Loading is not importing!

Recently, I posted some rumblings about my experience compiling Prolog modules as Logtalk objects. One of my pet peeves with Prolog module systems and Prolog module code is the unfortunate mix up between loading and importing. Is quite simple really, repeat after me:

Loading is not importing!

If a user wants to load a module, the ensure_loaded/1 directive shall be used. If the user want to import the public predicates of a module, the use_module/1-2 directives shall be used. Sadly, this basic distinction between these two orthogonal operations is not enforced by most Prolog compilers with a module system (despite some advice found in Prolog user manuals). Moreover, the ensure_loaded/1 directive shall only be used outside modules. Its semantics should simply be: load a file if not already loaded. There is no need for this directive to be the jack-of-all-trades. A simple test: if your Prolog compiler complains when you load two Prolog source files defining two different modules exporting the same predicate, then your Prolog compiler is broken. Complaining about conflicting imports only makes sense when importing. Otherwise a library developer would need to be omniscient about every other library developer work. Module encapsulation purpose is to avoid predicate name conflicts in the first place. Auto-importing public predicates when loading is equivalent to putting holes in module encapsulation. For what purpose? Is really that much trouble to use the use_module/1-2 directives for importing? Sure, these directives also load a file if not already loaded. They must. After all, we are saying that we want to use a module! That doesn’t mean that the ensure_loaded/1 directive should work in retribution as a use_module/1-2 directive in disguise!

As the bible says, why complicate implementation, complicate documentation, complicate semantics? In order to perpetuate mix-ups? To ruin encapsulation goals? Just to cope with sloppy user programming?

One Response to “Loading is not importing!”

  • Joachim Schimpf

    Agreed – that’s a weirdness from Quintus Prolog that has been perpetuated ever since. In ECLiPSe we have separated the functionality:

    ensure_loaded/1 loads
    import/1 imports
    use_module = ensure_loaded+import