x -> x + 2) [1..5] -- [3, 4, 5, 6, 7]
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2013-06-29 00:01:58 +00:00
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2013-06-29 03:53:43 +00:00
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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 15:33:25 +00:00
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-- list as the initial value for the accumulator.
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2013-06-29 00:01:58 +00:00
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foldl1 (\acc x -> acc + x) [1..5] -- 15
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----------------------------------------------------
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2013-06-29 00:09:34 +00:00
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-- 4. More functions
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2013-06-29 00:01:58 +00:00
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----------------------------------------------------
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2014-10-21 22:30:30 +00:00
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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 20:07:19 +00:00
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-- it gets "partially applied". That means it returns a function that takes the
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2013-06-29 00:09:34 +00:00
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-- rest of the arguments.
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2013-06-29 00:01:58 +00:00
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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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2015-10-23 21:31:10 +00:00
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foo = (10+)
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2013-06-29 00:01:58 +00:00
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foo 5 -- 15
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-- function composition
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2015-10-31 12:37:13 +00:00
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-- the operator `.` chains functions together.
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2013-06-29 00:09:34 +00:00
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-- For example, here foo is a function that takes a value. It adds 10 to it,
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2015-10-20 19:05:16 +00:00
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-- multiplies the result of that by 4, and then returns the final value.
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2015-10-23 21:31:10 +00:00
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foo = (4*) . (10+)
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2013-06-29 00:01:58 +00:00
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2017-02-09 15:27:29 +00:00
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-- 4*(10+5) = 60
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2015-10-20 19:05:16 +00:00
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foo 5 -- 60
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2013-06-29 00:01:58 +00:00
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-- fixing precedence
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2018-07-10 22:12:23 +00:00
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-- Haskell has an 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 13:25:33 +00:00
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-- has priority of 0 and is right-associative. Such a low priority means that
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2018-07-10 22:34:42 +00:00
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-- the expression on its right is applied as parameter to function on its left.
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2013-06-29 00:01:58 +00:00
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-- before
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2015-07-06 08:40:05 +00:00
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even (fib 7) -- false
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2013-06-29 00:01:58 +00:00
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2014-11-21 17:28:38 +00:00
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-- equivalently
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2015-03-16 19:24:21 +00:00
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even $ fib 7 -- false
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2014-11-21 17:28:38 +00:00
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2015-07-06 08:40:05 +00:00
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-- composing functions
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even . fib $ 7 -- false
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2013-06-29 00:01:58 +00:00
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----------------------------------------------------
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-- 5. Type signatures
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----------------------------------------------------
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2018-07-10 22:12:23 +00:00
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-- Haskell has a very strong type system, and every valid expression has a type.
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2013-06-29 00:01:58 +00:00
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2013-06-29 00:09:34 +00:00
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-- Some basic types:
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2013-06-29 00:01:58 +00:00
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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 00:09:34 +00:00
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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 17:03:42 +00:00
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-- not :: Bool -> Bool
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2013-06-29 00:01:58 +00:00
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2013-06-29 00:09:34 +00:00
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-- Here's a function that takes two arguments:
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2013-06-30 17:03:42 +00:00
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-- add :: Integer -> Integer -> Integer
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2013-07-01 15:50:25 +00:00
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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 17:03:42 +00:00
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double :: Integer -> Integer
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double x = x * 2
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2013-06-29 00:01:58 +00:00
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----------------------------------------------------
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2014-11-21 17:28:38 +00:00
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-- 6. Control Flow and If Expressions
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2013-06-29 00:01:58 +00:00
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----------------------------------------------------
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2017-02-09 15:26:11 +00:00
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-- if-expressions
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2013-06-29 00:01:58 +00:00
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haskell = if 1 == 1 then "awesome" else "awful" -- haskell = "awesome"
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2017-02-09 15:26:11 +00:00
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-- if-expressions can be on multiple lines too, indentation is important
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2013-06-29 00:01:58 +00:00
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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 17:28:38 +00:00
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-- case expressions: Here's how you could parse command line arguments
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2013-06-29 00:01:58 +00:00
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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 17:28:38 +00:00
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-- Haskell doesn't have loops; it uses recursion instead.
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2015-10-23 21:31:10 +00:00
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-- map applies a function over every element in a list
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2013-06-29 00:01:58 +00:00
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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
|
2013-06-29 00:09:34 +00:00
|
|
|
|
for [0..5] $ \i -> show i
|
2013-06-29 00:01:58 +00:00
|
|
|
|
|
2013-06-29 00:09:34 +00:00
|
|
|
|
-- we could've written that like this too:
|
|
|
|
|
for [0..5] show
|
2013-06-29 00:01:58 +00:00
|
|
|
|
|
2013-07-02 18:00:30 +00:00
|
|
|
|
-- 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)
|
|
|
|
|
|
2015-10-23 21:31:10 +00:00
|
|
|
|
-- foldl is left-handed, foldr is right-handed
|
2013-07-02 18:00:30 +00:00
|
|
|
|
foldr (\x y -> 2*x + y) 4 [1,2,3] -- 16
|
|
|
|
|
|
|
|
|
|
-- This is now the same as
|
2015-04-27 08:16:16 +00:00
|
|
|
|
(2 * 1 + (2 * 2 + (2 * 3 + 4)))
|
2013-07-02 18:00:30 +00:00
|
|
|
|
|
2013-06-29 00:01:58 +00:00
|
|
|
|
----------------------------------------------------
|
|
|
|
|
-- 7. Data Types
|
|
|
|
|
----------------------------------------------------
|
|
|
|
|
|
|
|
|
|
-- Here's how you make your own data type in Haskell
|
|
|
|
|
|
|
|
|
|
data Color = Red | Blue | Green
|
|
|
|
|
|
2013-06-29 00:09:34 +00:00
|
|
|
|
-- Now you can use it in a function:
|
2013-06-29 00:01:58 +00:00
|
|
|
|
|
2013-07-04 05:52:48 +00:00
|
|
|
|
say :: Color -> String
|
2017-02-09 15:26:11 +00:00
|
|
|
|
say Red = "You are Red!"
|
|
|
|
|
say Blue = "You are Blue!"
|
|
|
|
|
say Green = "You are Green!"
|
2013-06-29 00:01:58 +00:00
|
|
|
|
|
|
|
|
|
-- Your data types can have parameters too:
|
|
|
|
|
|
|
|
|
|
data Maybe a = Nothing | Just a
|
|
|
|
|
|
|
|
|
|
-- These are all of type Maybe
|
2013-07-04 06:12:53 +00:00
|
|
|
|
Just "hello" -- of type `Maybe String`
|
|
|
|
|
Just 1 -- of type `Maybe Int`
|
|
|
|
|
Nothing -- of type `Maybe a` for any `a`
|
2013-06-29 00:01:58 +00:00
|
|
|
|
|
|
|
|
|
----------------------------------------------------
|
2013-06-29 22:10:47 +00:00
|
|
|
|
-- 8. Haskell IO
|
|
|
|
|
----------------------------------------------------
|
|
|
|
|
|
2013-06-30 17:03:42 +00:00
|
|
|
|
-- While IO can't be explained fully without explaining monads,
|
|
|
|
|
-- it is not hard to explain enough to get going.
|
2013-06-29 22:10:47 +00:00
|
|
|
|
|
2014-02-03 23:09:52 +00:00
|
|
|
|
-- When a Haskell program is executed, `main` is
|
2015-10-23 21:31:10 +00:00
|
|
|
|
-- called. It must return a value of type `IO a` for some type `a`. For example:
|
2013-07-04 05:52:48 +00:00
|
|
|
|
|
|
|
|
|
main :: IO ()
|
2015-03-16 20:07:19 +00:00
|
|
|
|
main = putStrLn $ "Hello, sky! " ++ (say Blue)
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- putStrLn has type String -> IO ()
|
|
|
|
|
|
2015-03-16 20:07:19 +00:00
|
|
|
|
-- It is easiest to do IO if you can implement your program as
|
|
|
|
|
-- a function from String to String. The function
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- interact :: (String -> String) -> IO ()
|
2015-03-16 20:07:19 +00:00
|
|
|
|
-- inputs some text, runs a function on it, and prints out the
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- 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 ()
|
2015-03-16 20:07:19 +00:00
|
|
|
|
sayHello = do
|
2013-07-04 05:52:48 +00:00
|
|
|
|
putStrLn "What is your name?"
|
2013-09-20 05:25:36 +00:00
|
|
|
|
name <- getLine -- this gets a line and gives it the name "name"
|
2013-07-04 05:52:48 +00:00
|
|
|
|
putStrLn $ "Hello, " ++ name
|
2015-03-16 20:07:19 +00:00
|
|
|
|
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- Exercise: write your own version of `interact` that only reads
|
|
|
|
|
-- one line of input.
|
2015-03-16 20:07:19 +00:00
|
|
|
|
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- The code in `sayHello` will never be executed, however. The only
|
2015-03-16 20:07:19 +00:00
|
|
|
|
-- action that ever gets executed is the value of `main`.
|
|
|
|
|
-- To run `sayHello` comment out the above definition of `main`
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- and replace it with:
|
|
|
|
|
-- main = sayHello
|
|
|
|
|
|
2015-03-16 20:07:19 +00:00
|
|
|
|
-- Let's understand better how the function `getLine` we just
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- used works. Its type is:
|
|
|
|
|
-- getLine :: IO String
|
2013-07-04 06:12:53 +00:00
|
|
|
|
-- You can think of a value of type `IO a` as representing a
|
2015-03-16 20:07:19 +00:00
|
|
|
|
-- computer program that will generate a value of type `a`
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- when executed (in addition to anything else it does). We can
|
2015-10-23 21:31:10 +00:00
|
|
|
|
-- name and reuse this value using `<-`. We can also
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- make our own action of type `IO String`:
|
|
|
|
|
|
2013-06-30 17:03:42 +00:00
|
|
|
|
action :: IO String
|
2013-06-29 22:10:47 +00:00
|
|
|
|
action = do
|
|
|
|
|
putStrLn "This is a line. Duh"
|
2015-03-16 20:07:19 +00:00
|
|
|
|
input1 <- getLine
|
2013-06-29 22:10:47 +00:00
|
|
|
|
input2 <- getLine
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- The type of the `do` statement is that of its last line.
|
2015-03-16 20:07:19 +00:00
|
|
|
|
-- `return` is not a keyword, but merely a function
|
2013-07-04 05:52:48 +00:00
|
|
|
|
return (input1 ++ "\n" ++ input2) -- return :: String -> IO String
|
2013-06-29 22:10:47 +00:00
|
|
|
|
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- We can use this just like we used `getLine`:
|
2013-06-29 22:10:47 +00:00
|
|
|
|
|
2013-07-04 05:52:48 +00:00
|
|
|
|
main'' = do
|
|
|
|
|
putStrLn "I will echo two lines!"
|
2015-03-16 20:07:19 +00:00
|
|
|
|
result <- action
|
2013-06-29 22:10:47 +00:00
|
|
|
|
putStrLn result
|
|
|
|
|
putStrLn "This was all, folks!"
|
2013-06-30 17:03:42 +00:00
|
|
|
|
|
2013-07-04 06:01:47 +00:00
|
|
|
|
-- 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
|
2017-02-09 15:26:11 +00:00
|
|
|
|
-- type signature. This lets us reason about which functions are "pure" (don't
|
|
|
|
|
-- interact with the outside world or modify state) and which functions aren't.
|
2013-07-04 06:01:47 +00:00
|
|
|
|
|
|
|
|
|
-- This is a powerful feature, because it's easy to run pure functions
|
|
|
|
|
-- concurrently; so, concurrency in Haskell is very easy.
|
2013-06-29 22:10:47 +00:00
|
|
|
|
|
|
|
|
|
|
|
|
|
|
----------------------------------------------------
|
|
|
|
|
-- 9. The Haskell REPL
|
2013-06-29 00:01:58 +00:00
|
|
|
|
----------------------------------------------------
|
|
|
|
|
|
|
|
|
|
-- 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
|
|
|
|
|
|
2015-10-31 12:37:13 +00:00
|
|
|
|
-- You can see the type of any value or expression with `:t`:
|
2013-06-29 00:01:58 +00:00
|
|
|
|
|
2015-10-31 12:37:13 +00:00
|
|
|
|
> :t foo
|
2013-06-29 00:01:58 +00:00
|
|
|
|
foo :: Integer
|
2013-07-04 05:52:48 +00:00
|
|
|
|
|
2015-10-31 12:37:13 +00:00
|
|
|
|
-- Operators, such as `+`, `:` and `$`, are functions.
|
|
|
|
|
-- Their type can be inspected by putting the operator in parentheses:
|
|
|
|
|
|
|
|
|
|
> :t (:)
|
|
|
|
|
(:) :: a -> [a] -> [a]
|
|
|
|
|
|
|
|
|
|
-- You can get additional information on any `name` using `:i`:
|
|
|
|
|
|
|
|
|
|
> :i (+)
|
|
|
|
|
class Num a where
|
|
|
|
|
(+) :: a -> a -> a
|
|
|
|
|
...
|
|
|
|
|
-- Defined in ‘GHC.Num’
|
|
|
|
|
infixl 6 +
|
|
|
|
|
|
2013-07-04 05:52:48 +00:00
|
|
|
|
-- You can also run any action of type `IO ()`
|
|
|
|
|
|
|
|
|
|
> sayHello
|
|
|
|
|
What is your name?
|
|
|
|
|
Friend!
|
|
|
|
|
Hello, Friend!
|
|
|
|
|
|
2013-06-29 00:01:58 +00:00
|
|
|
|
```
|
|
|
|
|
|
2013-12-02 12:09:58 +00:00
|
|
|
|
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
|
2015-10-23 21:31:10 +00:00
|
|
|
|
Haskell example: an implementation of a quicksort variant in Haskell:
|
2013-06-29 00:01:58 +00:00
|
|
|
|
|
|
|
|
|
```haskell
|
|
|
|
|
qsort [] = []
|
|
|
|
|
qsort (p:xs) = qsort lesser ++ [p] ++ qsort greater
|
|
|
|
|
where lesser = filter (< p) xs
|
|
|
|
|
greater = filter (>= p) xs
|
|
|
|
|
```
|
2013-06-29 00:09:34 +00:00
|
|
|
|
|
2015-12-03 22:27:24 +00:00
|
|
|
|
There are two popular ways to install Haskell: The traditional [Cabal-based installation](http://www.haskell.org/platform/), and the newer [Stack-based process](https://www.stackage.org/install).
|
2013-06-29 03:53:43 +00:00
|
|
|
|
|
2013-07-09 14:42:37 +00:00
|
|
|
|
You can find a much gentler introduction from the excellent
|
2017-09-10 19:26:33 +00:00
|
|
|
|
[Learn you a Haskell](http://learnyouahaskell.com/),
|
|
|
|
|
[Happy Learn Haskell Tutorial](http://www.happylearnhaskelltutorial.com/) or
|
2013-07-09 14:42:37 +00:00
|
|
|
|
[Real World Haskell](http://book.realworldhaskell.org/).
|