TVar package:ghc-internal

Shared memory locations that support atomic memory transactions.

A MutVar# behaves like a single-element mutable array.

Modify the contents of a MutVar#, returning the previous contents x :: a and the result of applying the given function to the previous contents f x :: c. The data type c (not a newtype!) must be a record whose first field is of lifted type a :: Type and is not unpacked. For example, product types c ~ Solo a or c ~ (a, b) work well. If the record type is both monomorphic and strict in its first field, it's recommended to mark the latter {-# NOUNPACK #-} explicitly. Under the hood atomicModifyMutVar2# atomically replaces a pointer to an old x :: a with a pointer to a selector thunk fst r, where fst is a selector for the first field of the record and r is a function application thunk r = f x. atomicModifyIORef2Native from atomic-modify-general package makes an effort to reflect restrictions on c faithfully, providing a well-typed high-level wrapper.

Modify the contents of a MutVar#, returning the previous contents and the result of applying the given function to the previous contents.

Create a new TVar holding a value supplied

IO version of newTVar. This is useful for creating top-level TVars using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.

Return the current value stored in a TVar.

Return the current value stored in a TVar. This is equivalent to

readTVarIO = atomically . readTVar

but works much faster, because it doesn't perform a complete transaction, it just reads the current value of the TVar.

Write the supplied value into a TVar.

An implementation of the old atomicModifyMutVar# primop in terms of the new atomicModifyMutVar2# primop, for backwards compatibility. The type of this function is a bit bogus. It's best to think of it as having type

atomicModifyMutVar#
:: MutVar# s a
-> (a -> (a, b))
-> State# s
-> (# State# s, b #)

but there may be code that uses this with other two-field record types.