The data-accessor package

[ Tags: 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


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.


*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.Accessors rather than plain get functions. For now, the package data-accessor-template provides Template Haskell functions for automated generation of Data.Acesssor.Accessors.

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 define 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.


Versions 0.0.1, 0.1.1, 0.1.2, 0.1.3, 0.1.4, 0.2,,, 0.2.1,,,,,,,, 0.2.2,,,,,,,
Dependencies array (>=0.1 && <0.4), base (>=1 && <6), containers (>=0.1 && <0.4), transformers (==0.2.*) [details]
License BSD3
Author Henning Thielemann <>, Luke Palmer <>
Maintainer Henning Thielemann <>
Category Data
Home page
Uploaded Tue Jun 1 18:18:04 UTC 2010 by HenningThielemann
Distributions Arch:, Debian:, Fedora:, LTSHaskell:, NixOS:, Stackage:, Tumbleweed:
Downloads 21763 total (1262 in the last 30 days)
Rating 0.0 (0 ratings) [clear rating]
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Status Docs uploaded by user
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