[metapost] Fwd: Bug with the binary operator `point ... on ...` from Metafun

[Franck Pastor]

Début du message réexpédié :

> De : Franck Pastor <franck.pastor at me.com>
> Objet : Rép : [metapost] Bug with the binary operator `point ... on ...` from Metafun
> Date : 9 mars 2015 18:05:16 UTC+01:00
> À : Hans Hagen <pragma at wxs.nl>
> 
> 
>> Le 9 mars 2015 à 17:17, Hans Hagen <pragma at wxs.nl> a écrit :
>> 
>> On 3/9/2015 5:01 PM, Franck Pastor wrote:
>>> I’ve just encountered a very puzzling bug with the binary operator `point <len> of <path>` defined in the Metafun format.
>>> 
>>> In the Metafun manual (http://www.pragma-ade.com/general/manuals/metafun-p.pdf), p. 61, this binary operator is defined as such:
>>> 
>>> primarydef len of pat =
>>>  (arctime len of pat) of pat
>>> enddef;
>>> 
>>> So if I write `point 0 on pat`, it should return the starting point of the path <pat>, right? But n. To the contrary, it returns the end point of <pat>! Stranger yet, if I try `point (arctime 0 of pat) of pat`, it returns the starting point, as expected.
>>> 
>>> 

…

>> 
>> change > into >= in mp-tool.*
>> 
>> \startMPpage
>> 
>>   primarydef len on pat = % no outer ( ) .. somehow fails
>>       (arctime if len>=0 : len else : (arclength(pat)+len) fi of pat) of pat
>>   enddef ;

Hello everybody,

I've just noticed that the oversight above about the "on" binary operator  triggered another bug, concerning this time the "cutends" binary operator, which is defined as such in the mp-tool files:

% this cuts of a piece from both ends

tertiarydef pat cutends len =
    begingroup
    save tap ; path tap ;
    tap := pat cutbefore (point (xpart paired(len)) on pat) ;
    (tap cutafter (point -(ypart paired(len)) on tap))
    endgroup
enddef ;

More precisely, the unpleasant consequence of the "on"-bug on the "cutends" macro  it is that each time you use the cutends operator with 0 as the first coordinate of the second argument, it makes the path disappear completely! E.g.

path pat; pat = origin -- (4cm, 0);
draw pat cutends (0, 1cm) ;

makes the path simply vanish. (As a direct following from the instruction "tap := pat cutbefore (point (xpart paired(len)) on pat) ; ")

Another (much more anecdotical) consequence: if you enter

draw pat cutends (length pat, 0) ;

which should make the path vanish, then it is left unchanged. This comes this time from the instruction (tap cutafter (point -(ypart paired(len)) on tap)) in the cutends definition.

I stumbled on this phenomenon when working with one of my computers on which I had forgotten to correct the "on" bug. It took me a while to figure it out. 

So people, if you use "cutends" (or anything based on the "on" operator), do not hesitate to correct the "on"-bug in the mp-tool files!

Thanks,

Franck Pastor