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From: jinx@zurich.ai.mit.edu (Guillermo J. Rozas)
Newsgroups: comp.lang.scheme
Subject: Re: Ports & GC
Message-ID: <JINX.91Apr29152516@chamarti.ai.mit.edu>
Date: 29 Apr 91 19:25:16 GMT
References: <480@data.UUCP> <9104291751.AA10911@cymbal.reasoning.com.>
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In-reply-to: gyro@cymbal.reasoning.COM's message of 29 Apr 91 17:51:47 GMT

In article <9104291751.AA10911@cymbal.reasoning.com.> gyro@cymbal.reasoning.COM (Scott Layson Burson) writes:

   Somebody on this list recently alluded to "the well-known difficulty of
   defining UNWIND-PROTECT in the presence of firstclass continuations".

   I have long thought that what UNWIND-PROTECT ought to mean is that the
   cleanup actions are performed when the continuation in which they were
   established becomes garbage.

That is not clear at all.
The standard example of UNWIND-PROTECT (from CLtLII) is

(unwind-protect
 (progn (start-motor)
	(drill-hole))
 (stop-motor))

I don't think that waiting until the continuation is garbage to stop
the motor is what the user intended.

Sometimes you want it to happen on any exit from the protected action.
Sometimes you want it to happen on "final" exit, whatever that means.

   As Ken points out, this really isn't all that hard to implement.

The issue is not implementation, but what the correct behavior should
be.