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== Lists ==
== Lists ==
Revision as of 10:24, 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
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.
2.1 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 ListsIn Haskell, lists are what Arrays are in most other languages. Haskell has all of the general list manipulation functions, see also
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
Furthermore, Haskell supports some neat concepts.
3.1 Infinite lists
The list of all squares:
square x = x*x squares = map square [1..]
Prelude> take 10 squares [1,4,9,16,25,36,49,64,81,100]
3.2 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 ]
4 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'.
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
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
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
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
5 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.
6.1 Reading a string
Strings can be read as input using getLine.
Prelude> getLine Foo bar baz "Foo bar baz"
6.2 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"
6.3 Parsing command line arguments
7.1 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
7.2 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"
7.3 Creating a temporary file
7.4 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]
7.5 Logging to a file
8 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
9.2 Parsing XML
10 Databases access
There are two packages you can use to connect to MySQL, PostgreSQL, Sqlite3 and ODBC databases: HDBC and Hsql
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
[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]
11 Graphical user interfaces
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".
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
See also: Applications and libraries/Games
12 PDF files
For the following recipes you need to install HPDF.
12.1 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
12.2 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
13.1 How to interface with C
Magnus has written a nice example on how to call a C function operating on a user defined type.