dynamic-wind

[Christian Lynbech <chl@tbit.dk>]

A colleague asked me the followign about dynamic-wind:

  Will IN-GUARD and OUT-GUARD be called precisely the same number of
  times?

I think the answer is yes, but if anybody has a definitive insight
into this, it would be good.

The reference manual says:

     IN-GUARD is called, then THUNK, then OUT-GUARD.

     If, any time during the execution of THUNK, the continuation of
     the `dynamic-wind' expression is escaped non-locally, OUT-GUARD is
     called.   If the continuation of the dynamic-wind is re-entered,
     IN-GUARD is called.   Thus IN-GUARD and OUT-GUARD may be called
     any number of times.

My rationale for the yes is as follows:

If we have a non-local escape, OUT-GUARD is called once. If it is to
be called again, we need to enter the continuation again, in which
case IN-GUARD will be called again.


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Christian Lynbech          | Telebit Communications A/S                       
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Hit the philistines three times over the head with the Elisp reference manual.
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