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
> 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
) = 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.
> *Data.Accessor.Example> (first^:second^=10) (('b',7),"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
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.