Excerpts from Andrew Adams-Moran's message of Fri Mar 08 07:01:29 -0800 2013:

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This would correspond to the ability to "freeze" Haskell threads in mid-execution; no MVar blocking necessary. This is not possible with the current RTS. I don't know how hard it would be to do.

We have a way to interrupt Haskell threads mid-execution; could that machinery be used?

It is not difficult to imagine a circumstance where you would like to resume the thread after interrupting it; I don't think the current machinery can do that. Edward