Record and display MeTTa program traces

[ngeiswei]

A generally useful capability of MeTTa would be to record program traces. I suppose it would be implemented in a function that would take a space where to record the trace of a function call, itself describe by its function name and arguments, maybe something like

(: trace-eval (-> Space (-> $a $b) $b))

and would be used in the following way

;; Define foo that counts backward from $x to 0
(= (foo $x) (if (== $x 0) 0 (foo (- $x 1))))

;; Define `trace-space` to contain the trace of foo
!(bind &trace-space (new-space))

;; Run (foo 42) while storing a trace in `trace-space`
!(trace-eval &trace-space foo 42)

The result of that call would be [0] and the side effect would be that trace-space would contain the trace data. The exact format remains to be determined (maybe this could in fact be a parameter of trace-eval). An example of the content of trace-space after the call would be

(called-by (foo 42) foo.metta)
(called-by (foo 41) (foo 42))
(called-by (foo 40) (foo 41))
...

Then one could have rendering tools to display a tree of such data, like

|-(foo 41)
  |-(foo 40)
    |-(foo 39)
    ...

It would be good if such rendering represent non-deterministic branches clearly. Again the specifics remains to be defined.

[vsbogd]

It is a minor question probably, but why don't make trace-eval return a pair (<result> <trace-space>) instead of passing <trace-space> as an argument?
We always can build it from the trace-eval in the description though.

[ngeiswei]

It is a minor question probably, but why don't make trace-eval return a pair (<result> <trace-space>) instead of passing <trace-space> as an argument?

So <trace-space> would correspond to a space that has automatically been instantiated by trace-eval and contains the trace? Also, I was wondering, what if <result> is a superposition, would trace-eval return a superposition of pairs or would it return a single pair (<results> <trace-space>)?

I think the reason I suggest to provide a space as argument of trace-eval is so that the function behaves identically with or without traces.