Difference between revisions of "Cookbook"

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== Haskell Cookbook ==
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* [[Cookbook/Compilers and interpreters|Haskell compilers and interpreters]]
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* [[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]]
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* [[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]]
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== Similar projects for other programming languages ==
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* [http://cl-cookbook.sourceforge.net/ Common Lisp Cookbook]
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* [http://pleac.sourceforge.net/ PLEAC]
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* [http://www.zenspider.com/Languages/Ruby/Cookbook/index.html Ruby Cookbook]
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* [http://schemecookbook.org/Cookbook/WebHome Scheme Cookbook]
  +
* [http://fssnip.net/ F# Snippets]
  +
[[Category:FAQ]]
 
[[Category:How to]]
  
[[Category:How to]]
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'''We need to start a GOOD (aka, not a [http://pleac.sourceforge.net PLEAC] clone) Haskell cookbook.
  
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This page is based on the Scheme Cookbook at
 
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http://schemecookbook.org/Cookbook/WebHome'''
  
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== Prelude ==
  
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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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== 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 y
  
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y :: Integer
  
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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 -- 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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== Strings ==
  
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=== Input ===
  
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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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=== Output ===
  
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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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=== Concatenation ===
  
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Concatenation of strings (or any other list) is done with the `++' operator.
  
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<haskell>
  
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Prelude> "foo" ++ "bar"
  
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"foobar"
  
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</haskell>
  
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=== Regular expressions ===
  
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=== Interpolation ===
  
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=== Performance ===
  
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=== Unicode (?) ===
  
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== Numbers ==
  
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Numbers in Haskell can be of the type <hask>Int, Integer, Float, Double, or Rational</hask>.
  
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=== Random numbers ===
  
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== Dates and time ==
  
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Use [http://haskell.org/ghc/docs/latest/html/libraries/base/System-Time.html#v%3AgetClockTime System.Time.getClockTime] to get a properly formatted date stamp.
  
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<haskell>
  
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Prelude> System.Time.getClockTime
  
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Wed Feb 21 20:05:35 CST 2007
  
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</haskell>
  
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== Lists ==
  
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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>.
  
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<haskell>
  
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Prelude> head [1,2,3]
  
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1
  
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Prelude> tail [1,2,3]
  
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[2,3]
  
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Prelude> length [1,2,3]
  
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3
  
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</haskell>
  
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Furthermore, Haskell supports some neat concepts.
  
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===Infinite lists===
  
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<haskell>
  
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Prelude> [1..]
  
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</haskell>
  
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The list of all squares:
  
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<haskell>
  
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square x = x*x
  
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squares = map square [1..]
  
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</haskell>
  
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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>:
  
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<haskell>
  
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Prelude> take 10 squares
  
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[1,4,9,16,25,36,49,64,81,100]
  
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</haskell>
  
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===List Comprehensions===
  
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The list of all squares can also be written in a more comprehensive way, using list comprehensions:
  
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<haskell>
  
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squares = [x*x | x <- [1..]]
  
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</haskell>
  
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== Pattern matching ==
  
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Haskell does implicit pattern matching.
  
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A good example of pattern matching is done in the fact function for finding a factorial.
  
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<haskell>
  
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fact :: Integer -> Integer
  
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fact 0 = 1
  
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fact n = n * fact (n - 1)
  
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</haskell>
  
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In this function, <hask>fact :: Integer -> Integer</hask> is the functions type definition.
  
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The next line, <hask>fact 0 = 1</hask> is a pattern match, so when the argument to the function fact is 0, the return value is 1.
  
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The 3rd and final line of this function is another pattern match, which says that, whatever number was entered as the argument, is multiplied by the factorial of that number, minus 1. Notice this function is recursive.
  
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Pattern matching in Haskell evaluates the patterns in the order they are written, so <hask>fact 0 = 1</hask> is evaluated before <hask>fact n = n * fact (n - 1)</hask>.
  
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== Files ==
  
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=== Simple IO ===
  
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Using <hask>interact :: (String -> String) -> IO ()</hask>, 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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=== Reading from files ===
  
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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 files ===
  
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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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== Data Structures ==
  
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GHC comes with some handy data-structures by default. If you want to use a Map, use [http://haskell.org/ghc/docs/latest/html/libraries/base/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'.
  
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=== Map ===
  
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=== Set ===
  
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=== Tree ===
  
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=== ByteString ===
  
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=== Arrays ===
  
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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.
  
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[http://haskell.org/ghc/docs/latest/html/libraries/base/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.
  
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The following example groups a list of numbers according to their residual after division by <hask>n</hask> in one go.
  
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<haskell>
  
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bucketByResidual :: Int -> [Int] -> Array Int [Int]
  
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bucketByResidual n xs = accumArray (\xs x -> x:xs) [] (0,n-1) [(x `mod` n, x) | x <- xs]
  
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Data.Arra.IArray> bucketByResidual 4 [x*x | x <- [1..10]]
  
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array (0,3) [(0,[100,64,36,16,4]),(1,[81,49,25,9,1]),(2,[]),(3,[])]
  
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Data.Arra.IArray> amap reverse it
  
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array (0,3) [(0,[4,16,36,64,100]),(1,[1,9,25,49,81]),(2,[]),(3,[])]
  
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</haskell>
  
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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.
  
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<haskell>
  
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editDistance :: Eq a => [a] -> [a] -> Int
  
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editDistance xs ys = table ! (m,n)
  
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where
  
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(m,n) = (length xs, length ys)
  
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x = array (1,m) (zip [1..] xs)
  
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y = array (1,n) (zip [1..] ys)
  
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table :: Array (Int,Int) Int
  
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table = array bnds [(ij, dist ij) | ij <- range bnds]
  
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bnds = ((0,0),(m,n))
  
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dist (0,j) = j
  
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dist (i,0) = i
  
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dist (i,j) = minimum [table ! (i-1,j) + 1, table ! (i,j-1) + 1,
  
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if x ! i == y ! j then table ! (i-1,j-1) else table ! (i-1,j-1)]
  
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</haskell>
  
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[http://haskell.org/ghc/docs/latest/html/libraries/base/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://haskell.org/ghc/docs/latest/html/libraries/base/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.''
  
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<haskell>
  
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import Control.Monad.State
  
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type Node = Int
  
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data Color = White | Grey | Black
 
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hasCycle :: Array Node [Node] -> Bool
  
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hasCycle graph = runState (mapDfs $ indices g) initSeen
  
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where
  
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initSeen :: DiffArray Node Color
  
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initSeen = listArray (bounds graph) (repeat White)
  
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mapDfs = fmap or . mapM dfs
  
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dfs node = get >>= \seen -> case (seen ! node) of
  
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Black -> return False
  
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Grey -> return True -- we found a cycle
  
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White -> do
  
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modify $ \seen -> seen // [(node,Grey )]
  
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found <- mapDfs (graph ! node)
  
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modify $ \seen -> seen // [(node,Black)]
  
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return found
  
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</haskell>
  
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== Network Programming ==
  
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== XML ==
  
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=== Parsing XML ===
  
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== Databases ==
  
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=== MySQL ===
  
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=== PostgreSQL ===
  
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=== SQLite ===
  
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== FFI ==
  
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=== How to interface with C===
  
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== Testing ==
  
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=== QuickCheck ===
  
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=== HUnit ===
  

Latest revision as of 18:49, 26 May 2011

Haskell Cookbook

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