Prolog directives aren't consistent with logic and very misleading... right?

General
1

When we write
A :- B, C, D
We mean in logic
(B and C and D) -> A

What does this mean then?
:- B, C, D

I’ve spend 5 hours pondering the logical meaning and how to fit it all together. This is how the directives work in Prolog:

:- true. %no error
:- false. %Warning:    Goal (directive) failed: user:false

Let’s assume we imagine “false” into the left side:

false :- true % true->false, no error BUT it makes our kb logically inconsistent
false :- false % false->false, OK but error

If we put true:

true:- true %true -> true, OK
true:- false %false -> true, OK but error

In this case, nothing needs to be checked, but Prolog still checks the right part

There is a table on wiki, but I still can’t fit it all together.

Can you confirm the prolog directives syntax is wrong and misleading, so they should have used different format and symbols for procedural directives?

2

Directives are defined at SWI-Prolog -- Loading Prolog source files - here’s a snippet:

A directive is an instruction to the compiler. Directives are used to set (predicate) properties (see section 4.15), set flags (see set_prolog_flag/2) and load files (this section). Directives are terms of the form :- . .

It’s probably confusing to be attempting to redefine the built-in true/0 and false/0.

The built-in definitions can be seen (although true is implemented in C for performance, so no Prolog code is available):

?- listing(false).
system:false :-
    fail.

?- listing(true).
%   Foreign: system:true/0

As an example with non-conflicting names:

mytrue :- true.
mytrue :- false.

The mytrue :- false. declaration is of course useless, because it will never succeed.

The output will be:

?- mytrue.
true ;
false.

… which means that the true predicate succeeds, with a choicepoint to try the 2nd line, and the 2nd line (false) fails.

This is better illustrated with an argument (e.g. yes):

mytrue(yes) :- true.
mytrue(yes) :- false.

… which results in:

?- mytrue(Y).
Y = yes ;
false.
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