# Debugging¶

TraceStack(expression)

show calling stack after an error occurs

TraceStack() shows the calling stack after an error occurred. It shows the last few items on the stack, not to flood the screen. These are usually the only items of interest on the stack. This is probably by far the most useful debugging function in yacas. It shows the last few things it did just after an error was generated somewhere.

For each stack frame, it shows if the function evaluated was a built-in function or a user-defined function, and for the user-defined function, the number of the rule it is trying whether it was evaluating the pattern matcher of the rule, or the body code of the rule.

This functionality is not offered by default because it slows down the evaluation code.

Here is an example of a function calling itself recursively, causing yacas to flood its stack:

In> f(x):=f(Sin(x))
Out> True;
In> TraceStack(f(2))
Debug> 982 :  f (Rule # 0 in body)
Debug> 983 :  f (Rule # 0 in body)
Debug> 984 :  f (Rule # 0 in body)
Debug> 985 :  f (Rule # 0 in body)
Debug> 986 :  f (Rule # 0 in body)
Debug> 987 :  f (Rule # 0 in body)
Debug> 988 :  f (Rule # 0 in body)
Debug> 989 :  f (Rule # 0 in body)
Debug> 990 :  f (Rule # 0 in body)
Debug> 991 :  f (Rule # 0 in body)
Debug> 992 :  f (Rule # 0 in body)
Debug> 993 :  f (Rule # 0 in body)
Debug> 994 :  f (Rule # 0 in body)
Debug> 995 :  f (User function)
Debug> 996 :  Sin (Rule # 0 in pattern)
Debug> 997 :  IsList (Internal function)
Error on line 1 in file [CommandLine]
Max evaluation stack depth reached.
Please use MaxEvalDepth to increase the stack
size as needed.

TraceExp(expr)

evaluate with tracing enabled

The expression “expr” is evaluated with the tracing facility turned on. This means that every subexpression, which is evaluated, is shown before and after evaluation. Before evaluation, it is shown in the form {TrEnter(x)}, where {x} denotes the subexpression being evaluated. After the evaluation the line {TrLeave(x,y)} is printed, where {y} is the result of the evaluation. The indentation shows the nesting level.

Note that this command usually generates huge amounts of output. A more specific form of tracing (eg. {TraceRule}) is probably more useful for all but very simple expressions.

Example

In> TraceExp(2+3);
TrEnter(2+3);
TrEnter(2);
TrLeave(2, 2);
TrEnter(3);
TrLeave(3, 3);
TrEnter(IsNumber(x));
TrEnter(x);
TrLeave(x, 2);
TrLeave(IsNumber(x),True);
TrEnter(IsNumber(y));
TrEnter(y);
TrLeave(y, 3);
TrLeave(IsNumber(y),True);
TrEnter(True);
TrLeave(True, True);
TrEnter(x);
TrLeave(x, 2);
TrEnter(y);
TrLeave(y, 3);
TrLeave(2+3, 5);
Out> 5;

TraceRule(template) expr

turn on tracing for a particular function

Param template

template showing the operator to trace

Param expr

expression to evaluate with tracing on

The tracing facility is turned on for subexpressions of the form “template”, and the expression “expr” is evaluated. The template “template” is an example of the function to trace on. Specifically, all subexpressions with the same top-level operator and arity as “template” are shown. The subexpressions are displayed before (indicated with {TrEnter}) and after ({TrLeave}) evaluation. In between, the arguments are shown before and after evaluation ({TrArg}). Only functions defined in scripts can be traced.

This is useful for tracing a function that is called from within another function. This way you can see how your function behaves in the environment it is used in.

Example

In> TraceRule(x+y) 2+3*5+4;
TrEnter(2+3*5+4);
TrEnter(2+3*5);
TrArg(2, 2);
TrArg(3*5, 15);
TrLeave(2+3*5, 17);
TrArg(2+3*5, 17);
TrArg(4, 4);
TrLeave(2+3*5+4, 21);
Out> 21;