Can strict ST break referential transparency?

Ben Gamari ben at
Thu Nov 23 15:20:07 UTC 2017

Yuras Shumovich <shumovichy at> writes:

> Hello,

Sorry for the late reply; this required a bit of reflection. The
invariants surrounding the suspension of ST computations is a rather
delicate and poorly documented area.

I believe the asynchronous exception case which you point out is
precisely #13615. The solution there was, as David suggests, ensure that
no resulting thunk could be entered more than once by a very strict
blackholing protocol. Note that this isn't normal "eager" blackholing
protocol, which still might allow multiple entrancy. It's rather a more
strict variant, requiring two atomic operations.

I can't be certain that there aren't more cases like this, but I suspect
not since most asynchronous suspensions where the resulting thunk might
"leak" back into the program go through the raiseAsync codepath that was
fixed in #13615.


- ben

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