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Catch with backtrace

How do we get a backtrace and how does the exception term have to look to get one?

The backtrace is filled in according to SWI-Prolog conventions because the ISO Standard has nothing to say about this.

SWI-Prolog wants the second argument of the error/2 term (given in the ISO standard as Imp_def) to look like context(Location,Message). If Location is fresh and the catch is performed by catch_with_backtrace/3 (which happens either explicity in code or at the latest possible time at the Prolog Toplevel), Location is filled with a backtrace (as implemented by library(prolog_stack) in file swipl/lib/swipl/library/prolog_stack.pl). The Message is generally a cleartext message (string or atom).

Take this program:

call0(ExceptionTerm) :- 
   call1(ExceptionTerm).
   
call1(ExceptionTerm) :- 
   call2(ExceptionTerm).
   
call2(ExceptionTerm) :- 
   throw(ExceptionTerm).

Enable debugging to keep the compiler from optimizing-away stack frames.

?- debug.

Let's study the behaviour of "backtrace generation" by catch_with_backtrace/3 with various forms of ExceptionTerm. We will let the exception be caught at the Prolog Toplevel, which uses that predicate.

Non-ISO-standard exception term without placeholder

No backtrace is generated, there is minimal printing at toplevel:

?- call0("deep in a search tree").
ERROR: Unhandled exception: "deep in a search tree"

Quasi-ISO-standard exception term error(_,_)

An exception term that looks like error(_,_) matches the ISO Standard basic format, although the requirements regarding the formal term on the first position have to be followed too for full compliance.

The second argument is set to context(B,_) where B contains a backtrace.

The toplevel tries validly to print something in the first line, but has to admit that it found an Unknown error term:

[debug]  ?- call0(error("deep in a search tree",Context)).

ERROR: Unknown error term: "deep in a search tree"
ERROR: In:
ERROR:   [13] throw(error("deep in a search tree",_4126))
ERROR:   [12] call2(error("deep in a search tree",_4156)) at user://1:14
ERROR:   [11] call1(error("deep in a search tree",_4186)) at user://1:11
ERROR:   [10] call0(error("deep in a search tree",_4216)) at user://1:8
ERROR:    [9] <user>
   Exception: (13) throw(error("deep in a search tree", _3266)) ? Exception details
==

Asking for "exception details" using m reveals that Context has been filled in with a term context(prolog_stack(Frames),_) as the exception term looks as follows:

error("deep in a search tree",
      context(
         prolog_stack([
            frame(13,call(system:throw/1),throw(error("deep in a search tree",_4126))),
            frame(12,clause(<clause>(0x1a7ef30),4),call2(error("deep in a search tree",_4156))),
            frame(11,clause(<clause>(0x1a9ccd0),4),call1(error("deep in a search tree",_4186))),
            frame(10,clause(<clause>(0x1a368c0),4),call0(error("deep in a search tree",_4216))),
            frame(9,clause(<clause>(0x18f7450),3),'$toplevel':toplevel_call(user:user: ...))]),_4082))

Quasi-ISO-standard exception term with SWI-Prolog context term error(_,context(_,_))

The same as above, we just have SWI-Prolog specific context/2 subterm already in the ISO-standard specific error/2 term.

We can put a generic message in the second argument of context/2. In this example, a String:

[debug]  ?- call0(error("deep in a search tree",context(_,"get me outta here"))).
ERROR: Unknown error term: "deep in a search tree" (get me outta here)

ISO-standard exception term with SWI-Prolog context term error(IsoFormal,context(_,_))

As above, backtrace and all, except that now error message generation is correct as it is based on the list of valid ISO formal terms:

[debug]  ?- call0(error(instantiation_error,context(_,"get me outta here"))).
ERROR: Arguments are not sufficiently instantiated (get me outta here)
ERROR: In:
ERROR:   [13] throw(error(instantiation_error,context(_3028,"get me outta here")))
ERROR:   [12] call2(error(instantiation_error,context(_3064,"get me outta here"))) at user://1:14
ERROR:   [11] call1(error(instantiation_error,context(_3100,"get me outta here"))) at user://1:11
ERROR:   [10] call0(error(instantiation_error,context(_3136,"get me outta here"))) at user://1:8
ERROR:    [9] <user>