To me, it was somewhat confusing as to why and how I had to keep separate my program from my queries, since I was used to REPL-type interactive programming (e.g. irb for Ruby).

Later, in reading the SWI-Prolog manual, I saw reference to this odd snippet in the “Adding Arithmetic Functions” section:

?- [user].

:- other_stuff(x).
…

It appeared that [user] was changing the mode from query-mode to program-mode, and allowing me to define new predicates.

Well, close. It turns out the way to think about this (can’t seem to find it in the SWI-Prolog manual) is that the square-bracket notation is the “load file” shortcut, and Prolog comes with a virtual file known as “user.” When you query:

?- [user].

and then go on to get:

|:

as your new prompt, you’re loading the “user” file which is like opening a handle on STDIN. (Not precisely, but close enough.) You can then type your program entries, /but you must terminate them with EOF/ (ctrl-D).

This bit of prerequisite knowledge would have saved me a lot of puzzlement and trial-and-error. A shame that Prolog, despite a nearly 40-year history of continuous use and usefulness, has such a high propaedeutic load.

$ sudo port -v install swi-prolog

Ended up getting lots of BS errors on the lines of “ERROR: Could not load/find library -lXpm.”

*****************************************************************
* ERROR: Could not load/find library -lXpm.         *
* Please install the XPM library and rerun configure.       *
* The primary source for libXpm is at:              *
*                               *
*     http://www-sop.inria.fr/koala/lehors/xpm.html     *
*                               *
* NOTE: If your system has libXpm, check the following:     *
*                               *
*   - Do you have the development version of the library?   *
*   - Is the library you have of the right architecture?    *
*   - Could it be that other parts of X11 are missing?  *
*     (typically the case if -lXext is also missing)    *
*                               *
*   Examine packages/xpce/src/config.log for details    *
*   Linux users: check README.debian for dependencies   *
*   (note that the exact package names vary between     *
*   distributions).                     *
*****************************************************************

Long story short: For various reasons, the MacPorts installed software wasn’t finding my Apple Developer X11 libraries, and it also wasn’t appropriately requiring the software as dependencies for the swi-prolog package.  Had to do the following:

$ sudo port -v install xpm

$ sudo port -v install xorg-libXt

$ sudo port -v install Xft2

to get the prereqs installed (warning: those libs, in turn, have some mongo big dependencies, like Perl itself, so be ready for some serious compiling).  After that, installing swi-prolog worked just fine.