references: Generalization of lenses, folds and traversals to handle monads and addition.
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
From getter and setter, using one of the simplified functions (
Data.Traversalinstance on a datatype to generate a traversal of each element.
Using lenses from
Control.Lenspackage. 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
makeReferencesTemplate Haskell function.
|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.*), lens (==4.2.*), lifted-base (==0.2.*), monad-control (==0.3.*), mtl (==2.2.*), template-haskell (>=2.8 && <3), transformers (==0.4.*), transformers-base (==0.4.*) [details]|
|Revised||Revision 1 made by lazac at Thu Aug 14 12:41:51 UTC 2014|
|Uploaded||by lazac at Thu Aug 14 12:27:01 UTC 2014|
|Distributions||LTSHaskell:0.3.3.1, NixOS:0.3.3.1, Stackage:0.3.3.1|
|Downloads||5886 total (141 in the last 30 days)|
|Rating||(no votes yet) [estimated by rule of succession]|
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