[m-dev.] source-to-source debugger proposal

[Peter Wang]

As requested by Julien.

The idea is to implement a four port debugger by doing a
source-to-source transformation on the HLDS.  For the usual member
predicate, we would transform it to something roughly like the
following:

:- pred mem(T, list(T)).
:- mode mem(out, in) is nondet.

mem(E, L) :-
    GetVarsA = (pred(Vars::out) is det :-
	Vars = [
	    "E" - unbound,
	    "L" - bound(univ(L))
	]
    ),
    (
	impure handle_event(event_call, ... ),
	member(E0, L),  % body of member
	(
	    GetVarsB = (pred(Vars::out) is det :-
		Vars = [
		    "E" - bound(univ(E0)),
		    "L" - bound(univ(L))
		]
	    ),
	    ...
	    impure handle_event(event_exit, ... , allow_retry, RetryB),
	    (
		RetryB = yes,
		mem(E, L)
	    ;
		RetryB = no,
		E = E0
	    )
	;
	    impure handle_event(event_redo, ... ),
	    fail
	)
    ;
	impure handle_event(event_fail, ... , allow_retry, Retry),
	(
	    Retry = yes,
	    mem(E, L)
	;
	    Retry = no,
	    fail
	)
    ).

The handle_event calls would conditionally stop the program execution
and give the user a debugger prompt.  At exit and fail ports the user
can hit retry, causing the procedure to be reexecuted.

The GetVars closures are to be stored in a shadow stack (the maintenance
of which I did not show).  Then the user can print out and browse the
variables in each stack frame.

Ian also has an idea that the handle_event predicate can be overriden
to implement other tools.

Peter

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