# the debugger

We have a Macaulay2 source file with a pair of functions in it that we can use for demonstrating the debugger. Let's load it so we can run the functions in it.
 i1 : load "Macaulay2Doc/demo1.m2"
We can see what functions were provided to us with listUserSymbols.
 i2 : listUserSymbols o2 = symbol class value ------ ----- ----- g : FunctionClosure -- g ------------------------------------------------------------------------ location of symbol ------------------ /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:11:1-11:2
Let's peek at the code of the function g.
 i3 : code g o3 = /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:11:7-14:8: --source code: g = y -> ( c := f(y-1); d := f(y-2); c+d)
We see that the function g calls a function f, but f is not visible to us (because f is a local variable). In emacs' Macaulay2 Interaction Mode, pressing return (RET or enter) after positioning the cursor on the output line displaying the file name and line number will bring up the source code in a new buffer.

The first few times we use g, it seems to work.

 i4 : g 4 17 o4 = -- 6 o4 : QQ i5 : g 3 7 o5 = - 2 o5 : QQ
However, the following attempt results in an error, and the debugger starts up automatically.
 i6 : g 2 /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:8:12:(3):[2]: error: division by zero /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:8:12:(3):[2]: --entering debugger (type help to see debugger commands) /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:8:11-8:13: --source code: b := 1/x;
You may use help, as instructed, to view the commands available in the debugger. As suggested by the help display, we can use listLocalSymbols to list the local symbols and their values.
 ii7 : listLocalSymbols oo7 = symbol class value location of symbol ------ ----- ----- ------------------ a : String -- "hi there" /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:7:6-7:7 b : Nothing -- null /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:8:6-8:7 x : ZZ -- 0 /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:6:6-6:7 f : FunctionClosure -- ... /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:6:1-6:2
We see the the value of x is 0, and that explains the error message about division by zero. The other local symbols are the ones defined in the body of the function f, whose code can now be displayed with code.
 ii8 : code f oo8 = /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:6:8-9:8: --source code: f := x -> ( a := "hi there"; b := 1/x; b+1)
We can use step with argument 0 to bypass the current expression.
 ii9 : step 0 /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:8:12:(3):[2]: --stepping limit reached /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:8:12:(3):[2]: --entering debugger (type help to see debugger commands) /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:8:11-8:13: --source code: b := 1/x;
If we decide the problem is one level up, we can use end or the end-of-file character (which often is CTRL-D) to quit this instance of the debugger. In this case, the debugger will be entered again (triggered by the same error indication that caused it to be entered originally) at the point inside the function g from which the function f was called.
 ii10 : end /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:13:11:(3):[1]: --entering debugger (type help to see debugger commands) /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:13:11-13:15: --source code: d := f(y-2);
We can use listLocalSymbols again to see the local variables of g.
 ii11 : listLocalSymbols oo11 = symbol class value location of symbol ------ ----- ----- ------------------ c : QQ -- 2 /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:12:6-12:7 d : Nothing -- null /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:13:6-13:7 y : ZZ -- 2 /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:11:5-11:6 f : FunctionClosure -- ... /usr/share/Macaulay2/Macaulay2Doc/demo1.m2:6:1-6:2 
After we are done debugging, we can quit the debugger entirely and return to top level with break.
 ii12 : break
The variable errorDepth can be used to control how deep inside the code the debugger should be activated.

• break -- break from a loop
• step -- step by single lines in the debugger
• continue -- continue with the next iteration of a loop
• return -- return from a function
• listLocalSymbols -- display of local symbols and their values
• listUserSymbols -- display the user's symbols
• code -- display source code
• value -- evaluate
• disassemble -- disassemble pseudocode or a function
• errorDepth -- set the error printing depth