I have a really, really complicated set of string simplification rules (only it doesn't actually work on strings, I use stuff like


to get the results on the fly - also I never know beforehand which rule is applicable in a concrete situation). As you can imagine, 100 of these rules have at least one hidden typo.
Is it possible to partially "Hold" the execution with something like this pseudocode happening

>exec F[a,a] 3 steps
F[b,a], rule 1 used
F[b,b], rule 1 used
F[c], rule 2 used
>exec F[a,c] 1 steps
F[b,c], rule 1 used

so I can then control by hand where a substition went haywire (which is luckily easy)? I.e. print out all intermediate results of the replacements?

  • $\begingroup$ shouldn't F[x__,a,y__] be F[x___,a,y___]? and similarly, F[x__,b,b,y__] should be F[x___,b,b,y___]? $\endgroup$
    – kglr
    Jul 3, 2019 at 14:38
  • $\begingroup$ Correct. I confused the two as in the actual case, rarely something happens at the boundary. $\endgroup$ Jul 4, 2019 at 10:43

1 Answer 1


Perhaps Trace gives what you need:

F[x___, a, y___]:= F[x, b, y];
F[x___, b, b, y___]:= F[x, c, y];

Trace[F[a, a]]

{F[a, a], F[b, a], F[b, b], F[c]}

Trace[F[a, c]]

{F[a, c], F[b, c]}

  • 1
    $\begingroup$ I should have trusted my intuition which said "Surely, MATHEMATICA has something like a computation trace" and googled myself :-) Anyway, spot on. $\endgroup$ Jul 4, 2019 at 10:45

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