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Errors

Fabl does not yet have a graphical debugging environment, but does have a few facilities to aid in the interactive debugging of programs.

When the Fabl interpreter encounters an error, it prints an error message, and the prompt changes from:

fabl>

to something like this

7 fabl>

In this example, the "7" means that the call stack is 7 deep. (The call stack is the sequence of function calls that were in force when the error occured). To view the call stack, use the traceback command:

void tb()

An error can be invoked by the Fabl operator:

void function error(<anyType0>,,,<anyTypen>);

The arguments to error will be printed after an error message. Example:

void function f(int n){error('n = ',n);}
void function g(int n){f(2*n)}
g(10);
-->ERROR: n = 20
tb();
-->8 f
-->7 g

The above is an excerpt of the traceback containing the relevant lines; the rest of the traceback mentions Fabl internal functions, such as

5 evaluate

The operator:

void rs()

unwinds the call stack, and returns to the normal prompt.