From: anton@mips.complang.tuwien.ac.at
John Levine writes:
>In languages with nested scopes, label gotos
>can jump to an outer scope so they have to unwind some frames. Back when
>people used such things, a common use was on an error to jump out to some
>recovery code.
Pascal has that feature. Concerning error handling, jumping to an
error handler in a statically enclosing scope has fallen out of
favour, but throwing an exception to the next dynamically enclosing
exception handler is supported in a number of languages.
>Function pointers have a sort of similar problem in that they need to carry
>along pointers to all of the enclosing frames the function can see. That is
>reasonably well solved by displays, give or take the infamous Knuth man or boy
>program, 13 lines of Algol60 horror that Knuth himself got the results wrong.
Displays and static link chains are among the techniques that can be
used to implement static scoping correctly, i.e., where the man-or-boy
test produces the correct result. Knuth initially got the result
wrong, because he only had boy compilers, and the computation is too
involved to do it by hand.
The main horror in the original version is that for some of the Algol
60 syntax that is used, it is not obvious without studying the Algol
60 report what it means.
contains some
discussion, and one can find it in various other programming
languages, more or (often) less close to the original. The discussion
at and the
difference between the "proper job" version from the "crib the Common
Lisp or Scheme solution" version gives some insight.
The fact that "less close" also produces the correct result suggests
that the man-or-boy test is less discerning than Knuth probably
intended. That's a common problem with testing.
- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
Mitch Alsup,
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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