Evaluator with Trace

For this example we want to give our evaluator the power to create a trace of its execution. We add a function line/2 that takes a term and a value and creates a string that describes the term and shows that this evaluates to the value.

For example given the term {con, 42} and the value 42 it returns the string "eval({con,42}) <== 42" or similar.

Obviously this makes sense only if the value we pass is actually the value returned by the evaluation of the term we pass.  I mean, we won't call line({con, 42}), 99).

Then when the evaluator evaluates a term it also calls this function line/2 with the term and with the resulting value, thus showing the trace of that step, and returns this in a tuple with the actual return value.

Also I needed an extra function write_term/1 in there to turn a term into a string, for example to turn {con, 42} into the string "{con, 42}".

These extra functions are not exported from the module because we won't need to call them from the shell. Well, yes we would for testing, so you might opt to export them at first and then remove the export later.

So now each time I evaluate a term I include a trace of the evaluation in a tuple with the result, and I add to the trace each time it passes through.

Anyway here is the code now:

-module(eval_t).
-export([eval/1]).
-export([answer/0,error/0]).

% evaluator with trace

eval({con, A}) ->
    {line({con, A}, A), A};
eval({dv, T, U}) ->
    {X, A} = eval(T),
    {Y, B} = eval(U),
    {X ++ Y ++ line({dv, T, U}, (A div B)), A div B}.

write_term({con, A}) ->
    "{con, " ++ io_lib:print(A) ++ "}";
write_term({dv, T, U}) ->
    "{dv, " ++ write_term(T) ++ ", " ++ write_term(U) ++ "}".

line(Term, A) ->
    "eval (" ++ write_term(Term) ++ ") <==" ++ io_lib:print(A) ++ io_lib:nl().

answer() ->
    {dv, {dv, {con, 1972}, {con, 2}}, {con, 23}}.

error() ->
    {dv, {con, 1}, {con, 0}}.

So now when I evaluate the Answer I get back the value 42 tucked into a tuple with the string that constitutes the accumulated trace of the eval operations:

1> l(eval_t).
{module,eval_t}
2> eval_t:eval(eval_t:answer()).
{"eval ({con, 1972}) <==1972\neval ({con, 2}) <==2\neval ({dv, {con, 1972}, {con
, 2}}) <==986\neval ({con, 23}) <==23\neval ({dv, {dv, {con, 1972}, {con, 2}}, {
con, 23}}) <==42\n",
 42}

I need to spread that trace string out - split that string at each "\n" character:

eval ({con, 1972}) <==1972
eval ({con, 2}) <==2
eval ({dv, {con, 1972}, {con, 2}}) <==986
eval ({con, 23}) <==23
eval ({dv, {dv, {con, 1972}, {con, 2}}, {con, 23}}) <==42

Why does the arrow point from the result to the parameter and not the other way?  Don't know.  Hmm.