haskell@gentoo.org Gentoo Haskell 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.