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From: Kaz Kylheku <kaz@kylheku.com>
Newsgroups: comp.compilers
Subject: Re: catch and throw, was Is multi-level function return possible?
Date: Thu, 27 Mar 2014 05:44:15 +0000 (UTC)
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On 2014-03-27, Rob Warnock <rpw3@rpw3.org> wrote:
> According to Gabriel and Steele in "The Evolution of Lisp"[1],
> Lisp 1.5 [early 1960s] had (ERRSET form), which:
>
>     ...evaluates *form* in a context in which errors do not terminate
>     the program or enter the debugger. If *form* does not cause an
>     error, ERRSET returns a singleton list of the value. If execution
>     of *form* does cause an error, the ERRSET form quietly returns NIL.
> ...

Before this, Lisp-2, around 1965 or 1966, already had TRY and EXIT,
which looked like a predecessor to catch, and a close relative of this
ERRSET, while also hinting at unwind-protect also (although, in
contrast, unwind-protect doesn't serve as the ultimate termination
point of a control transfer, but only a "pit stop").

TRY went something like this:

  (TRY <var> <protected-statemenet> <cleanup-statement>)

TRY evaluates <protected-statement>. If an (EXIT <val>) does not occur
in that evaluation, then when <protected-statement> terminates, the
TRY terminates and control passes to whatever form is next.

If an (EXIT <val>) operation occurs in <protected-statement>, then
variable <var> is bound to the value <val>, and <cleanup-statement> is
executed in scope of that variable. The exit is thereby intercepted,
and control continues with whatever is next after the TRY.