gentoo-overlay/dev-haskell/data-accessor/metadata.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
<pkgmetadata>
	<maintainer type="project">
		<email>haskell@gentoo.org</email>
		<name>Gentoo Haskell</name>
	</maintainer>
	<longdescription>
		In Haskell 98 the name of a record field
		is automatically also the name of a function which gets the value
		of the according field.
		E.g. if we have
		
		data Pair a b = Pair
		first :: a, second :: b
		
		then
		
		&gt; first  :: Pair a b -&gt; a
		&gt; second :: Pair a b -&gt; b
		
		However for setting or modifying a field value
		we need to use some syntactic sugar, which is often clumsy.
		
		modifyFirst :: (a -&gt; a) -&gt; (Pair a b -&gt; Pair a b)
		modifyFirst f r\@(Pair
		first=a
		) = r
		first = f a
		
		With this package you can define record field accessors
		which allow setting, getting and modifying values easily.
		The package clearly demonstrates the power of the functional approach:
		You can combine accessors of a record and sub-records,
		to make the access look like the fields of the sub-record belong to the main record.
		
		Example:
		
		&gt; *Data.Accessor.Example&gt; (first^:second^=10) (('b',7),"hallo")
		&gt; (('b',10),"hallo")
		
		You can easily manipulate record fields in a 'Control.Monad.State.State' monad,
		you can easily code 'Show' instances that use the Accessor syntax
		and you can parse binary streams into records.
		See @Data.Accessor.Example@ for demonstration of all features.
		
		It would be great if in revised Haskell versions the names of record fields
		are automatically 'Data.Accessor.Accessor's
		rather than plain @get@ functions.
		For now, the package @data-accessor-template@ provides Template Haskell functions
		for automated generation of 'Data.Acesssor.Accessor's.
		See also the other @data-accessor@ packages
		that provide an Accessor interface to other data types.
		The package @enumset@ provides accessors to bit-packed records.
		
		For similar packages see @lenses@ and @fclabel@.
		A related concept are editors
		&lt;http://conal.net/blog/posts/semantic-editor-combinators/&gt;.
		Editors only consist of a modify method
		(and @modify@ applied to a 'const' function is a @set@ function).
		This way, they can modify all function values of a function at once,
		whereas an accessor can only change a single function value,
		say, it can change @f 0 = 1@ to @f 0 = 2@.
		This way, editors can even change the type of a record or a function.
		An Arrow instance can be defined for editors,
		but for accessors only a Category instance is possible ('(.)' method).
		The reason is the @arr@ method of the @Arrow@ class,
		that conflicts with the two-way nature (set and get) of accessors.
	</longdescription>
</pkgmetadata>