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.
References are more potent than lenses, folds and traversals:
* 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):
```haskell
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 number 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.