name: references version: 0.3.0.0 synopsis: Generalization of lenses, folds and traversals to handle monads and addition. description: References are data accessors that can read, write or update the accessed infromation through their context. They are first-class values, can be passed in functions, transformed, combined. References generalize lenses, folds and traversals for haskell (see: < https://hackage.haskell.org/package/lens>). . References are more general than field selectors in traditional languages. . * References are first-class values. If there is a struct in C, for example, with an `int` field `fl`, then fl can only be used as part of an expression. One can not generalize a function to take a field selector and transform the selected data or use it in other ways. . * They can have different meanings, while field accessors can only represent data-level containment. They can express uncertain containment (like field selectors of C unions), different viewpoints of the same data, and other concepts. . There are two things that references can do but the previously mentioned access methods don't. . * References can cooperate with monads, for example IO. This opens many new applications. . * References can be added using the @&+&@ operator, to create new lenses more easily. . Basic idea taken from the currently not maintained package . . An example use of the references (a logger application that spawns new threads to update a global log): . > logger = > (forever $ do > log <- logChan ^? chan&logRecord -- Extract the log record from the received log message > thrId <- forkIO (do time <- getTime > ioref&lastLogTime != time $ logDB -- Update the last logging time mutable log database > let logMsg = senderThread .- show -- Transform the thread id to a string and > $ loggingTime .= time -- update the time > $ log -- inside the log message > ioref&debugInfos !~ addLogEntry log $ logDB -- update the table of log entries > mvar !- (+1) $ count ) > mvar !- (thrId:) $ updaters -- Record the spawned thread > ) `catch` stopUpdaters updaters > where stopUpdaters updaters ThreadKilled = > mvar&traverse !| killThread $ updaters -- Kill all spawned threads before stopping . There are a bunch of predefined references for datatypes included in standard libraries. . New references can be created in several ways: . * From getter, setter and updater, using the @reference@ function. . * From getter and setter, using one of the simplified functions (@lens@, @simplePartial@, @partial@, ...). . * Using the `Data.Traversal` instance on a datatype to generate a traversal of each element. . * Using lenses from `Control.Lens` package. There are a lot of packages defining lenses, folds and traversals for various data structures, so it is very useful that all of them can simply be converted into a reference. . * Generating references for newly defined datatypes using the `makeReferences` Template Haskell function. . homepage: https://github.com/lazac/references license: BSD3 license-file: LICENSE author: Boldizsar Nemeth maintainer: nboldi@elte.hu -- copyright: category: Control build-type: Simple cabal-version: >=1.8 -- For some reason, cabal won't allow me to write this: -- -- source-repository: head -- type: git -- location: git://github.com/lazac/references.git -- source-repository: this -- type: git -- location: git://github.com/lazac/references.git -- tag: 0.2.1.2 library exposed-modules: Control.Reference , Control.Reference.TH.Records , Control.Reference.TH.Tuple , Control.Reference.Examples.TH , Control.Reference.Representation , Control.Reference.Operators , Control.Reference.Predefined , Control.Reference.Predefined.Containers , Control.Reference.Predefined.Containers.Tree , Control.Reference.TupleInstances , Control.Reference.InternalInterface build-depends: base >= 4.6 && < 5 , text == 1.1.* , array == 0.5.* , mtl == 2.2.* , transformers == 0.4.* , containers == 0.5.* , either == 4.3.* , template-haskell >= 2.8 && < 3 , instance-control == 0.1.* , directory == 1.2.* , filepath == 1.3.*