data-accessor
Version 0.2.1.2 revision 0 uploaded by HenningThielemann.
Package meta
- Synopsis
- Utilities for accessing and manipulating fields of records
- Description
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.Accessors rather than plain
get
functions. For now, the packagedata-accessor-template
provides Template Haskell functions for automated generation of Data.Acesssor.Accessors.For similar packages see
lenses
andfclabel
. A related concept are editors http://conal.net/blog/posts/semantic-editor-combinators/. Editors only consist of a modify method (andmodify
applied to a const function is aset
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 changef 0 = 1
tof 0 = 2
. This way, editors can even change the type of a record or a function. An Arrow instance can be define for editors, but for accessors only a Category instance is possible ((.) method). The reason is thearr
method of theArrow
class, that conflicts with the two-way nature (set and get) of accessors.- Author
- Henning Thielemann <haskell@henning-thielemann.de>, Luke Palmer <lrpalmer@gmail.com>
- Bug reports
- n/a
- Category
- Data
- Copyright
- n/a
- Homepage
- http://www.haskell.org/haskellwiki/Record_access
- Maintainer
- Henning Thielemann <haskell@henning-thielemann.de>
- Package URL
- http://code.haskell.org/data-accessor/
- Stability
- n/a