[mercury-users] Specific If Clause Goal/Expression problem

[Goncalo Jorge Coelho e Silva]

Hi again,

 I was working on code to check if a given string
contains a '/' character, and if so, produce a 
list of strings separated by the '/' char itself.
(Sort of a string__words predicate with a is_slash
predicate as it's 1st arg, instead of char__is_whitespace)
... but as a function.

 So far, so good but when I try to use the resulting
list to compare it with [], i get:

inter.m:094: In clause for predicate `inter:chooseInstruct/5':
inter.m:094:   error: the language construct =/2 should be
inter.m:094:   used as a goal, not as an expression.
inter.m:094:   If you are trying to use a goal as a boolean function,
inter.m:094:   you should write `if <goal> then yes else no' instead.
make: *** [run] Error 1


chooseInstruct(ChoppedString, ChoppedString2, ChoppedCommaList)-->
        (
         if {(checkLabel(ChoppedString)= ChoppedCommaList) \= []} then
		%
		% (!) Technically, I shouldn't have used the name
		%   'ChoppedCommaList' on this result but
		%   'ChoppedSlashList' or something...
		%
                io__write_string(" Label! ")
		%
                % pprint(ChoppedCommaList)
                %
                % doSomethingWith Label
                %
                % doSomethingWith ChoppedString2
            else
        if {ChoppedString = "put_structure"} then
		...


checkLabel(ChoppedString)= ChoppedSlashList :-
       string__words(is_slash, ChoppedString) = ChoppedSlashList.


is_slash(Char):-
        string__to_char_list("/") = [H|T],
        H = Char.



 Can't I use the following expression, or something similar
to this with the same effect in an if clause ? :

(checkLabel(ChoppedString)= ChoppedCommaList)\= []

% Compares the List resulting from applying the
% function to a string, against a empty list

Thanks,
Goncalo

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