Re: Adding if/then/else statement to GMPL

[Andrew Makhorin]

On Mon, 2020-08-24 at 14:00 +0000, Meketon, Marc wrote:
> I've always felt that GMPL needed if-then-else, for-loops, 'let'
> statements and the ability to re-solve to be a true modeling
> language.  And Andrew has always disagreed.
> 
> Many of the models that I create ultimately are 'iterative' where I
> need to take the results of one model and use it to setup another
> model.  To me, that is also modeling.  GMPL doesn't have it.
> 
> So often, I use GMPL for an initial model - it is a wonderful
> language, and I find it faster to code than alternatives.  But then
> when I 'get it right' I have to re-code it in PYOMO or PULP or write
> directly to an 'lp' file within a Python or C# or other language
> script.
> 
> Having the ability to run, adjust variables, add/take away
> constraints, re-run would be extremely useful, and make GMPL more of a
> one-stop modeling language.
> 

I agree that programming features like "goto" (as well as its structured
versions) sometimes are necessary, but in my opinion it should be
another language. Probably something like MPL (Math Programming
Language) developed by G.Dantzig in 70's is what you would like to have;
see https://dl.acm.org/doi/10.1145/800184.810495 .
The initial design of AMPL, which GNU MathProg is based on, is not
suitable to make AMPL a full-featured programming language, and in my
opinion all further additions just broke the design being incompatible
with it.

On the other hand, developing and implementing yet another (even
domain-specific) programming language is not a good idea. I think that
modeling features might be built *over* an appropriate programming
language. A good example of such approach is the GNU LilyPond (a music
engraving program; see https://lilypond.org/ ), where the domain-
specific part is built over the Scheme programming language (a dialect
of Lisp): in normal circumstances the user writes all things with
domain-specific constructions, but if something unusual is needed,
he/she may write things on a lower level directly in Scheme.


Andrew Makhorin