7.10 Debugging Recursion

The GAP interpreter monitors the level of nesting of GAP functions during execution. By default, whenever this nesting reaches a multiple of 5000, GAP enters a break loop (break loop) allowing you to terminate the calculation, or enter return; to continue it. 

gap> dive:= function(depth) if depth>1 then dive(depth-1); fi; return; end;
function( depth ) ... end
gap> dive(100);
gap> OnBreak:= function() Where(1); end; # shorter traceback
function(  ) ... end
gap> dive(6000);
recursion depth trap (5000)
 at
dive( depth - 1 );
dive( depth - 1 ); called from
...
Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
or you may return to continue
brk> return;
gap> dive(11000);
recursion depth trap (5000)
 at
dive( depth - 1 );
dive( depth - 1 ); called from
...
Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
or you may return to continue
brk> return;
recursion depth trap (10000)
 at
dive( depth - 1 );
dive( depth - 1 ); called from
...
Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
or you may return to continue
brk> return;
gap>

This behaviour can be controlled using the procedure

  • SetRecursionTrapInterval( interval ) F

    interval must be a non-negative small integer (between 0 and 228). An interval of 0 suppresses the monitoring of recursion altogether. In this case excessive recursion may cause GAP to crash. 

    gap> dive:= function(depth) if depth>1 then dive(depth-1); fi; return; end;
function( depth ) ... end
gap> SetRecursionTrapInterval(1000);
gap> dive(2500);
recursion depth trap (1000)
 at
dive( depth - 1 );
dive( depth - 1 ); called from
...
Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
or you may return to continue
brk> return;
recursion depth trap (2000)
 at
dive( depth - 1 );
dive( depth - 1 ); called from
...
Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
or you may return to continue
brk> return;
gap> SetRecursionTrapInterval(-1);
SetRecursionTrapInterval( <interval> ): <interval> must be a non-negative smal\
l integer
not in any function
Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
or You can return a non-negative small integer to continue
brk> return ();
SetRecursionTrapInterval( <interval> ): <interval> must be a non-negative smal\
l integer
not in any function
Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
or You can return a non-negative small integer to continue
brk> return 0;
gap> dive(20000);
gap> dive(2000000);
Segmentation fault

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    GAP 4 manual
    February 2000