Cookbook

(Difference between revisions)
 Revision as of 08:43, 23 April 2009 (edit) (→PDF files)← Previous diff Revision as of 10:18, 23 April 2009 (edit) (undo) (→Strings: start to split it up. saving takes way to long an restructuring is a pain)Next diff → Line 69: Line 69: - == Strings == + * [[Cookbook/Strings|Strings]] - + - Since strings are lists of characters, you can use any available list function. + - + - === Combining strings === + - + - {| class="wikitable" + - |- + - ! Problem + - ! Solution + - ! Examples + - |- + - | combining two strings + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A%2B%2B (++)] + - | + - "foo" ++ "bar" --> "foobar" + - + - |- + - | combining many strings + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:concat concat] + - | + - concat ["foo", "bar", "baz"] --> "foobarbaz" + - + - |} + - + - === Accessing substrings === + - + - {| class="wikitable" + - |- + - ! Problem + - ! Solution + - ! Examples + - |- + - | accessing the first character + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:head head] + - | + - head "foo bar baz" --> 'f' + - + - |- + - | accessing the last character + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Alast last] + - | + - last "foo bar baz" --> 'z' + - + - |- + - | accessing the character at a given index + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3A!! (!!)] + - | + - "foo bar baz" !! 4 --> 'b' + - + - |- + - | accessing the first n characters + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take] + - | + - take 3 "foo bar baz" --> "foo" + - + - |- + - | accessing the last n characters + - | TODO + - | TODO + - |- + - | accessing the n characters starting from index m + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:drop drop], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:take take] + - | + - take 4 $drop 2 "foo bar baz" --> "o ba" + - + - |} + - + - === Splitting strings === + - + - + - {| class="wikitable" + - |- + - ! Problem + - ! Solution + - ! Examples + - |- + - | splitting a string into a list of words + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:words words] + - | words "foo bar\t baz\n" --> ["foo","bar","baz"] + - + - |- + - | splitting a string into two parts + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3AsplitAt splitAt] + - | splitAt 3 "foo bar baz" --> ("foo"," bar baz") + - + - |} + - + - === Multiline strings === + - + - "foo\ + - \bar" --> "foobar" + - + - + - === Converting between characters and values === + - + - {| class="wikitable" + - |- + - ! Problem + - ! Solution + - ! Examples + - |- + - | converting a character to a numeric value + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v:ord ord] + - | + - import Char + - ord 'A' --> 65 + - + - |- + - | converting a numeric value to a character + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3Achr chr] + - | + - import Char + - chr 99 --> 'c' + - + - |} + - + - === Reversing a string by words or characters === + - + - {| class="wikitable" + - |- + - ! Problem + - ! Solution + - ! Examples + - |- + - | reversing a string by characters + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:reverse reverse] + - | + - reverse "foo bar baz" --> "zab rab oof" + - + - |- + - | reversing a string by words + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Awords words], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:reverse reverse], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Aunwords unwords] + - | + - unwords$ reverse $words "foo bar baz" --> "baz bar foo" + - + - |- + - | reversing a string by characters by words + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Awords words], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:reverse reverse], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Aunwords unwords] + - | + - unwords$ map reverse $words "foo bar baz" --> "oof rab zab" + - + - |} + - + - === Converting case === + - + - {| class="wikitable" + - |- + - ! Problem + - ! Solution + - ! Examples + - |- + - | converting a character to upper-case + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoUpper toUpper] + - | + - import Char + - toUpper 'a' --> "A" + - + - |- + - | converting a string to upper-case + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoUpper toUpper], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map] + - | + - import Char + - map toUpper "Foo Bar" --> "FOO BAR" + - + - |- + - | converting a character to lower-case + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoLower toLower] + - | + - import Char + - toLower 'A' --> "a" + - + - |- + - | converting a string to lower-case + - | [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Char.html#v%3AtoLower toLower], [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:map map] + - | + - import Char + - map toLower "Foo Bar" --> "foo bar" + - + - |} + - + - === Interpolation === + - + - TODO + - + - === Performance === + - + - For high performance requirements (where you would typically consider + - C), consider using [http://hackage.haskell.org/packages/archive/bytestring/latest/doc/html/Data-ByteString.html Data.ByteString]. + - + - === Unicode === + - + - TODO + == Numbers == == Numbers == Revision as of 10:18, 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.

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 Numbers

Numbers in Haskell can be of the type
Int, Integer, Float, Double, or Rational
.

3.1 Rounding numbers

Problem Solution Examples
rounding round
round 3.4      --> 3
round 3.5      --> 4
getting the least number not less than x ceiling
ceiling 3.1    --> 4
getting the greatest number not greater than x floor
floor 3.5      --> 3

3.2 Taking logarithms

log 2.718281828459045  --> 1.0
logBase 10 10000       --> 4.0

3.3 Generating random numbers

import System.Random

main = do
gen <- getStdGen
let ns = randoms gen :: [Int]

4.2 Adding to or subtracting from a date

Problem Solution Examples
import Date.Time
a = fromGregorian 2009 12 31    --> 2009-12-31
b = addDays 1 a                 --> 2010-01-01
subtracting days from a date addDays
import Date.Time
a = fromGregorian 2009 12 31    --> 2009-12-31
b = addDays (-7) a              --> 2009-12-24

4.3 Difference of two dates

Problem Solution Examples
calculating the difference of two dates diffDays
import Date.Time
a = fromGregorian 2009 12 31    --> 2009-12-31
b = fromGregorian 2010 12 32    --> 2010-12-31
diffDays b a                    --> 365

4.4 CPU time

Use System.CPUTime.getCPUTime to get the CPU time in picoseconds.

You can time a computation like this

getCPUTimeDouble :: IO Double
getCPUTimeDouble = do t <- System.CPUTime.getCPUTime; return ((fromInteger t) * 1e-12)

main = do
t1 <- getCPUTimeDouble
print (fib 30)
t2 <- getCPUTimeDouble
print (t2-t1)

5 Lists

Data.List
.
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.

5.1 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
take
:
Prelude> take 10 squares
[1,4,9,16,25,36,49,64,81,100]

5.2 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 ]

6 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'.

6.1 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

TODO

TODO

TODO

6.5 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 $\seen -> seen // [(node,Black)] return found 7 Pattern matching Regular expressions are useful in some situations where the Data.List library is unwieldy. Posix style regular expressions are available in the core libraries, and a suite of other regular expression libraries are [also available], including PCRE and TRE-style regexes. Bryan O'Sullivan has written a nice introduction to using the new regex libraries. 8 Interactivity 8.1 Reading a string Strings can be read as input using getLine. Prelude> getLine Foo bar baz "Foo bar baz" 8.2 Printing a string Strings can be output in a number of different ways. Prelude> putStr "Foo" FooPrelude> As you can see, putStr does not include the newline character \n'. We can either use putStr like this: Prelude> putStr "Foo\n" Foo Or use putStrLn, which is already in the Standard Prelude Prelude> putStrLn "Foo" Foo We can also use print to print a string, including the quotation marks. Prelude> print "Foo" "Foo" 8.3 Parsing command line arguments TODO 9 Files 9.1 Reading from a file The System.IO library contains the functions needed for file IO. The program below displays the contents of the file c:\test.txt. import System.IO main = do h <- openFile "c:\\test.txt" ReadMode contents <- hGetContents h putStrLn contents hClose h The same program, with some higher-lever functions: main = do contents <- readFile "c:\\test.txt" putStrLn contents 9.2 Writing to a file The following program writes the first 100 squares to a file: -- generate a list of squares with length 'num' in string-format. numbers num = unlines$ take num $map (show . \x -> x*x) [1..] main = do writeFile "test.txt" (numbers 100) putStrLn "successfully written" This will override the old contents of the file, or create a new file if the file doesn't exist yet. If you want to append to a file, you can use appendFile . 9.3 Creating a temporary file TODO 9.4 Writing a filter Using interact, you can easily do things with stdin and stdout. A program to sum up numbers: main = interact$ 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]

TODO

10 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 $do h <- connectTo "www.google.com" (PortNumber 80) hSetBuffering h LineBuffering hPutStr h "GET / HTTP/1.1\nhost: www.google.com\n\n" contents <- hGetContents h putStrLn contents hClose h 11 XML 11.1 Libraries There are multiple libraries available. In my own (limited) experience, I could only get HXT to do everything I wanted. It does make heavy use of [Arrows]. 11.2 Parsing XML TODO 12 Databases access There are two packages you can use to connect to MySQL, PostgreSQL, Sqlite3 and ODBC databases: HDBC and Hsql 12.1 MySQL TODO 12.2 PostgreSQL TODO 12.3 SQLite Suppose you have created a 'test.db' database like this,$ 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

$./sqlite output: [SqlString "1",SqlString "This is sample data",SqlString "3.0",SqlNull] [SqlString "2",SqlString "More sample data",SqlString "6.0",SqlNull] [SqlString "3",SqlString "And a little more",SqlString "9.0",SqlNull] 13 Graphical user interfaces 13.1 wxHaskell wxHaskell is a portable and native GUI library for Haskell based on the wxWidgets Library. Hello World example: module Main where import Graphics.UI.WX main :: IO () main = start hello hello :: IO () hello = do f <- frame [text := "Hello!"] quit <- button f [text := "Quit", on command := close f] set f [layout := widget quit] This code was taken from "a quick start with wxHaskell". 13.2 Gtk2Hs Gtk2Hs is a GUI Library for Haskell based on GTK. Gtk2Hs Tutorial. Hello world example: import Graphics.UI.Gtk main :: IO () main = do initGUI w <- windowNew b <- buttonNew set b [buttonLabel := "Quit"] onClicked b$ widgetDestroy w
set w [windowTitle := "Hello", containerBorderWidth := 10]
onDestroy w mainQuit
widgetShowAll w
mainGUI

For more examples, see: Applications and libraries/Games

13.3 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.

13.4 SDL

There are some Haskell bindings to SDL at Hackage.

14 PDF files

For the following recipes you need to install HPDF.

14.1 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 $do addPage Nothing 14.2 Pages with different sizes If you pass "Nothing" to the function addPage, the default page size will be used for the size of the new page. Let’s create three pages, the last two pages with different dimensions: import Graphics.PDF main :: IO () main = do let outputFileName= "test2.pdf" let defaultPageSize = PDFRect 0 0 200 300 runPdf outputFileName standardDocInfo defaultPageSize$ do
addPage $Just$ PDFRect 0 0 100 100
addPage $Just$ PDFRect 0 0 150 150`

15 FFI

15.1 How to interface with C

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

TODO

TODO