references

Version 0.2.1.1 revision 1 uploaded 2014-08-27T10:31:58.000Z by lazac.

Package meta

Synopsis

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

Description

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.

Author

Boldizsar Nemeth

Bug reports

n/a

Category

Control

Copyright

n/a

Homepage

https://github.com/lazac/references

Maintainer

nboldi@elte.hu

Package URL

n/a

Stability

n/a

Components

• lib:references