ghc-references

Selectors for reading and updating data

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<https://hackage.haskell.org/package/yall>. 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. .

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