Evaluator with State.

Leaving the error processing aside the next example incorporates an updateable state into our evaluator.  As an example we say that we wish to count how often the division operation has been called.  We could do this by creating a global variable that we increment in the division code.  In our functional version we do this by tagging our values with a state, our counter, by including it in a tuple. So instead of our evaluator returning an integer A it will return return {A, N} where A is the integer result and N is is the count of division operations that we have done.

So we adapt the evaluator to set and maintain this counter, as so:

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

% evaluator with state

eval({{con, A}, X}) ->
    {A, X};
eval({{dv, T, U}, X}) ->
    {A, Y} = eval({T, X}),
    {B, Z} = eval({U, Y}),
    {A div B, Z + 1}.

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

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

The counter is passed through each evaluation and on the line where we do the division we add one to it.

Then when we evaluate Answer we also pass a starting value zero for the counter:  the result contains the answer 42 and the final value of the counter, 2.

1> l(eval_s).
{module,eval_s}
2> eval_s:eval({eval_s:answer(), 0}).
{42,2}
3> eval_s:eval({eval_s:error(), 0}).
** exception error: an error occurred when evaluating an arithmetic expression
     in function  eval_s:eval/1 (c:/Users/polly/Erlang/eval_s.erl, line 12)

Evaluating Error still gives you an exception of course.