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* [[Cookbook/Strings|Lists]]
   
 
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Revision as of 10:27, 23 April 2009


This article is a draft, with further revisions actively invited. Drafts are typically different than stubs in that these articles are in an active edit process. Feel free to help by expanding the article.

We need to start a Haskell centered cookbook (aka, not a PLEAC clone)

This page is based on the Scheme Cookbook at http://schemecookbook.org/Cookbook/WebHome

Contents

1 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.

2 GHCi/Hugs

2.1 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>

Prelude is the "base" library of Haskell.

To create variables at the GHCi prompt, use `let'

Prelude> let x = 5
Prelude> x
5
Prelude> let y = 3
Prelude> y
3
Prelude> x + y
8

`let' is also the way to create simple functions at the GHCi prompt

Prelude> let fact n = product [1..n]
Prelude> fact 5
120


2.2 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


3 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'.

3.1 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
qualified
to avoid name-clashing with the Prelude. See Import for more information.

3.2 Set

TODO

3.3 Tree

TODO

3.4 ByteString

TODO

3.5 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
Data.Array.IArray.Array i e
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
n
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)
    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)]


Mutable arrays like
Data.Array.IO.IOArray i e
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
Data.Array.Diff.DiffArray i e
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
    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

4 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.

5 Interactivity

5.1 Reading a string

Strings can be read as input using getLine.

Prelude> getLine
Foo bar baz
"Foo bar baz"

5.2 Printing a string

Strings can be output in a number of different ways.

Prelude> putStr "Foo"
FooPrelude>

As you can see, putStr does not include the newline character `\n'. We can either use putStr like this:

Prelude> putStr "Foo\n"
Foo

Or use putStrLn, which is already in the Standard Prelude

Prelude> putStrLn "Foo"
Foo

We can also use print to print a string, including the quotation marks.

Prelude> print "Foo"
"Foo"

5.3 Parsing command line arguments

TODO

6 Files

6.1 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

6.2 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
appendFile
.

6.3 Creating a temporary file

TODO

6.4 Writing a filter

Using interact, you can easily do things with stdin and stdout.

A program to sum up numbers:

main = interact $ show . sum . map read . lines

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]

6.5 Logging to a file

TODO

7 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 "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

8 XML

8.1 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].

8.2 Parsing XML

TODO

9 Databases access

There are two packages you can use to connect to MySQL, PostgreSQL, Sqlite3 and ODBC databases: HDBC and Hsql

9.1 MySQL

TODO

9.2 PostgreSQL

TODO

9.3 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

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]

10 Graphical user interfaces

10.1 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 ()
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]

This code was taken from "a quick start with wxHaskell".

10.2 Gtk2Hs

Gtk2Hs is a GUI Library for Haskell based on GTK. Gtk2Hs Tutorial.

Hello world example:

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

For more examples, see: Applications and libraries/Games

10.3 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 [[1]] by Jim.

See also: Applications and libraries/Games

10.4 SDL

There are some Haskell bindings to SDL at Hackage.

11 PDF files

For the following recipes you need to install HPDF.

11.1 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

11.2 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

12 FFI

12.1 How to interface with C

Magnus has written a nice example on how to call a C function operating on a user defined type.

13 Testing

13.1 QuickCheck

TODO

13.2 HUnit

TODO