[Haskell-cafe] Timing out a pure evaluation of an expression I did not write myself

Ryan Reich ryan.reich at gmail.com
Sat Nov 17 23:21:53 UTC 2018

I want to time out a pure computation.  My experience, and that described
in various previous questions here and elsewhere (the best of which is
is that this doesn't always work: for instance,

>>> timeout 1 $ evaluate $ let x = 0 : x in last x

does not time out because, apparently, the fact that the expression
evaluates in constant space (i.e. never allocates) means that it never
yields to the timeout monitor thread that would kill it.

The solution that is described in the other iterations is to embed
checkpoints in the expression that do allocate, giving the RTS a chance to
switch contexts.  However, in my application, the expression is /arbitrary/
and I do not have the freedom to inject alterations into it.  (Don't argue
this point, please.  The expression is arbitrary.)

How can I time out a tight loop like the above?  Clearly, it can be done,
because I can, say, alt-tab over to another terminal and kill the process,
which exploits the operating system's more aggressively pre-emptive
scheduling.  Is there a solution using bound threads, say 'forkOS' instead
of 'forkIO' in the implementation of 'timeout'?  Unix signals?  Some
FFI-based workaround?  Etc.  Keep in mind that notwithstanding that
comment, I don't actually want to kill the whole process, but just the one

Thanks in advance,
Ryan Reich
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