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,
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
There are two things that references can do but the previously mentioned access
References can cooperate with monads, for example IO. This opens many new
References can be added using the '&+&' operator, to create new lenses more
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
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', ...).
Data.Traversal instance on a datatype to generate a traversal of
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
Template Haskell function. .