An experience for understanding module transparency finally

General
1

I feel I have understood finally about module transparency, only one hour ago by practicing as following log. I am glad if it clarifies some confusion on module transparency for some prologers like me.

In the log below,

  • zdd/1 is a meta predicate defined in module zmod.

  • udg_path_count/3 is defined in module frtvec, which is loaded in module zmod.

?- context_module(X).
X = zmod.

?- zdd udg_path_count(a-b, [a-b,b-c,a-c], C).
 ERROR: Unknown procedure: zmod:udg_path_count/3
<snip>

?- frtvec:(zdd udg_path_count(a-b, [a-b,b-c,a-c], C)).
 C = 2.		% as expected.

?- context_module(M).
 M = zmod.

?- module(frtvec).  %  change to module `frtvec`
 true.

?- context_module(M).
 M = frtvec.

?- zdd udg_path_count(a-b, [a-b,b-c,a-c], C).
C = 2.		%  as expected without module prefix 'frtvec'.

What was that confused me on module transparency ? It is
a “secret”, which should be cleared only through practice.

EDIT Sorry. I forgot to export udg_path_count/3. With exporting the predicate, every query in the log turned out “YES” without error.