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349 lines
9.6 KiB
Markdown
349 lines
9.6 KiB
Markdown
---
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language: haskell
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author: Adit Bhargava
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author_url: http://adit.io
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---
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Haskell was designed as a practical, purely functional programming language. It's famous for
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it's monads and it's type system, but I keep coming back to it because of it's elegance. Haskell
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makes coding a real joy for me.
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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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in a block like this.
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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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-- 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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-- 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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-- Strings can be concatenated
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"Hello " ++ "world!" -- "Hello world!"
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-- A string is a list of characters
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"This is a string" !! 0 -- 'T'
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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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-- Two lists that are the same
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[1, 2, 3, 4, 5]
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[1..5]
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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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-- 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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- joining two lists
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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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-- indexing into a list
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[0..] !! 5 -- 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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[x*2 | x <- [1..5], x*2 > 4] -- [6, 8, 10]
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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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-- A tuple:
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("haskell", 1)
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-- accessing elements of a tuple
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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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-- 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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-- Using the function
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add 1 2 -- 3
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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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1 `add` 2 -- 3
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-- You can also define functions that have no characters! This lets
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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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(//) 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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| x < 2 = x
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| otherwise = fib (x - 1) + fib (x - 2)
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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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-- function that matches the pattern of the value.
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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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-- Pattern matching on arrays. Here `x` is the first element
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-- in the array, and `xs` is the rest of the array. We can write
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-- our own map function:
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myMap func [x] = [func x]
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myMap func (x:xs) = func x:(myMap func xs)
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-- Anonymous functions are created with a backslash followed by
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-- all the arguments.
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myMap (\x -> x + 2) [1..5] -- [3, 4, 5, 6, 7]
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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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-- array as the initial value for the accumulator.
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foldl1 (\acc x -> acc + x) [1..5] -- 15
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----------------------------------------------------
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-- 4. More functions
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----------------------------------------------------
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-- currying: if you don't pass in all the arguments to a function,
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-- it gets "curried". That means it returns a function that takes the
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-- rest of the arguments.
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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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-- 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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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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-- Haskell has another function called `$`. This changes the precedence
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-- so that everything to the left of it gets computed first and then applied
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-- to everything on the right. You can use `.` and `$` to get rid of a lot
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-- of parentheses:
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-- before
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(even (fib 7)) -- true
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-- after
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even . fib $ 7 -- true
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----------------------------------------------------
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-- 5. Type signatures
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----------------------------------------------------
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-- Haskell has a very strong type system, and everything has a type signature.
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-- Some basic types:
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5 :: Integer
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"hello" :: String
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True :: Bool
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-- Functions have types too.
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-- `not` takes a boolean and returns a boolean:
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-- not :: Bool -> Bool
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-- Here's a function that takes two arguments:
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-- add :: Integer -> Integer -> Integer
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-- When you define a value, it's good practice to write it's type above it:
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double :: Integer -> Integer
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double x = x * 2
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----------------------------------------------------
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-- 6. Control Flow and If Statements
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----------------------------------------------------
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-- if statements
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haskell = if 1 == 1 then "awesome" else "awful" -- haskell = "awesome"
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-- if statements can be on multiple lines too, indentation is important
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haskell = if 1 == 1
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then "awesome"
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else "awful"
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-- case statements: Here's how you could parse command line arguments
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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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-- Haskell doesn't have loops because it uses recursion instead.
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-- map a function over every element in an array
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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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for [0..5] $ \i -> show i
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-- we could've written that like this too:
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for [0..5] show
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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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-- Now you can use it in a function:
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say :: Color -> IO String
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say Red = putStrLn "You are Red!"
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say Blue = putStrLn "You are Blue!"
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say Green = putStrLn "You are Green!"
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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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Nothing
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Just "hello"
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Just 1
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----------------------------------------------------
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-- 8. Haskell IO
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----------------------------------------------------
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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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-- An `IO a` value is an IO action: you can chain them with do blocks
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action :: IO String
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action = do
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putStrLn "This is a line. Duh"
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input <- getLine -- this gets a line and gives it the name "input"
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input2 <- getLine
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return (input1 ++ "\n" ++ input2) -- This is the result of the whole action
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-- This didn't actually do anything. When a haskell program is executed
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-- an IO action called "main" is read and interpreted.
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main = do
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putStrLn "Our first program. How exciting!"
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result <- action -- our defined action is just like the default ones
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putStrLn result
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putStrLn "This was all, folks!"
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-- Haskell does IO through a monad because this allows it to be a purely
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-- functional language. Our `action` function had a type signature of `IO String`.
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-- In general any function that interacts with the outside world (i.e. does IO)
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-- gets marked as `IO` in it's type signature. This lets us reason about what
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-- functions are "pure" (don't interact with the outside world or modify state)
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-- and what functions aren't.
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-- This is a powerful feature, because it's easy to run pure functions concurrently
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-- so concurrency in Haskell is very easy.
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----------------------------------------------------
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-- 9. The Haskell REPL
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----------------------------------------------------
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-- Start the repl by typing `ghci`.
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-- Now you can type in Haskell code. Any new values
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-- need to be created with `let`:
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let foo = 5
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-- You can see the type of any value with `:t`:
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>:t foo
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foo :: Integer
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```
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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:
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```haskell
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qsort [] = []
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qsort (p:xs) = qsort lesser ++ [p] ++ qsort greater
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where lesser = filter (< p) xs
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greater = filter (>= p) xs
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```
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Haskell is easy to install. Get it [here](http://www.haskell.org/platform/).
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You can find a much gentler introduction from the excellent [Learn you a Haskell](http://learnyouahaskell.com/)
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