Re: lib(ech) and label_with : maybe a trivial question

Hi,

>I'm looking for someone would can explain me why this code does not compile.

>:- lib(ech).
...
>file try_CHR.pl, line 16: syntax error: postfix/infix operator expected
>| label_with my_plus(P,N,R) if true.

The label_with declaration that was in the old chr library is not
supported by lib(ech). [See the section `The Extended CHR Implementation'
in the CHR chapter of the Library Manual for a list of differences between
lib(chr) and lib(ech).]                            

The idea is that labelling can be done explicitly by the user. However, if
you want to duplicate the effect of chr_labeling/0 of lib(chr), you can
define it as follow:

:- constraints chr_labeling/0.

chr_labeling, my_plus(P,N,R) <=> (P=zero;N=zero), chr_labeling.

Note that you need to declare chr_labeling, as it is also not defined in
lib(ech). 

Hope this helps,

Kish