Create an initially empty StrictSVar

NOTE: Since readSVarSTM allows to read the StrictSVar even when it is empty, we need an initial value of a even though the StrictSVar starts out empty. However, we are NOT strict in this value, to allow it to be error.

Read the possibly-stale value of the SVar

Will return the current value of the SVar if it non-empty, or the last known value otherwise.

Swap value of a StrictSVar

NOTE: Since swapping the value can't leave the StrictSVar empty, we could check the invariant first and only then swap. We nonetheless swap first and check the invariant after to keep the semantics the same with putSVar, otherwise it will be difficult to understand when a StrictSVar is updated and when it is not.

Strict SVar (modelled using a lazy TMVar under the hood)

The StrictSVar API is slightly stronger than the usual SVar one, as we offer a primitive to read the value of the SVar even if it is empty (in which case we will return the oldest known stale one). See readSVarSTM.

There is a weaker invariant for a StrictSVar than for a StrictTVar: although all functions that modify the StrictSVar check the invariant, we do not guarantee that the value inside the StrictSVar always satisfies the invariant. Instead, we do guarantee that if the StrictSVar is updated with a value that does not satisfy the invariant, an exception is thrown. The reason for this weaker guarantee is that leaving an SVar empty can lead to very hard to debug "blocked indefinitely" problems.

This is also the reason we do not offer support for an invariant in StrictTMVar: if we throw an exception from an STM transaction, the STM transaction is not executed, and so we would not even be able to provide the weaker guarantee that we provide for StrictSVar.