[Simh] An obscure dialect of Pascal: MULTICS roots?

[Leo Broukhis]

Hello Adam,

Thank you for the references! May I copy your answer to my SE post, or
would you like to answer there as well under your name?

I have a binary of a game (Mankala) written in that dialect of Pascal; I
might get to decompiling it to see if the branch operator was used.

Thank you,
Leo

On Aug 26, 2017 15:30, "Adam Sampson" <ats at offog.org> wrote:

Leo Broukhis <leob at mailcom.com> writes:

> I'm specifically interested in the branch/back feature. What
> theoretical paper does it come from?

I think it's fairly likely that the designers of your 1979 dialect of
Pascal were thinking of this as a feature to support (what was then
called) "backtrack programming" for AI-like applications, rather than as
an exception handling mechanism...

There were several programming languages experimenting with approaches
to backtracking in the mid-late 70s, Prolog probably being the
best-known result ("branch" is equivalent to a Prolog predicate with
multiple rules, where failure inside one rule causes control flow to
backtrack to the next rule). Your branch/back construct would be pretty
handy if you were trying to write a tree search algorithm with pruning
(e.g. a classic board game AI), or a backtracking parser.

Here's a 1974 survey which sketches the backtracking idea (p157) and
describes some early implementations:
http://dl.acm.org/citation.cfm?id=356632

This 1977 paper gives a denotational semantics for backtracking, and is
fairly widely cited by later work:
https://link.springer.com/article/10.1007/BF00289245

(Note in particular that it calls its equivalent of "branch" the
"alternative" operator -- which might explain your "exit on ALT"
message?)

This 1979 paper takes the idea and generalises it to work with
coroutines (using Pascal for examples, although with different syntax):
http://dl.acm.org/citation.cfm?id=357062.357063

Thanks,

--
Adam Sampson <ats at offog.org>                         <http://offog.org/>
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