BSD-3-Clause licensed by Alexey Kuleshevich
Maintained by [email protected]
This version can be pinned in stack with:pvar-,1893

Module documentation for

Used by 1 package in lts-20.26(full list with versions):


Interface for a mutable variable PVar that can hold values that have Prim instance.


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Main features include:

  • Performance. There is practically no overhead when compared to operating on pure values, although there is a higher memory overhead, since PVar is backed by a MutableByteArray#
  • Atomic operations for PVars with Int values. This includes a unique function that is not available in ghc-prim out of the box:
atomicModifyIntPVar :: PrimMonad m => PVar m Int -> (Int -> (Int, a)) -> m a
  • Works in PrimMonad, therefore it is usable with ST, IO and various transformer monads.
  • Easy access to PVar contents with Storable
  • isByteArrayPinned, isMutableByteArrayPinned function that work on ghc-7.10 and ghc-8.0 as well as all the newer ones.


Changelog for pvar

  • Switch to MonadPrim type classes that was introduced in primitive-
  • Fix definition PVar m a -> PVar a s. Parametrization on a monad is not general enough for some cases and making state token s last allows to use it with forall easier.
  • Relax monad in conversion to ForeignPtr

  • Rename modifyPVar to fetchModifyPVar and modifyPVarM to fetchModifyPVarM. This is a breaking change in favor of consistency with other librarries.
  • New implementation for modifyPVar and modifyPVarM that can return some artifact.
  • Addition of modifyFetchPVar and modifyFetchPVarM
  • Addition of atomicModifyFetchIntPVar and atomicFetchModifyIntPVar

  • Addition of backwards compatible:
    • isByteArrayPinned, isMutableByteArrayPinned (that work on ghc-7.10 and ghc-8.0)
    • Primitive versions isByteArrayPinned#, isMutableByteArrayPinned#
  • Support for GHC 7.10 and GHC 8.0
  • Re-export sizeOf and alignment for easier compatibility with older primitive versions.

  • Initial release