[derefr]

Programming languages are specificational: you say everything up-front, then the computer interprets all your statements at once and executes them.

Natural language, however, is conversational. You say A, the person you're talking to interprets that as Z and asks for clarification, you explain the difference between A and Z, now the other person thinks M and asks more clarifying questions, etc.

Computers are fully capable of working conversationally, instead of specificationally; it just requires that instructions be stored in a form that's a bit more complicated than a linear tape (e.g. a database of constraints, like Prolog.)

[silentbicycle]

In more conventional programming terms, some languages have REPLs* for a conversation, and support declarative programming ("Here's what I want, figure it out"), but most are procedural ("Do this, than this, than this, then give me the result").

* Read/eval(uate)/print loops

Prolog (also Erlang and some others) is mixed; you reload a file of rules read as a whole, but can easily prompt the system for easy testing, and reloading is very fast. It's very convenient with a Prolog shell terminal and a vi window, two buffers in Emacs, etc.

[derefr]

Prolog has a REPL, but only new facts can be declared through it efficiently, not new rules. If you (re-)declare a rule (through `assert`), the entire constraint database is actually re-evaluated behind the scenes. The big problem in conversational declarative programming is how to start with general-purpose rules, and work downward with more and more special-case exceptions, without each new assertion taking longer to integrate into the database than the last.

Inform is an exapmple of a natural-language-ish, rule-based system (for programming text adventures), that could efficiently re-declare rules in a REPL (if not for its basis in virtual machine image formats that expect to be compiled from complete specifications.) Inform guarantees efficiency by using a hub-and-spoke system of rules: rather than every rule having the possibility to interact with every other rule, rules can only interact with rules in their own "rulebook" (module), the core rulebooks (standard library), and the "meta" rulebook (monkeypatches to re-specify libraries.) Thus, integrating a new definition only takes O(k + n + e) time—where k, n, and e should all be small—rather than O(n^2). This works well for Inform, but I'm not sure whether it would be as effective in a general-purpose programming environment.

[aaronblohowiak]

Do rules need to be global in scope?

[silentbicycle]

Not necessarily. Some Prolog implementations have module/packaging systems, some don't. Prolog is a weird language - many details feel very antiquated* , yet on the whole it's way ahead of its time (esp. constraint programming). I think it would fare much better as an embedded library (like e.g. Lua or SQLite), rather than a freestanding language. Working on it, though I will likely finish other projects first.

* Case in point: Loading a file is "consult"; I assume this is historically because Prolog was originally a language for doing NLP in French. (See e.g. HoPL-2.)