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

[Gerben Wierda]

Maybe a nice little explanation (educational with good examples) of ‘how to use 
pass by reference’ and ‘building and using complex data structures’ would be 
useful for people. Say, some ‘patterns’.

G  

> On 16 Mar 2020, at 10:38, Hans Hagen  wrote:
> 
> 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)

___
If your question is of interest to others as well, please add an entry to the 
Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
archive  : https://bitbucket.org/phg/context-mirror/commits/
wiki : http://contextgarden.net
___

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

[Gerben Wierda]

> On 16 Mar 2020, at 10:23, 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.

Coffee! Thanks anyway, Taco, your help is really appreciated.

> 
>  vardef Foo(text y) = 
>scantokens(y&":=5”);
>  enddef;
>  Foo("x");
> 
> This would work, but that is weird.

Which is somewhat intuitive to parse as a human. I understand it as that after 
the last statement x has the value 5.

> 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);

This was initially complete gobbledegook to me. But I now think that after the 
last statement foo[1] equals 5. Very weird/counterintuitive that y[a] := 5 
works here without the use of scantokens. The first one is less weird to me 
because scantokens actually calls the METAPOST parser on a string as if read 
from a file.

> 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.

I actually have that book and did a try small bit of METAFONT long, long ago to 
create a logo in it. Maybe that explains things better. (Unearthing the book 
for later use…)

> As you can clearly see, even experienced users are likely to make mistakes.

The main problem with all of this (ConTeXt, METAPOST, etc.) remains the 
learning curve and the lack of good and up-to-date educational materials. I.e. 
the METAPOST manual is terse, probably because it relies on people 
understanding the background that is in the METAFONT book.

And, if one is used to thinking in terms of functional programming, all that 
‘expansion’ stuff (‘replacement’  programming?) is very counterintuitive for 
me. Lua then adds a functional model to the mix (or I think it does). Very hard 
to make progress. I learned Python in a day and produced a complex program in a 
matter of weeks. This is far less easy. And not being able so far to see what 
intermediate results lea to errors adds to the slow going.

I.e., the whole chapter on vardef in the METAFONT book is 

G

> 
> 
> Best wishes,
> Taco
> 
> 
> 
> 
> ___
> If your question is of interest to others as well, please add an entry to the 
> Wiki!
> 
> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
> archive  : https://bitbucket.org/phg/context-mirror/commits/
> wiki : http://contextgarden.net
> ___

___
If your question is of interest to others as well, please add an entry to the 
Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
archive  : https://bitbucket.org/phg/context-mirror/commits/
wiki : http://contextgarden.net
___

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

[Hans Hagen]

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
-
___
If your question is of interest to others as well, please add an entry to the 
Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
archive  : https://bitbucket.org/phg/context-mirror/commits/
wiki : http://contextgarden.net
___

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

[Taco Hoekwater]

> 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.


Best wishes,
Taco




___
If your question is of interest to others as well, please add an entry to the 
Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
archive  : https://bitbucket.org/phg/context-mirror/commits/
wiki : http://contextgarden.net
___

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

[Taco Hoekwater]

> 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)

Taco


___
If your question is of interest to others as well, please add an entry to the 
Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
archive  : https://bitbucket.org/phg/context-mirror/commits/
wiki : http://contextgarden.net
___

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

[Gerben Wierda]

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?

G

> On 16 Mar 2020, at 09:31, Taco Hoekwater  wrote:
> 
> 
> 
>> On 15 Mar 2020, at 11:22, Gerben Wierda  wrote:
>> 
>> I would like a vardef macro to return the pair of a picture and a bounding 
>> box. I don't want to use the setbound operator, because as soon as I do 
>> that, I cannot access the components of the picture anymore with pathpart.
>> 
>> I found metapost vardef returning multiple values on StackExchange but that 
>> is about returning multiple values of the same numeric type. Is it possible 
>> to have vardef return a set of (picture, path) or (picture, picture) in some 
>> way? If not, I need to make two vardef macros for each result type.
> 
> Sure, using that same endgroup,begingroup trick. But since there is no 
> multi-assignment in metapost that doesn’t you help much.
> Either two separate vardefs or passing the to-be-assigned variable(s) as an 
> extra argument(s) are generally more practical.
> 
> Best wishes,
> Taco
> 
> 
> 
> ___
> If your question is of interest to others as well, please add an entry to the 
> Wiki!
> 
> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
> archive  : https://bitbucket.org/phg/context-mirror/commits/
> wiki : http://contextgarden.net
> ___

___
If your question is of interest to others as well, please add an entry to the 
Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
archive  : https://bitbucket.org/phg/context-mirror/commits/
wiki : http://contextgarden.net
___

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

[Taco Hoekwater]

> On 15 Mar 2020, at 11:22, Gerben Wierda  wrote:
> 
> I would like a vardef macro to return the pair of a picture and a bounding 
> box. I don't want to use the setbound operator, because as soon as I do that, 
> I cannot access the components of the picture anymore with pathpart.
> 
> I found metapost vardef returning multiple values on StackExchange but that 
> is about returning multiple values of the same numeric type. Is it possible 
> to have vardef return a set of (picture, path) or (picture, picture) in some 
> way? If not, I need to make two vardef macros for each result type.

Sure, using that same endgroup,begingroup trick. But since there is no 
multi-assignment in metapost that doesn’t you help much.
Either two separate vardefs or passing the to-be-assigned variable(s) as an 
extra argument(s) are generally more practical.

Best wishes,
Taco



___
If your question is of interest to others as well, please add an entry to the 
Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
archive  : https://bitbucket.org/phg/context-mirror/commits/
wiki : http://contextgarden.net
___

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

[Gerben Wierda]

I would like a vardef macro to return the pair of a picture and a bounding box. 
I don't want to use the setbound operator, because as soon as I do that, I 
cannot access the components of the picture anymore with pathpart.

I found metapost vardef returning multiple values 

 on StackExchange but that is about returning multiple values of the same 
numeric type. Is it possible to have vardef return a set of (picture, path) or 
(picture, picture) in some way? If not, I need to make two vardef macros for 
each result type.

I tried many things, amongst which:

vardef Foo( expr w, h)
% define pic
pic
end group,begingroup
% define pic
pic
endif

picture foo[]; foo = Foo( 2, 4);

___
If your question is of interest to others as well, please add an entry to the 
Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
archive  : https://bitbucket.org/phg/context-mirror/commits/
wiki : http://contextgarden.net
___