# data-accessor: Utilities for accessing and manipulating fields of records

[ bsd3, data, library ] [ Propose Tags ]

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. The package data-accessor-template provides Template Haskell functions for automated generation of Data.Acesssor.Accessors.