Re: [NTG-context] METAPOST vardef returning multiple and non-numeric types as a single answer?

On 3/16/2020 10:23 AM, Taco Hoekwater wrote:

...

On 16 Mar 2020, at 10:10, Taco Hoekwater wrote:

...

On 16 Mar 2020, at 09:53, Gerben Wierda wrote:

Hola! So, the arguments passed to macros are by reference and not by value? I could have known of course, they are simple expansions, but I’d like to be sure. IfI assign to a variable inside a vardef macro and that variable is not ’save’d I’m changing the original?

Yes. (A simple test would have confirmed that)

Oops, sorry, no! I was wrong on that… it must be too early for me.

vardef Foo(text y) = scantokens(y&":=5”); enddef; Foo("x");

This would work, but that is weird. This also works, and that is why I had the erroneous memory that it would work always:

vardef Foo(expr a)(text y) = y[a] := 5; enddef; numeric foo[]; foo[1] = 6; Foo(1,foo);

or you could use global variables, of course.

Main point: Metapost is not an easy language to grasp. If you want to really understand how it works, you should study the Metafont book by DEK. > As you can clearly see, even experienced users are likely to make

mistakes. it's the mixture of delayed parsing and expansion and ... no other language like that i guess (which at the same time is its charm) passing "text y" is (i think) not that that far from the scantokens (running over already tokenized vs running over a string while tokenizing it ... but these input models (are we in scanning file input, string input, tokenlist) can be kind of confusing that said, after muy share of mp programming i would not have thought about the text y as pass by reference, also because it's only true for these pseudo arrays (maybe suffixes also work) Hans ----------------------------------------------------------------- Hans Hagen | PRAGMA ADE Ridderstraat 27 | 8061 GH Hasselt | The Netherlands tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl -----------------------------------------------------------------