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[[Category:How to]]
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== Haskell Cookbook ==
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* [[Cookbook/Compilers and interpreters|Haskell compilers and interpreters]]
* [[Cookbook/Strings|Strings]]
 
 
* [[Cookbook/Numbers|Numbers]]
 
* [[Cookbook/Numbers|Numbers]]
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* [[Cookbook/Lists and strings|Lists and strings]]
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* [[Cookbook/Other data structures|Other data structures]]
 
* [[Cookbook/Dates And Time|Dates and time]]
 
* [[Cookbook/Dates And Time|Dates and time]]
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* [[Cookbook/Pattern matching|Pattern matching]]
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* [[Cookbook/Interactivity|Interactivity]]
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* [[Cookbook/Files|Files]]
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* [[Cookbook/Network programming|Network programming]]
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* [[Cookbook/XML|XML]]
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* [[Cookbook/Databases access|Databases access]]
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* [[Cookbook/Graphical user interfaces|Graphical user interfaces]]
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* [[Cookbook/PDF files|PDF files]]
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* [[Cookbook/FFI|FFI]]
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* [[Cookbook/Testing|Testing]]
   
{{Template:Anonymousdraft}}
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== Similar projects for other programming languages ==
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* [http://cl-cookbook.sourceforge.net/ Common Lisp Cookbook]
'''We need to start a Haskell centered cookbook (aka, not a [http://pleac.sourceforge.net/ PLEAC] clone)
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* [http://pleac.sourceforge.net/ PLEAC]
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* [http://www.zenspider.com/Languages/Ruby/Cookbook/index.html Ruby Cookbook]
This page is based on the Scheme Cookbook at
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* [http://schemecookbook.org/Cookbook/WebHome Scheme Cookbook]
http://schemecookbook.org/Cookbook/WebHome'''
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* [http://fssnip.net/ F# Snippets]
== Prelude ==
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[[Category:FAQ]]
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[[Category:How to]]
A lot of functions are defined in the "[http://www.haskell.org/hoogle/?q=Prelude Prelude]". Also, if you ever want to search for a function, based on the name, type or module, take a look at the excellent [http://www.haskell.org/hoogle/ 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.
 
/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
 
\____/\/ /_/\____/|_| Type :? for help.
 
 
Loading package base ... linking ... done.
 
Prelude>
 
 
[http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html Prelude] is the "base" library of Haskell.
 
 
To create variables at the GHCi prompt, use `let'
 
<haskell>
 
Prelude> let x = 5
 
Prelude> x
 
5
 
Prelude> let y = 3
 
Prelude> y
 
3
 
Prelude> x + y
 
8
 
</haskell>
 
 
`let' is also the way to create simple functions at the GHCi prompt
 
<haskell>
 
Prelude> let fact n = product [1..n]
 
Prelude> fact 5
 
120
 
</haskell>
 
 
 
=== Checking Types ===
 
To check the type of an expression or function, use the command `:t'
 
<haskell>
 
Prelude> :t x
 
x :: Integer
 
Prelude> :t "Hello"
 
"Hello" :: [Char]
 
</haskell>
 
Haskell has the following types defined in the [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html Standard Prelude].
 
<haskell>
 
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
 
</haskell>
 
 
== Lists ==
 
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>.
 
 
<haskell>
 
head [1,2,3] --> 1
 
tail [1,2,3] --> [2,3]
 
length [1,2,3] --> 3
 
init [1,2,3] --> [1,2]
 
last [1,2,3] --> 3
 
</haskell>
 
 
Furthermore, Haskell supports some neat concepts.
 
 
===Infinite lists===
 
<haskell>
 
Prelude> [1..]
 
</haskell>
 
 
The list of all squares:
 
<haskell>
 
square x = x*x
 
squares = map square [1..]
 
</haskell>
 
 
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>:
 
 
<haskell>
 
Prelude> take 10 squares
 
[1,4,9,16,25,36,49,64,81,100]
 
</haskell>
 
 
===List comprehensions===
 
 
The list of all squares can also be written in a more comprehensive way, using list comprehensions:
 
 
<haskell>
 
squares = [x*x | x <- [1..]]
 
</haskell>
 
 
List comprehensions allow for constraints as well:
 
 
<haskell>
 
-- multiples of 3 or 5
 
mults = [ x | x <- [1..], mod x 3 == 0 || mod x 5 == 0 ]
 
</haskell>
 
 
== Other data structures ==
 
 
GHC comes with some handy data-structures by default. If you want to use a Map, use [http://hackage.haskell.org/packages/archive/containers/latest/doc/html/Data-Map.html 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 [http://haskell.org/ghc/docs/latest/html/libraries/index.html 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 [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Map.html Data.Map] we can construct a fast map using this data-structure:
 
 
<haskell>
 
import qualified Data.Map as Map
 
 
myMap :: Map.Map String Int
 
myMap = Map.fromList [("alice", 111), ("bob", 333), ("douglas", 42)]
 
</haskell>
 
 
We can then do quick lookups:
 
<haskell>
 
bobsPhone :: Maybe Int
 
bobsPhone = Map.lookup "bob" myMap
 
</haskell>
 
 
Map is often imported <hask>qualified</hask> to avoid name-clashing with the Prelude. See [[Import]] for more information.
 
 
=== Set ===
 
 
TODO
 
 
=== Tree ===
 
 
TODO
 
 
=== ByteString ===
 
 
TODO
 
 
=== 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.
 
 
[http://hackage.haskell.org/packages/archive/array/latest/doc/html/Data-Array-IArray.html 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.
 
<haskell>
 
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,[])]
 
</haskell>
 
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.
 
<haskell>
 
editDistance :: Eq a => [a] -> [a] -> Int
 
editDistance xs ys = table ! (m,n)
 
where
 
(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)]
 
</haskell>
 
 
 
[http://hackage.haskell.org/packages/archive/array/latest/doc/html/Data-Array-MArray.html 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 [http://hackage.haskell.org/packages/archive/array/latest/doc/html/Data-Array-Diff.html 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.''
 
<haskell>
 
import Control.Monad.State
 
type Node = Int
 
data Color = White | Grey | Black
 
 
hasCycle :: Array Node [Node] -> Bool
 
hasCycle graph = runState (mapDfs $ indices g) initSeen
 
where
 
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
 
</haskell>
 
 
== 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 [http://www.serpentine.com/blog/2007/02/27/a-haskell-regular-expression-tutorial/ a nice introduction] to using the new regex libraries.
 
 
== Interactivity ==
 
 
=== Reading a string ===
 
Strings can be read as input using [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3AgetLine getLine].
 
<haskell>
 
Prelude> getLine
 
Foo bar baz
 
"Foo bar baz"
 
</haskell>
 
 
=== Printing a string ===
 
Strings can be output in a number of different ways.
 
<haskell>
 
Prelude> putStr "Foo"
 
FooPrelude>
 
</haskell>
 
As you can see, [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3AputStr putStr] does not include the newline character `\n'. We can either use putStr like this:
 
<haskell>
 
Prelude> putStr "Foo\n"
 
Foo
 
</haskell>
 
Or use [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3AputStrLn putStrLn], which is already in the Standard Prelude
 
<haskell>
 
Prelude> putStrLn "Foo"
 
Foo
 
</haskell>
 
We can also use [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Aprint print] to print a string, '''including the quotation marks.'''
 
<haskell>
 
Prelude> print "Foo"
 
"Foo"
 
</haskell>
 
 
=== Parsing command line arguments ===
 
 
TODO
 
 
== 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.
 
 
<haskell>
 
import System.IO
 
 
main = do
 
h <- openFile "c:\\test.txt" ReadMode
 
contents <- hGetContents h
 
putStrLn contents
 
hClose h
 
</haskell>
 
 
The same program, with some higher-lever functions:
 
 
<haskell>
 
main = do
 
contents <- readFile "c:\\test.txt"
 
putStrLn contents
 
</haskell>
 
 
=== Writing to a file ===
 
 
The following program writes the first 100 squares to a file:
 
<haskell>
 
-- 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"
 
</haskell>
 
 
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 ===
 
 
TODO
 
 
=== Writing a filter ===
 
Using [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:interact 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:
 
 
<haskell>
 
main = interact numberLines
 
numberLines = unlines . zipWith combine [1..] . lines
 
where combine lineNumber text = concat [show lineNumber, " ", text]
 
</haskell>
 
 
=== Logging to a file ===
 
 
TODO
 
 
== 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.
 
 
<haskell>
 
import Network;
 
import System.IO;
 
 
main = withSocketsDo $ do
 
h <- connectTo "www.google.com" (PortNumber 80)
 
hSetBuffering h LineBuffering
 
hPutStr h "GET / HTTP/1.1\nhost: www.google.com\n\n"
 
contents <- hGetContents h
 
putStrLn contents
 
hClose h
 
</haskell>
 
== 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 [[http://haskell.org/arrows/ Arrows]].
 
 
=== Parsing XML ===
 
 
TODO
 
 
== Databases access ==
 
There are two packages you can use to connect to MySQL, PostgreSQL, Sqlite3 and ODBC databases: [http://software.complete.org/software/projects/show/hdbc HDBC] and Hsql
 
 
=== MySQL ===
 
 
TODO
 
 
=== PostgreSQL ===
 
 
TODO
 
 
=== 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:
 
<haskell>
 
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
 
</haskell>
 
 
 
$ ghc --make sqlite.hs
 
 
$ ./sqlite
 
 
output:
 
 
[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:
 
 
<haskell>
 
module Main where
 
import Graphics.UI.WX
 
 
main :: IO ()
 
main
 
= start hello
 
 
hello :: IO ()
 
hello
 
= do f <- frame [text := "Hello!"]
 
quit <- button f [text := "Quit", on command := close f]
 
set f [layout := widget quit]
 
</haskell>
 
 
This code was taken from [[WxHaskell/Quick_start | "a quick start with wxHaskell"]].
 
 
=== Gtk2Hs ===
 
[http://haskell.org/gtk2hs/screenshots/ Gtk2Hs] is a GUI Library for
 
Haskell based on GTK. [http://home.telfort.nl/sp969709/gtk2hs/ Gtk2Hs Tutorial].
 
 
Hello world example:
 
 
<haskell>
 
import Graphics.UI.Gtk
 
 
main :: IO ()
 
main = do
 
initGUI
 
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
 
mainGUI
 
</haskell>
 
 
For more examples, see: [[Applications and libraries/Games]]
 
 
=== HOpenGL ===
 
[http://www.haskell.org/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
 
[[http://haskell-tetris.pbwiki.com/Main]] by Jim.
 
 
See also: [[Applications and libraries/Games]]
 
 
=== SDL ===
 
There are some Haskell bindings to [http://libsdl.org/ SDL] at [http://hackage.haskell.org/packages/archive/pkg-list.html Hackage].
 
 
== PDF files ==
 
 
For the following recipes you need to install [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HPDF HPDF].
 
 
=== Creating an empty PDF file ===
 
 
The following code creates an empty PDF file with the name "test1.pdf":
 
 
<haskell>
 
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
 
</haskell>
 
 
=== Pages with different sizes ===
 
 
If you pass "Nothing" to the function [http://hackage.haskell.org/packages/archive/HPDF/latest/doc/html/Graphics-PDF-Document.html#v%3AaddPage 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:
 
 
<haskell>
 
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
 
</haskell>
 
 
== FFI ==
 
=== How to interface with C===
 
 
Magnus has written [http://therning.org/magnus/archives/315 a nice example ] on how to call a C function operating on a user defined type.
 
 
== Testing ==
 
 
=== QuickCheck ===
 
 
TODO
 
 
=== HUnit ===
 
 
TODO
 

Latest revision as of 18:49, 26 May 2011

[edit] 1 Haskell Cookbook

[edit] 2 Similar projects for other programming languages