6.4 Variable Access in a Break Loop

Moves up nr steps in the environment and allows one to inspect variables on this level. If nr is negative it steps down in the environment again. nr defaults to 1 if not given.

gap> test:= function( n )
>    if n > 3 then Error( "!" ); fi; test( n+1 ); end;;
gap> test( 1 );
Error ! at
Error( "!" );
Entering break read-eval-print loop,
you can 'quit;' to quit to outer loop,
or you can return to continue
brk> Where();
test( n + 1 ); called from
test( n + 1 ); called from
test( n + 1 ); called from
<function>( <arguments> ) called from read-eval-loop
brk> n;
4
brk> DownEnv();
brk> n;
3
brk> Where();
test( n + 1 ); called from
test( n + 1 ); called from
<function>( <arguments> ) called from read-eval-loop
brk> DownEnv( 2 );
brk> n;
1
brk> Where();
<function>( <arguments> ) called from read-eval-loop
brk> DownEnv( -2 );
brk> n;
3

Note that the change of the environment caused by DownEnv only affects variable access in the break loop. If you use return to continue a calculation GAP automatically jumps to the right environment level again.

Note also that search for variables looks first in the chain of outer functions which enclosed the definition of a currently executing function, before it looks at the chain of calling functions which led to the current invocation of the function.

gap> foo := function() 
> local x; x := 1; 
> return function() local y; y := x*x; Error("!!"); end; 
> end;  
function(  ) ... end
gap> bar := foo();
function(  ) ... end
gap> fun := function() local x; x := 3; bar(); end;
function(  ) ... end
gap> fun();
Error !! at
Error( "!!" );
bar(  ); called from
<function>( <arguments> ) called from read-eval-loop
Entering break read-eval-print loop, you can 'quit;' to quit to outer loop,
or you can return to continue
brk> x;
1
brk> DownEnv(1);
brk> x;
3

Here the x of foo which contained the definition of bar is found before that of fun which caused its execution. Using DownEnv we can access the x from fun.

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