Headline

An XML data binding technology for Language:Haskell

Discussion

XML picklers come with the Haskell XML Toolbox. They allow one to "pickle" values of an algebraic data type to Language:XML data, and "unpickle" this data back to Haskell values <cite>haskellWikiHX</cite>. Both transformations are realized by using arrows:

xunpickleDocument :: PU a -> SysConfigList -> String -> IOStateArrow s b a

That is, given a pickler

PU a

a

b

a

Headline

A functional programming idiom for composing computations

Details

With arrows, like monads, one can compose computations as defined by <cite>hughes2000generalising</cite>. Arrows are more versatile than monads. For instance, arrows can be independent of the input or take multiple inputs. <cite>haskellOrgArrows</cite> In Haskell, arrows are supported by the. type class

Arrow

Arrow a => arr :: (b -> c) -> a b c

b -> c

Arrow a => first :: a b c -> a (b, d) (c, d)

Headline

A type for the construction of terms

Illustration

Algebraic data types are typically supported by functional programming languages. For instance, Language:Haskell, Language:Scala, and Language:SML support algebraic data types. Illustrations are given here for Language:Haskell.

A data type for shapes can be defined as follows:

data Shape = Circle Float
           | Rectangle Float Float

The data contructor Circle serves for the representation of circles; the one and only constructor component serves for the radius. The data constructor Rectangle serves for the representation of rectangles; the two constructor components serve for width and height.

Constructors can be used as functions to construct terms:

myCircle :: Shape
myCircle = Circle 42

myRectangle :: Shape
myRectangle = Rectangle 77 88

In fact, constructors are functions with the types of the constructor components as argument types and the type of algebraic data type as the result type:

> :t Circle
Circle :: Float -> Shape
> :t Rectangle 
Rectangle :: Float -> Float -> Shape

Pattern matching can be applied to terms of algebraic data types:

-- Test whether the shape is a circle
isCircle (Circle _) = True
isCircle (Rectangle _ _) = False

See Contribution:haskellData for a more profound illustration of algebraic data types.

All predefined, compound types of Haskell are essentially algebraic data types. For instance, Haskell's Booleans could be defined by an algebraic data type with two data constructors as follows:

data Bool = True | False

Haskell's lists could be defined by an algebraic data type with two construtors as follows:

data List a = Nil | Cons a (List a)

The constructors Nil and Cons are meant to correspond to the empty list and ":" of Haskell's built-in lists. The Nil constructor has no constructor component, whereas the Cons constructor has two constructor components.

See also Maybe types for yet another illustration of algebraic data types.

Headline

The functional programming language Haskell

Details

101wiki hosts plenty of Haskell-based contributions. This is evident from corresponding back-links. More selective sets of Haskell-based contributions are organized in themes: Theme:Haskell data, Theme:Haskell potpourri, and Theme:Haskell genericity. Haskell is also the language of choice for a course supported by 101wiki: Course:Lambdas_in_Koblenz.

Illustration

The following expression takes the first 42 elements of the infinite list of natural numbers:

> take 42 [0..]
[0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41]

In this example, we leverage Haskell's lazy evaluation.

Headline

Mapping between XML types and types of a programming language

Synonyms

• Object-XML mapping

Details

XML data binding stands for the general notion of mediating between XML types (e.g., based on Language:XSD) and programming languages types (e.g., OO classes or algebraic data types in functional programming). Careful, one can easily run into the O/X impedance mismatch.^[2]

Headline

The extensible markup language