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dev-haskell/data-accessor: access library to record fields, a depend …
…of dev-haskell/idiii Package-Manager: portage-2.3.0
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Sergei Trofimovich
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Sep 11, 2016
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DIST data-accessor-0.2.2.7.tar.gz 9670 SHA256 3465227ad5f81059a885d354e2f3c108d550287580e6939e18350fa65e78c2ed SHA512 42f78d5be738e83e2bc64a2f38bc70748ecb741433916c4e527355f5e808a844e43b14607cfff149a5951f557c73a947469c878340738244e3778f475ccd021d WHIRLPOOL 5d9cb0eeda4d89b6e5b5e22820431b20faa9736cf539705afdd3bdc9accadec1138ff8acd33d00389755f1b803b79cfd240b13037cafdb27576478084e813ec1 |
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# Copyright 1999-2016 Gentoo Foundation | ||
# Distributed under the terms of the GNU General Public License v2 | ||
# $Id$ | ||
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EAPI=5 | ||
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# ebuild generated by hackport 0.4.6.9999 | ||
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CABAL_FEATURES="lib profile haddock hoogle hscolour" | ||
inherit haskell-cabal | ||
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DESCRIPTION="Utilities for accessing and manipulating fields of records" | ||
HOMEPAGE="http://www.haskell.org/haskellwiki/Record_access" | ||
SRC_URI="mirror://hackage/packages/archive/${PN}/${PV}/${P}.tar.gz" | ||
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LICENSE="BSD" | ||
SLOT="0/${PV}" | ||
KEYWORDS="~amd64 ~x86" | ||
IUSE="" | ||
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RDEPEND=">=dev-haskell/transformers-0.2:=[profile?] <dev-haskell/transformers-0.6:=[profile?] | ||
>=dev-lang/ghc-7.4.1:= | ||
" | ||
DEPEND="${RDEPEND} | ||
>=dev-haskell/cabal-1.6 | ||
" |
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<?xml version="1.0" encoding="UTF-8"?> | ||
<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd"> | ||
<pkgmetadata> | ||
<maintainer type="project"> | ||
<email>[email protected]</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 | ||
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data Pair a b = Pair | ||
first :: a, second :: b | ||
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then | ||
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> first :: Pair a b -> a | ||
> second :: Pair a b -> b | ||
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However for setting or modifying a field value | ||
we need to use some syntactic sugar, which is often clumsy. | ||
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modifyFirst :: (a -> a) -> (Pair a b -> Pair a b) | ||
modifyFirst f r\@(Pair | ||
first=a | ||
) = r | ||
first = f a | ||
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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. | ||
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Example: | ||
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> *Data.Accessor.Example> (first^:second^=10) (('b',7),"hallo") | ||
> (('b',10),"hallo") | ||
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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. | ||
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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. | ||
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For similar packages see @lenses@ and @fclabel@. | ||
A related concept are editors | ||
<http://conal.net/blog/posts/semantic-editor-combinators/>. | ||
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> |