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Macaulay2Doc :: the debugger

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
Let's peek at the code of the function g.
i3 : code g

o3 = /home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:11:7-14:8: --source code:
     g = y -> (
          c := f(y-1);
          d := f(y-2);
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

o4 = --

o4 : QQ
i5 : g 3

o5 = -

o5 : QQ
However, the following attempt results in an error, and the debugger starts up automatically.
i6 : g 2
/home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:8:12:(3):[2]: error: division by zero
/home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:8:12:(3):[2]: --entering debugger (type help to see debugger commands)
/home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:8:11-8:13: --source code:
     b := 1/x;
We use help, as instructed, to view the commands available in the debugger.
ii7 : help

oo7 = --debugger activation depth control:
          errorDepth=3        -- activate at positions in user code (default)
          errorDepth=2        -- activate also at positions in packages
          errorDepth=1        -- activate also at positions in Core
          errorDepth=0        -- activate also at positions in the loader
      --debugging control:
          return              -- bypass current expression, return null, stop
          return x            -- bypass current expression, return x, stop
          step                -- step 1 line
          step n              -- step n lines
          step (-n)           -- trace n microsteps
          end (or eof char)   -- enter debugger one level up
          continue            -- leave the debugger, continuing execution
                              -- with current expression
          break               -- leave the debugger, returning to top level
      --debugging information:
          listLocalSymbols    -- display local symbols and their values
          listUserSymbols     -- display user symbols and their values
          current             -- the current expression; initially, the one
                              -- that produced an error
          code current        -- source code of current expression
          value current       -- execute current expression, obtain value
          disassemble current -- display microcode of current expression
          currentString       -- the string being evaluated by 'value', if
                              -- an error occurred within it
      -- emacs commands in *M2* buffer:
          RET                 -- on an file/position line, go to source
As suggested, we can use listLocalSymbols to list the local symbols and their values.
ii8 : listLocalSymbols

oo8 = symbol   class              value                                                   location of symbol
      ------   -----              -----                                                   ------------------                                                   
      a      : String          -- "hi there"                                              /home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:7:6-7:7
      b      : Nothing         -- null                                                    /home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:8:6-8:7
      x      : ZZ              -- 0                                                       /home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:6:6-6:7
      f      : FunctionClosure -- -*Function[/home2/dan/src/M2/M2/Macaulay2/packages/Mac. /home2/dan/src/M2/M2/Macaulay2/packages/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.
ii9 : code f

oo9 = /home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:6:8-9:8: --source code:
      f := x -> (
           a := "hi there";
           b := 1/x;
We can use step with argument 0 to bypass the current expression.
ii10 : step 0
/home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:8:12:(3):[2]: --stepping limit reached
/home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:8:12:(3):[2]: --entering debugger (type help to see debugger commands)
/home2/dan/src/M2/M2/Macaulay2/packages/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.
ii11 : end
/home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:13:11:(3):[1]: --entering debugger (type help to see debugger commands)
/home2/dan/src/M2/M2/Macaulay2/packages/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.
ii12 : listLocalSymbols

oo12 = symbol   class              value                                                   location of symbol
       ------   -----              -----                                                   ------------------                                                     
       c      : QQ              -- 2                                                       /home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:12:6-12:7
       d      : Nothing         -- null                                                    /home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:13:6-13:7
       y      : ZZ              -- 2                                                       /home2/dan/src/M2/M2/Macaulay2/packages/Macaulay2Doc/demo1.m2:11:5-11:6
       f      : FunctionClosure -- -*Function[/home2/dan/src/M2/M2/Macaulay2/packages/Mac. /home2/dan/src/M2/M2/Macaulay2/packages/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.
ii13 : break
The variable errorDepth can be used to control how deep inside the code the debugger should be activated.

See also