Re: [Simh] An obscure dialect of Pascal: MULTICS roots?
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" wrote:
Leo Broukhis 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
Re: [Simh] An obscure dialect of Pascal: MULTICS roots?
Leo Broukhis 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
[Simh] An obscure dialect of Pascal: MULTICS roots?
While exploring implementations of Pascal on BESM-6, I've encountered two peculiar language extensions about which I've asked on SE: BRANCH/BACK: an analog of try/catch: https://retrocomputing.stackexchange.com/q/3150/4025 Structural labels as a way to implement generalized break/continue: https://retrocomputing.stackexchange.com/q/3296/4025 Both posts are quite verbose so I'm not quoting them here. Regarding the first feature, I was told in a private conversation that the message "exit on ALT" for an uncaught "exception" - the equivalent of C++ terminate() - might have its roots in MULTICS. Does any of the above ring any bells? I'm specifically interested in the branch/back feature. What theoretical paper does it come from? Thanks, Leo ___ Simh mailing list Simh@trailing-edge.com http://mailman.trailing-edge.com/mailman/listinfo/simh
