data-accessor: Utilities for accessing and manipulating fields of records
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
first :: Pair a b -> a second :: Pair a b -> b
However for setting or modifying a field value we need to use some syntactic sugar, which is often clumsy.
modifyFirst :: (a -> a) -> (Pair a b -> 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:
*Data.Accessor.Example> (first^:second^=10) (('b',7),"hallo") (('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
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.
Modules
[Index]
Flags
Automatic Flags
Name | Description | Default |
---|---|---|
category | Check whether Arrow class is split into Arrow and Category. | Enabled |
splitbase | Choose the smaller, split-up base package from version 2 on. | Enabled |
Use -f <flag> to enable a flag, or -f -<flag> to disable that flag. More info
Downloads
- data-accessor-0.2.2.1.tar.gz [browse] (Cabal source package)
- Package description (as included in the package)
Maintainer's Corner
For package maintainers and hackage trustees
Candidates
Versions [RSS] | 0.0.1, 0.1.1, 0.1.2, 0.1.3, 0.1.4, 0.2, 0.2.0.1, 0.2.0.2, 0.2.1, 0.2.1.2, 0.2.1.3, 0.2.1.4, 0.2.1.5, 0.2.1.6, 0.2.1.7, 0.2.1.8, 0.2.2, 0.2.2.1, 0.2.2.2, 0.2.2.3, 0.2.2.4, 0.2.2.5, 0.2.2.6, 0.2.2.7, 0.2.2.8, 0.2.3, 0.2.3.1 |
---|---|
Dependencies | array (>=0.1 && <0.5), base (>=1 && <5), containers (>=0.1 && <0.5), transformers (>=0.2 && <0.3) [details] |
Tested with | ghc ==6.4.1, ghc ==6.8.2, ghc ==6.10.4, ghc ==6.12.3, ghc ==7.0.1, ghc ==7.2.1, ghc ==7.4.1, jhc ==0.7.3 |
License | BSD-3-Clause |
Author | Henning Thielemann <haskell@henning-thielemann.de>, Luke Palmer <lrpalmer@gmail.com> |
Maintainer | Henning Thielemann <haskell@henning-thielemann.de> |
Category | Data |
Home page | http://www.haskell.org/haskellwiki/Record_access |
Source repo | head: darcs get http://code.haskell.org/data-accessor/core/ |
Uploaded | by HenningThielemann at 2011-12-31T10:45:45Z |
Distributions | Arch:0.2.3.1, Debian:0.2.3, Fedora:0.2.3.1, LTSHaskell:0.2.3.1, NixOS:0.2.3.1, Stackage:0.2.3.1 |
Reverse Dependencies | 62 direct, 3710 indirect [details] |
Downloads | 52544 total (172 in the last 30 days) |
Rating | (no votes yet) [estimated by Bayesian average] |
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Status | Docs uploaded by user Build status unknown [no reports yet] |