references: Generalization of lenses, folds and traversals to handle monads and addition.

[ bsd3, control, library ] [ Propose Tags ]

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.

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:

Versions [faq] 0.1.0.0, 0.2.0.0, 0.2.1.0, 0.2.1.1, 0.2.1.2, 0.3.0.0, 0.3.0.1, 0.3.1.0, 0.3.1.1, 0.3.2.0, 0.3.2.1, 0.3.2.2, 0.3.3.0, 0.3.3.1
Dependencies base (>=4.6 && <5), containers (==0.5.*), either (==4.3.*), lifted-base (==0.2.*), monad-control (==0.3.*), mtl (==2.2.*), template-haskell (>=2.8 && <3), transformers (==0.4.*), transformers-base (==0.4.*) [details]
License BSD-3-Clause
Author Boldizsar Nemeth
Maintainer nboldi@elte.hu
Revised Revision 1 made by lazac at Wed Aug 27 10:31:58 UTC 2014
Category Control
Home page https://github.com/lazac/references
Uploaded by lazac at Mon Aug 25 07:26:34 UTC 2014
Distributions LTSHaskell:0.3.3.1, NixOS:0.3.3.1, Stackage:0.3.3.1
Downloads 5714 total (238 in the last 30 days)
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Status Hackage Matrix CI
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Successful builds reported [all 1 reports]

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