| Name : ghc-references
| |
| Version : 0.3.2.1
| Vendor : openSUSE
|
| Release : lp150.1.7
| Date : 2018-04-18 21:04:59
|
| Group : Development/Languages/Other
| Source RPM : ghc-references-0.3.2.1-lp150.1.7.src.rpm
|
| Size : 0.79 MB
| |
| Packager : https://bugs_opensuse_org
| |
| Summary : Selectors for reading and updating data
|
Description :
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,
with an `int` field `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
concepts.
There are two things that references can do but the previously mentioned access
methods don\'t.
* References can cooperate with monads, for example IO. This opens many new
applications.
* 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 datatypes using the `makeReferences`
Template Haskell function. .
|
RPM found in directory: /vol/rzm3/linux-opensuse/distribution/leap/15.0/repo/oss/x86_64 |
