2008 November 28th Friday (十一月 二十八日 金曜日)

   There is a prolog program.  Let's look at it again.

%% exercise

assign(X, Y) -->
    left(X), [:=], right(Y), [;].

left(X, Start, End) :-
    [ X | End ] = Start,
    atomic(X).

right(Y, Start, End) :-
    [ Y | End ] = Start,
    number(Y).

  I run it as follow.

| ?- assign(x, 3, A, []).

A = [x,:=,3,;]

yes
| ?- assign(L, R, [x, :=, 3, ;], T).

L = x
R = 3
T = []

yes
| ?- assign(L, R, [x, :=, 3, ;, y, :=, 2, ;], T).

L = x
R = 3
T = [y,:=,2,;]

yes

  Every predicate has a alike following control flow.

        +-----------+
call -->|           |--> exit
        | predicate |
fail <--|           |<-- redo
        +-----------+
               |
exception <----+

repeat:

        +-----------+
call -->|        +->|--> exit
        | repeat |  |
fail <--|        +--|<-- redo
        +-----------+
               |
exception <----+

fail:

        +-----------+
call -->|--+         |--> exit
        |  | fail    |
fail <--|<-+         |<-- redo
        +-----------+
               |
exception <----+

  So, repeat, ..., fail. will cause a loop