Re: Union Type Object problem

[Frank Heckenbach]

Laurence Finston wrote:

> One of the most clever things about Metafont is that it can solve linear
> equations and perform certain other operations with unknown quantities.  For
> example, the following is valid MF code:
> 
> a = 2b;
> b = .5c + 1;
> c = 3;
> show a;
> --> 5 
> show b;
> --> 2.5 
> 
> Since `a', `b' and `c' aren't declared, MF considers them to be of type
> `numeric'.  Operations on variables of other types with unknown values are
> also allowed.

That's not really solving linear equations, just plain constant
arithmetic, with automatic reordering of declarations, or lazy
evaluation. Maybe it's just this example, though, and it can solve
linear equations in fact -- does this work?

a = 2b;
b = a - 3;
show a;
--> 6

> In MF, `path a' can be a `declaration' or a
> `boolean expression', e.g.,
> 
> path a; % `declaration'
> 
> if (path a) % `boolean expression'
> ...
> fi
> 
> I'm not even going to try to implement this using Bison.  Maybe it would work,
> but I think it's so likely not to, that I'm going to just use `is_path', etc.,
> for the `boolean expressions' instead.

Since the context it clearly distinct (provided `if' is a keyword),
this should be no problem at all.

Frank

-- 
Frank Heckenbach, address@hidden
http://fjf.gnu.de/
GnuPG and PGP keys: http://fjf.gnu.de/plan (7977168E)