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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]]
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* [[Cookbook/Numbers|Numbers]]
 
* [[Cookbook/Numbers|Numbers]]
 +
* [[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]]
* [[Cookbook/Lists|Lists]]
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* [[Cookbook/Pattern matching|Pattern matching]]
* [[Cookbook/Other data structures|Other data structures]]
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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.  
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== GHCi/Hugs ==
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=== GHCi interaction ===
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To start GHCi from a command prompt, simply type `ghci'
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    $ ghci
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      ___        ___ _
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      / _ \ /\  /\/ __(_)
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    / /_\// /_/ / /  | |      GHC Interactive, version 6.6, for Haskell 98.
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    / /_\\/ __  / /___| |      http://www.haskell.org/ghc/
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    \____/\/ /_/\____/|_|      Type :? for help.
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    Loading package base ... linking ... done.
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    Prelude>
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[http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html Prelude] is the "base" library of Haskell.
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To create variables at the GHCi prompt, use `let'
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<haskell>
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Prelude> let x = 5
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Prelude> x
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5
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Prelude> let y = 3
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Prelude> y
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3
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Prelude> x + y
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8
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</haskell>
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`let' is also the way to create simple functions at the GHCi prompt
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<haskell>
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Prelude> let fact n = product [1..n]
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Prelude> fact 5
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120
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</haskell>
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=== Checking Types ===
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To check the type of an expression or function, use the command `:t'
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<haskell>
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Prelude> :t x
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x :: Integer
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Prelude> :t "Hello"
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"Hello" :: [Char]
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</haskell>
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Haskell has the following types defined in the [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html Standard Prelude].
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<haskell>
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    Int        -- bounded, word-sized integers
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    Integer    -- unbounded integers
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    Double      -- floating point values
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    Char        -- characters
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    String      -- equivalent to [Char], strings are lists of characters
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    ()          -- the unit type
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    Bool        -- booleans
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    [a]        -- lists
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    (a,b)      -- tuples / product types
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    Either a b  -- sum types
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    Maybe a    -- optional values
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</haskell>
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== Pattern matching ==
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Regular expressions are useful in some situations where the Data.List
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library is unwieldy. Posix style regular expressions are available in
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the core libraries, and a suite of other regular expression libraries
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are [also available], including PCRE and TRE-style regexes.
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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.
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== Interactivity ==
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=== Reading a string ===
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Strings can be read as input using [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3AgetLine getLine].
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<haskell>
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Prelude> getLine
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Foo bar baz
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"Foo bar baz"
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</haskell>
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=== Printing a string ===
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Strings can be output in a number of different ways.
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<haskell>
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Prelude> putStr "Foo"
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FooPrelude>
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</haskell>
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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:
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<haskell>
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Prelude> putStr "Foo\n"
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Foo
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</haskell>
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Or use [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3AputStrLn putStrLn], which is already in the Standard Prelude
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<haskell>
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Prelude> putStrLn "Foo"
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Foo
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</haskell>
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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.'''
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<haskell>
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Prelude> print "Foo"
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"Foo"
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</haskell>
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=== Parsing command line arguments ===
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TODO
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== Files ==
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=== Reading from a file ===
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The System.IO library contains the functions needed for file IO.  The program
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below displays the contents of the file c:\test.txt.
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<haskell>
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import System.IO
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main = do
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  h <- openFile "c:\\test.txt" ReadMode
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  contents <- hGetContents h
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  putStrLn contents
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  hClose h
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</haskell>
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The same program, with some higher-lever functions:
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<haskell>
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main = do
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  contents <- readFile "c:\\test.txt"
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  putStrLn contents
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</haskell>
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=== Writing to a file ===
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The following program writes the first 100 squares to a file:
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<haskell>
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-- generate a list of squares with length 'num' in string-format.
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numbers num = unlines $ take num $ map (show . \x -> x*x) [1..]
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main = do
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  writeFile "test.txt" (numbers 100)
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  putStrLn "successfully written"
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</haskell>
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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>.
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=== Creating a temporary file ===
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TODO
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=== Writing a filter  ===
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Using [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:interact interact], you can easily do things with stdin and stdout.
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A program to sum up numbers:
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<haskell>main = interact $ show . sum . map read . lines</haskell>
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A program that adds line numbers to each line:
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<haskell>
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main = interact numberLines
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numberLines = unlines . zipWith combine [1..] . lines
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where combine lineNumber text = concat [show lineNumber, " ", text]
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</haskell>
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=== Logging to a file ===
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TODO
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== Network programming ==
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The following example makes use of the Network and System.IO libraries to open
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a socket connection to Google and retrieve the Google home page.
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<haskell>
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    import Network;
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    import System.IO;
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    main = withSocketsDo $ do
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h <- connectTo "www.google.com" (PortNumber 80)
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hSetBuffering h LineBuffering
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hPutStr h "GET / HTTP/1.1\nhost: www.google.com\n\n"
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contents <- hGetContents h
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putStrLn contents
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hClose h
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</haskell>
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== XML ==
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=== Libraries ===
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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]].
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=== Parsing XML ===
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TODO
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== Databases access ==
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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
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=== MySQL ===
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TODO
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=== PostgreSQL ===
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TODO
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=== SQLite ===
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Suppose you have created a 'test.db' database like this,
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$ sqlite3 test.db  "create table t1 (t1key INTEGER PRIMARY KEY,data TEXT,num double,timeEnter DATE);"
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$ sqlite3 test.db  "insert into t1 (data,num) values ('This is sample data',3);"
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$ sqlite3 test.db  "insert into t1 (data,num) values ('More sample data',6);"
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$ sqlite3 test.db  "insert into t1 (data,num) values ('And a little more',9);"
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Using HDBC and HDBC-sqlite3 packages, you can connect and query it like this:
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<haskell>
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import Control.Monad
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import Database.HDBC
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import Database.HDBC.Sqlite3
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main = do conn <- connectSqlite3 "test.db"
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          rows <- quickQuery' conn "SELECT * from t1" []
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          forM_ rows $ \row -> putStrLn $ show row
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</haskell>
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$ ghc --make sqlite.hs
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$ ./sqlite
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output:
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[SqlString "1",SqlString "This is sample data",SqlString "3.0",SqlNull]
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[SqlString "2",SqlString "More sample data",SqlString "6.0",SqlNull]
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[SqlString "3",SqlString "And a little more",SqlString "9.0",SqlNull]
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== Graphical user interfaces ==
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=== wxHaskell ===
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[[WxHaskell|wxHaskell]] is a portable and native GUI library for Haskell based on the wxWidgets Library.
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Hello World example:
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<haskell>
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module Main where
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import Graphics.UI.WX
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main :: IO ()
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main
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  = start hello
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hello :: IO ()
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hello
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  = do f    <- frame    [text := "Hello!"]
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      quit <- button f [text := "Quit", on command := close f]
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      set f [layout := widget quit]
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</haskell>
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This code was taken from [[WxHaskell/Quick_start | "a quick start with wxHaskell"]].
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=== Gtk2Hs ===
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[http://haskell.org/gtk2hs/screenshots/ Gtk2Hs] is a GUI Library for
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Haskell based on GTK. [http://home.telfort.nl/sp969709/gtk2hs/ Gtk2Hs Tutorial].
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Hello world example:
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<haskell>
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import Graphics.UI.Gtk
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main :: IO ()
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main = do
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    initGUI
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    w <- windowNew
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    b <- buttonNew
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    set b [buttonLabel := "Quit"]
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    onClicked b $ widgetDestroy w
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    set w [windowTitle := "Hello", containerBorderWidth := 10]
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    containerAdd w b
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    onDestroy w mainQuit
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    widgetShowAll w
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    mainGUI
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</haskell>
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For more examples, see: [[Applications and libraries/Games]]
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=== HOpenGL ===
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[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.
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There is a Haskell OpenGL Tetris program at
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[[http://haskell-tetris.pbwiki.com/Main]] by Jim.
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See also: [[Applications and libraries/Games]]
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=== SDL ===
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There are some Haskell bindings to [http://libsdl.org/ SDL] at [http://hackage.haskell.org/packages/archive/pkg-list.html Hackage].
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== PDF files ==
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For the following recipes you need to install [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HPDF HPDF].
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=== Creating an empty PDF file ===
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The following code creates an empty PDF file with the name "test1.pdf":
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<haskell>
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import Graphics.PDF
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main :: IO ()
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main = do
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  let outputFileName= "test1.pdf"
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  let defaultPageSize = PDFRect 0 0 200 300
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  runPdf outputFileName standardDocInfo defaultPageSize $ do
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    addPage Nothing
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</haskell>
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=== Pages with different sizes ===
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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.
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Let’s create three pages, the last two pages with different dimensions:
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<haskell>
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import Graphics.PDF
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main :: IO ()
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main = do
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  let outputFileName= "test2.pdf"
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  let defaultPageSize = PDFRect 0 0 200 300
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  runPdf outputFileName standardDocInfo defaultPageSize $ do
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    addPage Nothing
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    addPage $ Just $ PDFRect 0 0 100 100
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    addPage $ Just $ PDFRect 0 0 150 150
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</haskell>
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== FFI ==
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=== How to interface with C===
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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.
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== Testing ==
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=== QuickCheck ===
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TODO
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=== HUnit ===
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TODO
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Latest revision as of 18:49, 26 May 2011

[edit] 1 Haskell Cookbook

[edit] 2 Similar projects for other programming languages