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[[Category:How to]]
== Haskell Cookbook ==
* [[Cookbook/Compilers and interpreters|Haskell compilers and interpreters]]
'''We need to start a Haskell centered cookbook (aka, not a [ PLEAC] clone)
This page is based on the Scheme Cookbook at'''
== Prelude ==
A lot of functions are defined in the "[ Prelude]". Also, if you ever want to search for a function, based on the name, type or module, take a look at the excellent [ Hoogle]. This is for a lot of people a must-have while debugging and writing Haskell programs.
== GHCi/Hugs ==
=== GHCi interaction ===
To start GHCi from a command prompt, simply type `ghci'
$ ghci
___ ___ _
/ _ \ /\ /\/ __(_)
/ /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98.
/ /_\\/ __ / /___| |
\____/\/ /_/\____/|_| Type :? for help.
Loading package base ... linking ... done.
[ Prelude] is the "base" library of Haskell.
To create variables at the GHCi prompt, use `let'
Prelude> let x = 5
Prelude> x
Prelude> let y = 3
Prelude> y
Prelude> x + y
`let' is also the way to create simple functions at the GHCi prompt
Prelude> let fact n = product [1..n]
Prelude> fact 5
=== Checking Types ===
To check the type of an expression or function, use the command `:t'
Prelude> :t x
x :: Integer
Prelude> :t "Hello"
"Hello" :: [Char]
Haskell has the following types defined in the [ Standard Prelude].
Int -- bounded, word-sized integers
Integer -- unbounded integers
Double -- floating point values
Char -- characters
String -- equivalent to [Char], strings are lists of characters
() -- the unit type
Bool -- booleans
[a] -- lists
(a,b) -- tuples / product types
Either a b -- sum types
Maybe a -- optional values
* [[Cookbook/Strings|Strings]]
* [[Cookbook/Numbers|Numbers]]
* [[Cookbook/Numbers|Numbers]]
* [[Cookbook/Lists and strings|Lists and strings]]
* [[Cookbook/Other data structures|Other data structures]]
* [[Cookbook/Dates And Time|Dates and time]]
* [[Cookbook/Pattern matching|Pattern matching]]
* [[Cookbook/Interactivity|Interactivity]]
* [[Cookbook/Files|Files]]
* [[Cookbook/Network programming|Network programming]]
* [[Cookbook/XML|XML]]
* [[Cookbook/Databases access|Databases access]]
* [[Cookbook/Graphical user interfaces|Graphical user interfaces]]
* [[Cookbook/PDF files|PDF files]]
* [[Cookbook/FFI|FFI]]
* [[Cookbook/Testing|Testing]]
== Similar projects for other programming languages ==
* [ Common Lisp Cookbook]
== Lists ==
* [ PLEAC]
In Haskell, lists are what Arrays are in most other languages. Haskell has all of the general list manipulation functions, see also <hask>Data.List</hask>.
* [ Ruby Cookbook]
* [ Scheme Cookbook]
* [ F# Snippets]
head [1,2,3] --> 1
tail [1,2,3] --> [2,3]
[[Category:How to]]
length [1,2,3] --> 3
init [1,2,3] --> [1,2]
last [1,2,3] --> 3
Furthermore, Haskell supports some neat concepts.
===Infinite lists===
Prelude> [1..]
The list of all squares:
square x = x*x
squares = map square [1..]
But in the end, you probably don't want to use infinite lists, but make them finite. You can do this with <hask>take</hask>:
Prelude> take 10 squares
===List comprehensions===
The list of all squares can also be written in a more comprehensive way, using list comprehensions:
squares = [x*x | x <- [1..]]
List comprehensions allow for constraints as well:
-- multiples of 3 or 5
mults = [ x | x <- [1..], mod x 3 == 0 || mod x 5 == 0 ]
== Other data structures ==
GHC comes with some handy data-structures by default. If you want to use a Map, use [ Data.Map]. For sets, you can use Data.Set. A good way to find efficient data-structures is to take a look at the hierarchical libraries, see [ Haskell Hierarchical Libraries] and scroll down to 'Data'.
=== Map ===
A naive implementation of a map would be using a list of tuples in the form of (key, value). This is used a lot, but has the big disadvantage that most operations take O(n) time.
Using [ Data.Map] we can construct a fast map using this data-structure:
import qualified Data.Map as Map
myMap :: Map.Map String Int
myMap = Map.fromList [("alice", 111), ("bob", 333), ("douglas", 42)]
We can then do quick lookups:
bobsPhone :: Maybe Int
bobsPhone = Map.lookup "bob" myMap
Map is often imported <hask>qualified</hask> to avoid name-clashing with the Prelude. See [[Import]] for more information.
=== Set ===
=== Tree ===
=== ByteString ===
=== Arrays ===
Arrays are generally eschewed in Haskell. However, they are useful if you desperately need constant lookup or update or if you have huge amounts of raw data.
[ Immutable arrays] like <hask>Data.Array.IArray.Array i e</hask> offer lookup in constant time but they get copied when you update an element. Use them if they can be filled in one go.
The following example groups a list of numbers according to their residual after division by <hask>n</hask> in one go.
bucketByResidual :: Int -> [Int] -> Array Int [Int]
bucketByResidual n xs = accumArray (\xs x -> x:xs) [] (0,n-1) [(x `mod` n, x) | x <- xs]
Data.Arra.IArray> bucketByResidual 4 [x*x | x <- [1..10]]
array (0,3) [(0,[100,64,36,16,4]),(1,[81,49,25,9,1]),(2,[]),(3,[])]
Data.Arra.IArray> amap reverse it
array (0,3) [(0,[4,16,36,64,100]),(1,[1,9,25,49,81]),(2,[]),(3,[])]
Note that the array can fill itself up in a circular fashion. Useful for dynamic programming. Here is the [[Edit distance]] between two strings without array updates.
editDistance :: Eq a => [a] -> [a] -> Int
editDistance xs ys = table ! (m,n)
(m,n) = (length xs, length ys)
x = array (1,m) (zip [1..] xs)
y = array (1,n) (zip [1..] ys)
table :: Array (Int,Int) Int
table = array bnds [(ij, dist ij) | ij <- range bnds]
bnds = ((0,0),(m,n))
dist (0,j) = j
dist (i,0) = i
dist (i,j) = minimum [table ! (i-1,j) + 1, table ! (i,j-1) + 1,
if x ! i == y ! j then table ! (i-1,j-1) else 1 + table ! (i-1,j-1)]
[ Mutable arrays] like <hask>Data.Array.IO.IOArray i e</hask> are updated in place, but they have to live in the IO-monad or the ST-monad in order to not destroy referential transparency. There are also [ diff arrays] like <hask>Data.Array.Diff.DiffArray i e</hask> that look like immutable arrays but do updates in place if used in a single threaded way. Here is depth first search with diff arrays that checks whether a directed graph contains a cycle. ''Note: this example really belongs to Map or Set.''
import Control.Monad.State
type Node = Int
data Color = White | Grey | Black
hasCycle :: Array Node [Node] -> Bool
hasCycle graph = runState (mapDfs $ indices g) initSeen
initSeen :: DiffArray Node Color
initSeen = listArray (bounds graph) (repeat White)
mapDfs = fmap or . mapM dfs
dfs node = get >>= \seen -> case (seen ! node) of
Black -> return False
Grey -> return True -- we found a cycle
White -> do
modify $ \seen -> seen // [(node,Grey )]
found <- mapDfs (graph ! node)
modify $ \seen -> seen // [(node,Black)]
return found
== Pattern matching ==
Regular expressions are useful in some situations where the Data.List
library is unwieldy. Posix style regular expressions are available in
the core libraries, and a suite of other regular expression libraries
are [also available], including PCRE and TRE-style regexes.
Bryan O'Sullivan has written [ a nice introduction] to using the new regex libraries.
== Interactivity ==
=== Reading a string ===
Strings can be read as input using [ getLine].
Prelude> getLine
Foo bar baz
"Foo bar baz"
=== Printing a string ===
Strings can be output in a number of different ways.
Prelude> putStr "Foo"
As you can see, [ putStr] does not include the newline character `\n'. We can either use putStr like this:
Prelude> putStr "Foo\n"
Or use [ putStrLn], which is already in the Standard Prelude
Prelude> putStrLn "Foo"
We can also use [ print] to print a string, '''including the quotation marks.'''
Prelude> print "Foo"
=== Parsing command line arguments ===
== Files ==
=== Reading from a file ===
The System.IO library contains the functions needed for file IO. The program
below displays the contents of the file c:\test.txt.
import System.IO
main = do
h <- openFile "c:\\test.txt" ReadMode
contents <- hGetContents h
putStrLn contents
hClose h
The same program, with some higher-lever functions:
main = do
contents <- readFile "c:\\test.txt"
putStrLn contents
=== Writing to a file ===
The following program writes the first 100 squares to a file:
-- generate a list of squares with length 'num' in string-format.
numbers num = unlines $ take num $ map (show . \x -> x*x) [1..]
main = do
writeFile "test.txt" (numbers 100)
putStrLn "successfully written"
This will override the old contents of the file, or create a new file if the file doesn't exist yet. If you want to append to a file, you can use <hask>appendFile</hask>.
=== Creating a temporary file ===
=== Writing a filter ===
Using [ interact], you can easily do things with stdin and stdout.
A program to sum up numbers:
<haskell>main = interact $ show . sum . map read . lines</haskell>
A program that adds line numbers to each line:
main = interact numberLines
numberLines = unlines . zipWith combine [1..] . lines
where combine lineNumber text = concat [show lineNumber, " ", text]
=== Logging to a file ===
== Network programming ==
The following example makes use of the Network and System.IO libraries to open
a socket connection to Google and retrieve the Google home page.
import Network;
import System.IO;
main = withSocketsDo $ do
h <- connectTo "" (PortNumber 80)
hSetBuffering h LineBuffering
hPutStr h "GET / HTTP/1.1\nhost:\n\n"
contents <- hGetContents h
putStrLn contents
hClose h
== XML ==
=== Libraries ===
There are multiple libraries available. In my own (limited) experience, I could only get [[HXT]] to do everything I wanted. It does make heavy use of [[ Arrows]].
=== Parsing XML ===
== Databases access ==
There are two packages you can use to connect to MySQL, PostgreSQL, Sqlite3 and ODBC databases: [ HDBC] and Hsql
=== MySQL ===
=== PostgreSQL ===
=== SQLite ===
Suppose you have created a 'test.db' database like this,
$ sqlite3 test.db "create table t1 (t1key INTEGER PRIMARY KEY,data TEXT,num double,timeEnter DATE);"
$ sqlite3 test.db "insert into t1 (data,num) values ('This is sample data',3);"
$ sqlite3 test.db "insert into t1 (data,num) values ('More sample data',6);"
$ sqlite3 test.db "insert into t1 (data,num) values ('And a little more',9);"
Using HDBC and HDBC-sqlite3 packages, you can connect and query it like this:
import Control.Monad
import Database.HDBC
import Database.HDBC.Sqlite3
main = do conn <- connectSqlite3 "test.db"
rows <- quickQuery' conn "SELECT * from t1" []
forM_ rows $ \row -> putStrLn $ show row
$ ghc --make sqlite.hs
$ ./sqlite
[SqlString "1",SqlString "This is sample data",SqlString "3.0",SqlNull]
[SqlString "2",SqlString "More sample data",SqlString "6.0",SqlNull]
[SqlString "3",SqlString "And a little more",SqlString "9.0",SqlNull]
== Graphical user interfaces ==
=== wxHaskell ===
[[WxHaskell|wxHaskell]] is a portable and native GUI library for Haskell based on the wxWidgets Library.
Hello World example:
module Main where
import Graphics.UI.WX
main :: IO ()
= start hello
hello :: IO ()
= do f <- frame [text := "Hello!"]
quit <- button f [text := "Quit", on command := close f]
set f [layout := widget quit]
This code was taken from [[WxHaskell/Quick_start | "a quick start with wxHaskell"]].
=== Gtk2Hs ===
[ Gtk2Hs] is a GUI Library for
Haskell based on GTK. [ Gtk2Hs Tutorial].
Hello world example:
import Graphics.UI.Gtk
main :: IO ()
main = do
w <- windowNew
b <- buttonNew
set b [buttonLabel := "Quit"]
onClicked b $ widgetDestroy w
set w [windowTitle := "Hello", containerBorderWidth := 10]
containerAdd w b
onDestroy w mainQuit
widgetShowAll w
For more examples, see: [[Applications and libraries/Games]]
=== HOpenGL ===
[ HOpenGL] is a Haskell binding for the OpenGL graphics API (GL 1.2.1 / GLU 1.3) and the portable OpenGL utility toolkit GLUT.
There is a Haskell OpenGL Tetris program at
[[]] by Jim.
See also: [[Applications and libraries/Games]]
=== SDL ===
There are some Haskell bindings to [ SDL] at [ Hackage].
== PDF files ==
For the following recipes you need to install [ HPDF].
=== Creating an empty PDF file ===
The following code creates an empty PDF file with the name "test1.pdf":
import Graphics.PDF
main :: IO ()
main = do
let outputFileName= "test1.pdf"
let defaultPageSize = PDFRect 0 0 200 300
runPdf outputFileName standardDocInfo defaultPageSize $ do
addPage Nothing
=== Pages with different sizes ===
If you pass "Nothing" to the function [ addPage], the default page size will be used for the size of the new page.
Let’s create three pages, the last two pages with different dimensions:
import Graphics.PDF
main :: IO ()
main = do
let outputFileName= "test2.pdf"
let defaultPageSize = PDFRect 0 0 200 300
runPdf outputFileName standardDocInfo defaultPageSize $ do
addPage Nothing
addPage $ Just $ PDFRect 0 0 100 100
addPage $ Just $ PDFRect 0 0 150 150
== FFI ==
=== How to interface with C===
Magnus has written [ a nice example ] on how to call a C function operating on a user defined type.
== Testing ==
=== QuickCheck ===
=== HUnit ===

Latest revision as of 18:49, 26 May 2011

[edit] 1 Haskell Cookbook

[edit] 2 Similar projects for other programming languages