In order to define new predicates in the top level you need to furst consult the pseudo-file user to specify the terminal. Press ctrl-D when finished. http://www.gprolog.org/manual/html_node/gprolog007.html
For example, you can see that I can reproduce your error, but then fix things. Note that the ctrl-D entered before the "user compiled..." line is not visible, but that is what exits the pseudo-file and causes what has been typed to be compiled. $ gprolog GNU Prolog 1.4.5 (32 bits) Compiled Dec 3 2020, 00:37:14 with gcc By Daniel Diaz Copyright (C) 1999-2020 Daniel Diaz | ?- p(a). uncaught exception: error(existence_error(procedure,p/1),top_level/0) | ?- [user]. compiling user for byte code... p(a). user compiled, 1 lines read - 168 bytes written, 20696 ms (4 ms) yes | ?- p(X). X = a yes | ?- This is great for learning, but more typically you would enter your predicates in a file and then consult that file in the top-level, or specify it on the gprolog command line, or even just compile it with gplc. ________________________________ From: Users-prolog on behalf of William Clodius via Users list for the GNU implementation of Prolog Sent: Monday, May 3, 2021 6:58 PM To: users-prolog@gnu.org Subject: uncaught exception: error(existence_error(procedure, p/1), top_level/0) I am trying to teach myself prolog starting with the J. R. Fisher tutorial https://www.cpp.edu/~jrfisher/www/prolog_tutorial/pt_framer.html. I worked through the first few exercises using gprolog with no problems. I exited prolog for a break, but now when I restarted it I find myself unable to define procedures and am getting an uncaught exception: gprolog GNU Prolog 1.4.5 (64 bits) Compiled Jul 14 2018, 16:25:15 with /usr/bin/clang By Daniel Diaz Copyright (C) 1999-2018 Daniel Diaz | ?- p(a). uncaught exception: error(existence_error(procedure,p/1),top_level/0) I suspect I exited prolog improperly and there is an invalid state somewhere in a cache. FWIW I am running on a MacBook with OS X Big Sur version 11.3. Are there any suggestions about how to fix this?
