@hackage references0.2.1.2

References can read, write or update parts of the data. 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).

There are two things that references can do but the previously mentioned access methods don't.

• References can cooperate with monads, for example IO.

• 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 records using the makeReferences Template Haskell function.