The ivar-simple package

[Tags:library, mit]

IVars are write-once (immutable) variables.

They can be read, an operation that will block until a value was written to the variable. They can be written to exactly once.

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Versions,, 0.2, 0.3, 0.3.1, 0.3.2
Change log Changelog
Dependencies base (==4.*) [details]
License MIT
Copyright (c) 2008-2015 Bertram Felgenhauer
Maintainer Bertram Felgenhauer <>
Category Concurrency
Source repository head: git clone
Uploaded Wed Oct 28 23:04:47 UTC 2015 by BertramFelgenhauer
Distributions NixOS:0.3.2
Downloads 1143 total (17 in the last 30 days)
0 []
Status Docs available [build log]
Last success reported on 2015-10-30 [all 1 reports]
Hackage Matrix CI




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Readme for ivar-simple

Readme for ivar-simple-0.3.2

What is this?

ivar-simple provides immutable, write-once variables (IVars) in the Data.IVar.Simple module.

It also provides two more experimental channel implementations built on top of IVars,

  • Data.IVar.Simple.IChan: multi-cast channels with write-once semantics.
  • Data.IVar.Simple.MIChan: channels with write semantics similar to Control.Concurrent.Chan

Comparison to data-ivar

Both data-ivar and ivar-simple provide a write-once variable. That's where the similarities end:

  • Reading an IVar with data-ivar is an IO operation. In ivar-simple it's a pure function.
  • data-ivar provides a Reader monoid, monad, etc. for reading from one of several IVars. ivar-simple has no such functionality.
  • The data-ivar implementation can, in principle, add arbitrary IO actions that are called when an IVar is written. (This detail is not exposed, however)
  • Technically, ivar-simple tries for efficiency by exploiting the existing locking structures in the RTS; in particular, reading a full IVar for a second time is as cheap as evaluating a record selector. (I have no performance numbers for this.)