Re: question about vardef variables

Re: question about vardef variables

[Jeong Dal]

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Dear Damien Thiriet,

If you change ‘vardef’ to ‘def’, you will get the figure.
There is a difference between ‘vardef’ and ‘def’.
AFAIK, vardef returns the value of the last statement in the macro.
Please see  the examples in Metafun manual from page 32 ~ .
I hope that you find your answer there.

I am sorry that I am not good enough to explain the variables.

Best regards,

Dalyoung

> 2021. 1. 17. 오후 8:00, ntg-context-request@ntg.nl 작성:
> 
> \starttext
> \startMPpage
> 
> vardef Test (expr ab,cd)=
>   %numeric ab;
>   %numeric cd;
>   draw unitsquare scaled 10 xshifted ab;
>   draw unitsquare scaled 10 yshifted cd;
> enddef;
> 
> Test (50,100);
> \stopMPpage
> \stoptext


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question about vardef variables

[Damien Thiriet]

Hi,


I have some troubles with understanding metafun's macro mechanism.

Given this MWE:

\starttext
\startMPpage

vardef Test (expr ab,cd)=
   %numeric ab;
   %numeric cd;
   draw unitsquare scaled 10 xshifted ab;
   draw unitsquare scaled 10 yshifted cd;
enddef;

Test (50,100);
\stopMPpage
\stoptext

Uncommenting the two declarations makes me run intro a silent error. I was 
lucky enough yesterday to get an explicit error message with a more 
complex macro.  It made me figure out where my problems came from.

So here are my two questions:
- Why shouldn't we tell metafun if expr is a path, a pair or so on?
- Would it be possible to issue an error message each time this wrong
declaration is made?

Thanks,


Damien Thiriet 
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Re: question about vardef variables

[Taco Hoekwater]

> On 17 Jan 2021, at 11:17, Damien Thiriet <damien@thiriet.web4me.fr> wrote:
> 
> Hi,
> 
> 
> I have some troubles with understanding metafun's macro mechanism.
> 
> Given this MWE:
> 
> \starttext
> \startMPpage
> 
> vardef Test (expr ab,cd)=
>   %numeric ab;
>   %numeric cd;
>   draw unitsquare scaled 10 xshifted ab;
>   draw unitsquare scaled 10 yshifted cd;
> enddef;
> 
> Test (50,100);
> \stopMPpage
> \stoptext
> 
> Uncommenting the two declarations makes me run intro a silent error. I was 
> lucky enough yesterday to get an explicit error message with a more 
> complex macro.  It made me figure out where my problems came from.
> 
> So here are my two questions:
> - Why shouldn't we tell metafun if expr is a path, a pair or so on?

You were (attempting to) redefine the macro’s arguments, but they are read-only.

A somewhat longer answer: the type of macro arguments in this case is “expr”. 
Metapost argument types are based solely on what the internal language parser 
needs to interpret the code; the language is not strongly typed.

Also, during macro expansion, your code is replaced by:

  %numeric (READONLY TEMP VALUE 1);
  %numeric (READONLY TEMP VALUE 2);
  draw unitsquare scaled 10 xshifted (READONLY TEMP VALUE 1);
  draw unitsquare scaled 10 yshifted (READONLY TEMP VALUE 2);

Best wishes,
Taco

— 
Taco Hoekwater              E: taco@bittext.nl
genderfluid (all pronouns)







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