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[[Category:How to]]
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== Haskell Cookbook ==
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{{Template:Anonymousdraft}}
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* [[Cookbook/Compilers and interpreters|Haskell compilers and interpreters]]
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'''We need to start a Haskell centered cookbook (aka, not a [http://pleac.sourceforge.net/ PLEAC] clone)
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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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`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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* [[Cookbook/Strings|Strings]]
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* [[Cookbook/Numbers|Numbers]]
* [[Cookbook/Numbers|Numbers]]
 +
* [[Cookbook/Lists and strings|Lists and strings]]
 +
* [[Cookbook/Other data structures|Other data structures]]
* [[Cookbook/Dates And Time|Dates and time]]
* [[Cookbook/Dates And Time|Dates and time]]
 +
* [[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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== Lists ==
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== Similar projects for other programming languages ==
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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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* [http://cl-cookbook.sourceforge.net/ Common Lisp Cookbook]
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* [http://pleac.sourceforge.net/ PLEAC]
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<haskell>
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* [http://www.zenspider.com/Languages/Ruby/Cookbook/index.html Ruby Cookbook]
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head [1,2,3] --> 1
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* [http://schemecookbook.org/Cookbook/WebHome Scheme Cookbook]
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tail [1,2,3] --> [2,3]
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* [http://fssnip.net/ F# Snippets]
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length [1,2,3] --> 3
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[[Category:FAQ]]
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init [1,2,3] --> [1,2]
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[[Category:How to]]
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last [1,2,3] --> 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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List comprehensions allow for constraints as well:
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<haskell>
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-- multiples of 3 or 5
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mults = [ x | x <- [1..], mod x 3 == 0 || mod x 5 == 0 ]
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</haskell>
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== Other data structures ==
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GHC comes with some handy data-structures by default. If you want to use a Map, use [http://hackage.haskell.org/packages/archive/containers/latest/doc/html/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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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.
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Using [http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Map.html Data.Map] we can construct a fast map using this data-structure:
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<haskell>
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import qualified Data.Map as Map
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myMap :: Map.Map String Int
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myMap = Map.fromList [("alice", 111), ("bob", 333), ("douglas", 42)]
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</haskell>
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We can then do quick lookups:
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<haskell>
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bobsPhone :: Maybe Int
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bobsPhone = Map.lookup "bob" myMap
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</haskell>
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Map is often imported <hask>qualified</hask> to avoid name-clashing with the Prelude. See [[Import]] for more information.
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=== Set ===
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TODO
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=== Tree ===
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TODO
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=== ByteString ===
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TODO
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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://hackage.haskell.org/packages/archive/array/latest/doc/html/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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+
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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 1 + table ! (i-1,j-1)]
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</haskell>
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[http://hackage.haskell.org/packages/archive/array/latest/doc/html/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://hackage.haskell.org/packages/archive/array/latest/doc/html/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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+
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hasCycle :: Array Node [Node] -> Bool
+
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hasCycle graph = runState (mapDfs $ indices g) initSeen
+
-
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
+
-
</haskell>
+
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+
-
== Pattern matching ==
+
-
 
+
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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 [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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+
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=== Reading a string ===
+
-
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>
+
-
Prelude> getLine
+
-
Foo bar baz
+
-
"Foo bar baz"
+
-
</haskell>
+
-
 
+
-
=== Printing a string ===
+
-
Strings can be output in a number of different ways.
+
-
<haskell>
+
-
Prelude> putStr "Foo"
+
-
FooPrelude>
+
-
</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
+
-
</haskell>
+
-
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>
+
-
Prelude> putStrLn "Foo"
+
-
Foo
+
-
</haskell>
+
-
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>
+
-
Prelude> print "Foo"
+
-
"Foo"
+
-
</haskell>
+
-
 
+
-
=== Parsing command line arguments ===
+
-
 
+
-
TODO
+
-
 
+
-
== Files ==
+
-
 
+
-
=== 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.
+
-
 
+
-
<haskell>
+
-
import System.IO
+
-
 
+
-
main = do
+
-
h <- openFile "c:\\test.txt" ReadMode
+
-
contents <- hGetContents h
+
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putStrLn contents
+
-
hClose h
+
-
</haskell>
+
-
 
+
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The same program, with some higher-lever functions:
+
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+
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<haskell>
+
-
main = do
+
-
contents <- readFile "c:\\test.txt"
+
-
putStrLn contents
+
-
</haskell>
+
-
 
+
-
=== Writing to a file ===
+
-
 
+
-
The following program writes the first 100 squares to a file:
+
-
<haskell>
+
-
-- 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"
+
-
</haskell>
+
-
 
+
-
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>.
+
-
 
+
-
=== Creating a temporary file ===
+
-
 
+
-
TODO
+
-
 
+
-
=== Writing a filter ===
+
-
Using [http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:interact interact], you can easily do things with stdin and stdout.
+
-
 
+
-
A program to sum up numbers:
+
-
 
+
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<haskell>main = interact $ show . sum . map read . lines</haskell>
+
-
 
+
-
A program that adds line numbers to each line:
+
-
 
+
-
<haskell>
+
-
main = interact numberLines
+
-
numberLines = unlines . zipWith combine [1..] . lines
+
-
where combine lineNumber text = concat [show lineNumber, " ", text]
+
-
</haskell>
+
-
 
+
-
=== Logging to a file ===
+
-
 
+
-
TODO
+
-
 
+
-
== 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.
+
-
 
+
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<haskell>
+
-
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)
+
-
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
+
-
hClose h
+
-
</haskell>
+
-
== XML ==
+
-
=== 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 [[http://haskell.org/arrows/ Arrows]].
+
-
 
+
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=== Parsing XML ===
+
-
 
+
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TODO
+
-
 
+
-
== Databases access ==
+
-
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 ===
+
-
 
+
-
TODO
+
-
 
+
-
=== 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:
+
-
<haskell>
+
-
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
+
-
</haskell>
+
-
 
+
-
 
+
-
$ 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]
+
-
 
+
-
== Graphical user interfaces ==
+
-
 
+
-
=== wxHaskell ===
+
-
[[WxHaskell|wxHaskell]] is a portable and native GUI library for Haskell based on the wxWidgets Library.
+
-
 
+
-
Hello World example:
+
-
 
+
-
<haskell>
+
-
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]
+
-
</haskell>
+
-
 
+
-
This code was taken from [[WxHaskell/Quick_start | "a quick start with wxHaskell"]].
+
-
 
+
-
=== Gtk2Hs ===
+
-
[http://haskell.org/gtk2hs/screenshots/ Gtk2Hs] is a GUI Library for
+
-
Haskell based on GTK. [http://home.telfort.nl/sp969709/gtk2hs/ Gtk2Hs Tutorial].
+
-
 
+
-
Hello world example:
+
-
 
+
-
<haskell>
+
-
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
+
-
</haskell>
+
-
 
+
-
For more examples, see: [[Applications and libraries/Games]]
+
-
 
+
-
=== HOpenGL ===
+
-
[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.
+
-
There is a Haskell OpenGL Tetris program at
+
-
[[http://haskell-tetris.pbwiki.com/Main]] by Jim.
+
-
 
+
-
See also: [[Applications and libraries/Games]]
+
-
 
+
-
=== SDL ===
+
-
There are some Haskell bindings to [http://libsdl.org/ SDL] at [http://hackage.haskell.org/packages/archive/pkg-list.html Hackage].
+
-
 
+
-
== PDF files ==
+
-
 
+
-
For the following recipes you need to install [http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HPDF HPDF].
+
-
 
+
-
=== Creating an empty PDF file ===
+
-
 
+
-
The following code creates an empty PDF file with the name "test1.pdf":
+
-
 
+
-
<haskell>
+
-
import Graphics.PDF
+
-
 
+
-
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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-
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runPdf outputFileName standardDocInfo defaultPageSize $ do
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-
addPage Nothing
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-
</haskell>
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-
 
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=== Pages with different sizes ===
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-
 
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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:
+
-
 
+
-
<haskell>
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-
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
+
-
</haskell>
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-
 
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-
== FFI ==
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=== How to interface with C===
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-
 
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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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+
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== Testing ==
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-
 
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=== QuickCheck ===
+
-
 
+
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TODO
+
-
 
+
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=== HUnit ===
+
-
 
+
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TODO
+

Current revision

1 Haskell Cookbook

2 Similar projects for other programming languages

Retrieved from "http://www.haskell.org/haskellwiki/Cookbook"

Categories: FAQ | How to