yap prolog read predicate

Question

I am experimenting with prolog, reading "Programming in prolog using the ISO standard, fith edition". I have installed yap (yet another prolog) on my ubuntu 10.10 Maverick RC system, installed using synaptic. I am running prolog from within emacs23 using prolog-mode.

The following code (from chapter five of book) does not give results as in book:

/*   FILE   history_base.pl   */                                                    
use_module(library(lists)) /* to use member/2    */                   
event(1505,['Euclid',translated,into,'Latin']).                        
event(1510,['Reuchlin-Pfefferkorn',controversy]).                      
event(1523,['Christian','II',flies,from,'Denmark']).                        

mywhen(X,Y):-event(Y,Z),member(X,Z).

% Restoring file /usr/lib/Yap/startup
YAP version Yap-5.1.3

< reading the above file into yap>

  ?- mywhen("Denmark",D).
no

which is not what the book gives!

Now adding to the file above the line (from the book):

hello1(Event):- read(Date), event(Date,Event).

Gives this error when reading the file into yap

(using "consult buffer" in the prolog menu in emacs):

  ?-  % reconsulting /tmp/prolcomp14814QRf.pl...    
     SYNTAX ERROR at /tmp/prolcomp14814QRf.pl, near line 3:   
 use_module( library( lists ) )                                                    
<    ==== HERE ====>                              
 event( 1505 , [ Euclid , translated , into
 , Latin ] ).
 % reconsulted /tmp/prolcomp14814QRf.pl in module user, 0 msec 752 bytes
yes
   ?- 

¿Any comments?

Answer 1

Perhaps you should terminate the use_module(library(lists)) statement with a . and declare it as a directive, i.e.:

:- use_module(library(lists)).

Answer 2

You have to write Denmark between single quotes instead of double quotes, i.e.:

?- mywhen('Denmark', D).

When you put Denmark between double quotes, the prolog interprets it as a list of character codes instead of an atom, but in the definition of event it is written as an atom (between single quotes).