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Error handling: split stack frames over two lines; adjust more manual examples #6263

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011dec8
error: split stack frames over two lines
fingolfin Mar 11, 2026
04e6294
avoid setjmp/longjmp clobbering
fingolfin Mar 11, 2026
4a0e38f
Adjust some more stacktrace / break loop relevant docs
fingolfin Mar 12, 2026
bbe95f1
Update some comments
fingolfin Mar 13, 2026
6cc08be
fixup! error: split stack frames over two lines
fingolfin Mar 13, 2026
6b15d58
Avoid a trailing space
fingolfin Mar 13, 2026
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96 changes: 39 additions & 57 deletions doc/ref/debug.xml
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Expand Up @@ -915,41 +915,33 @@ to terminate the calculation, or enter <B>Return</B><C>;</C> 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> OnBreak:= function() Where(2); end;; # shorter traceback
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@fingolfin fingolfin Mar 11, 2026

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the "1" showed 2 level due to a bug that I fixed in #6257, so I've now changed it to 2 levels to more closely match was the example showed before

gap> dive(6000);
recursion depth trap (5000)
at
dive( depth - 1 );
called from
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
Error, recursion depth trap (5000)
*[1] dive( depth - 1 );
@ *stdin*:1
[2] dive( depth - 1 );
@ *stdin*:1
... at *stdin*:4
you may 'return;'
brk> return;
gap> dive(11000);
recursion depth trap (5000)
at
dive( depth - 1 );
called from
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
Error, recursion depth trap (5000)
*[1] dive( depth - 1 );
@ *stdin*:1
[2] dive( depth - 1 );
@ *stdin*:1
... at *stdin*:5
you may 'return;'
brk> return;
recursion depth trap (10000)
at
dive( depth - 1 );
called from
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>
Error, recursion depth trap (10000)
*[1] dive( depth - 1 );
@ *stdin*:1
[2] dive( depth - 1 );
@ *stdin*:1
... at *stdin*:5
you may 'return;'
brk> return;gap>
]]></Log>
<P/>

Expand Down Expand Up @@ -979,39 +971,29 @@ gap> dive := function(depth)
> fi;
> end;;
gap> SetRecursionTrapInterval(1000);
gap> OnBreak:= function() Where(2); end;; # shorter traceback
gap> dive(100);
Depth 100
gap> dive(2500);
recursion depth trap (1000)
at
dive( depth - 1 );
called from
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
Error, recursion depth trap (1000)
*[1] dive( depth - 1 );
@ *stdin*:4
[2] dive( depth - 1 );
@ *stdin*:4
... at *stdin*:12
you may 'return;'
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@fingolfin fingolfin Mar 11, 2026

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actually I think one still can quit; but the message changed. Maybe a bug? But I think not caused by this PR. Someone should investigate (I might, later)

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@limakzi limakzi Mar 11, 2026

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I think its a bug, but I am sure its not cause but this PR as well and its irrelevant to the changes.

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@fingolfin fingolfin Mar 13, 2026

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I just checked: in GAP 4.4 it still printed:

Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can 'return;' after assigning a value to continue

but starting with 4.5, the "quit" line was gone. I think I know why (certain changes to kernel error handling), but am too lazy to dig up the exact commit: it'll not be in the public history anyway, plus I don't see a point: clearly we want to show this info, yet error handling has substantially changed since then, so we are better of just adding it back now without caring how it used to be done. I'll do that

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@fingolfin fingolfin Mar 13, 2026

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The problem is not so small, so for now I filed an issue, let's focus on the rest of this PR first.

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@limakzi limakzi Mar 14, 2026

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OK.

brk> return;
recursion depth trap (2000)
at
dive( depth - 1 );
called from
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
Error, recursion depth trap (2000)
*[1] dive( depth - 1 );
@ *stdin*:4
[2] dive( depth - 1 );
@ *stdin*:4
... at *stdin*:12
you may 'return;'
brk> GetRecursionDepth();
0
brk> return;
gap> SetRecursionTrapInterval(-1);
Error, SetRecursionTrapInterval: <interval> must be a small integer greater than 5 (n\
ot the integer -1)
not in any function
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can replace <interval> via 'return <interval>;' to continue
brk> return 0;
gap> SetRecursionTrapInterval(2^50);
gap> dive(20000);
Depth 20000
gap> dive(2000000);
Expand Down
6 changes: 1 addition & 5 deletions doc/ref/grpperm.xml
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Expand Up @@ -652,11 +652,7 @@ gap> h:=Group(permutationlist);
<permutation group with 5 generators>
gap> StabChainOptions(h).random:=1;;
gap> Size(h);
exceeded the permitted memory (`-o' command line option) at
mlimit := 1; called from
SCRMakeStabStrong( S.stabilizer, [ g ], param, orbits, where, basesize,
base, correct, missing, false ); called from
SCRMakeStabStrong( S.stabilizer, [ g ], param, orbits, where, basesize,
exceeded the permitted memory (`-o' command line option)
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@fingolfin fingolfin Mar 11, 2026

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I decided to drop the stacktrace: the example can't be reproduced (missing inputs) but it is also irrelevant for the point this section is trying to make

...
]]></Log>
<P/>
Expand Down
19 changes: 7 additions & 12 deletions doc/ref/language.xml
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Expand Up @@ -1556,24 +1556,19 @@ If the expression <A>expr</A> is a function call then this function must
return a value. If the function does not return a value an error is
signalled and you enter a break loop (see <Ref Sect="Break Loops"/>).
As usual you can leave the break loop with <C>quit;</C>.
If you enter <C>return <A>return-expr</A>;</C> the value of the expression
<A>return-expr</A> is assigned to the variable,
and execution continues after the assignment.
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@fingolfin fingolfin Mar 11, 2026

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That was straight out wrong (we removed this capability years ago as it seemed to be only a source of grief)

<P/>
<Log><![CDATA[
gap> f1:= function( x ) Print( "value: ", x, "\n" ); end;;
gap> f2:= function( x ) return f1( x ); end;;
gap> f2( 4 );
value: 4
Function Calls: <func> must return a value at
return f1( x );
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 supply one by 'return <value>;' to continue
brk> return "hello";
"hello"
Error, Function Calls: <func> must return a value
*[1] f1( x )
@ *stdin*:2
<function "f2">( <arguments> )
called from read-eval loop at *stdin*:3
type 'quit;' to quit to outer loop
brk>
]]></Log>
<P/>
In the above example, the function <C>f2</C> calls <C>f1</C> with argument
Expand Down
17 changes: 6 additions & 11 deletions doc/ref/methsel.xml
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Expand Up @@ -125,19 +125,15 @@ objecty, a "No Method Found" error will be returned.
<Log><![CDATA[
gap> XCons(IsFullTransformationMonoid,4);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `XCons' on 2 arguments called from
<function "HANDLE_METHOD_NOT_FOUND">( <arguments> )
called from read-eval loop at line 8 of *stdin*
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
Error, no 1st choice method found for `XCons' on 2 arguments
called from read-eval loop at *stdin*:8
type 'quit;' to quit to outer loop
brk> quit;
gap> XCons(IsNilpotentGroup,4);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `XCons' on 2 arguments called from
<function "HANDLE_METHOD_NOT_FOUND">( <arguments> )
called from read-eval loop at line 9 of *stdin*
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
Error, no 1st choice method found for `XCons' on 2 arguments
called from read-eval loop at *stdin*:8
type 'quit;' to quit to outer loop
brk>
]]></Log>

Expand Down Expand Up @@ -435,4 +431,3 @@ the mathematical term <Q>centre</Q>.
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<!-- %% -->
<!-- %E -->

108 changes: 62 additions & 46 deletions doc/ref/mloop.xml
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Expand Up @@ -476,24 +476,20 @@ indicate that you are in a break loop.
<P/>
<Log><![CDATA[
gap> 1/0;
Rational operations: <divisor> must not be zero
not in any function
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can replace <divisor> via 'return <divisor>;' to continue
Error, Rational operations: <divisor> must not be zero
not in any function at *stdin*:2
type 'quit;' to quit to outer loop
]]></Log>
<P/>
If errors occur within a break loop &GAP; enters another break loop at a
<E>deeper level</E>. This is indicated by a number appended to <C>brk</C>:
<P/>
<Log><![CDATA[
brk> 1/0;
Rational operations: <divisor> must not be zero
not in any function
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can replace <divisor> via 'return <divisor>;' to continue
brk_02>
Error, Rational operations: <divisor> must not be zero
not in any function at *stdin*:2
type 'quit;' to quit to outer loop
brk_2>
]]></Log>
<P/>
There are two ways to leave a break loop,
Expand All @@ -511,7 +507,7 @@ Note that &GAP; code between <C>quit;</C> and the end of the input line
is ignored.
<P/>
<Log><![CDATA[
brk_02> quit;
brk_2> quit;
brk>
]]></Log>
<P/>
Expand Down Expand Up @@ -750,25 +746,30 @@ same as <Ref Func="Where"/> while also showing the arguments and local
variables of each function.
<P/>
<Log><![CDATA[
gap> StabChain(SymmetricGroup(100)); # After this we typed ^C
user interrupt at
bpt := S.orbit[1];
called from
SiftedPermutation( S, (g * rep) ^ -1 ) called from
StabChainStrong( S.stabilizer, [ sch ], options ); called from
StabChainStrong( S.stabilizer, [ sch ], options ); called from
StabChainStrong( S, GeneratorsOfGroup( G ), options ); called from
StabChainOp( G, rec(
) ) called from
...
Entering break read-eval-print loop ...
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
gap> StabChain(SymmetricGroup(1000)); # After this we typed ^C
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@fingolfin fingolfin Mar 11, 2026

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had to go from 100 to 1000 as I otherwise couldn't press Ctrl-C quickly enough 😁

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@limakzi limakzi Mar 14, 2026

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I would resolve that too.
I'm happy with that.

Error, user interrupt
*[1] genlabels := Filtered( genlabels, function ( x )
return pnt ^ sgs[x] = pnt;
end );
@ GAPROOT/lib/stbc.gi:426
[2] StabChainBaseStrongGenerators( dom{[ 1 .. Length( dom ) - 1 ]}, sgs, () )
@ GAPROOT/lib/gpprmsya.gi:2006
[3] StabChainOp( G, rec(
) )
@ GAPROOT/lib/stbc.gi:28
[4] StabChainImmutable( arg[1] )
@ GAPROOT/lib/stbc.gi:18
<function "StabChain">( <arguments> )
called from read-eval loop at *stdin*:2
you can 'return;'
brk> Where(2);
called from
SiftedPermutation( S, (g * rep) ^ -1 ) called from
StabChainStrong( S.stabilizer, [ sch ], options ); called from
...
*[1] genlabels := Filtered( genlabels, function ( x )
return pnt ^ sgs[x] = pnt;
end );
@ GAPROOT/lib/stbc.gi:426
[2] StabChainBaseStrongGenerators( dom{[ 1 .. Length( dom ) - 1 ]}, sgs, () )
@ GAPROOT/lib/gpprmsya.gi:2006
... at *errin*:1
]]></Log>
<P/>
Note that the variables displayed even in the first line of the
Expand Down Expand Up @@ -836,10 +837,14 @@ Error, !
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> Where();
*[1] Error( "!\n" ); at *stdin*:3 called from
[2] test( n + 1 ); at *stdin*:3 called from
[3] test( n + 1 ); at *stdin*:3 called from
[4] test( n + 1 ); at *stdin*:3 called from
*[1] Error( "!\n" );
@ *stdin*:4
[2] test( n + 1 );
@ *stdin*:4
[3] test( n + 1 );
@ *stdin*:4
[4] test( n + 1 );
@ *stdin*:4
<function "test">( <arguments> )
called from read-eval loop at *errin*:1
brk> n;
Expand All @@ -848,20 +853,28 @@ brk> DownEnv();
brk> n;
3
brk> Where();
[1] Error( "!\n" ); at *stdin*:3 called from
*[2] test( n + 1 ); at *stdin*:3 called from
[3] test( n + 1 ); at *stdin*:3 called from
[4] test( n + 1 ); at *stdin*:3 called from
[1] Error( "!\n" );
@ *stdin*:4
*[2] test( n + 1 );
@ *stdin*:4
[3] test( n + 1 );
@ *stdin*:4
[4] test( n + 1 );
@ *stdin*:4
<function "test">( <arguments> )
called from read-eval loop at *errin*:5
brk> DownEnv( 2 );
brk> n;
1
brk> Where();
[1] Error( "!\n" ); at *stdin*:3 called from
[2] test( n + 1 ); at *stdin*:3 called from
[3] test( n + 1 ); at *stdin*:3 called from
*[4] test( n + 1 ); at *stdin*:3 called from
[1] Error( "!\n" );
@ *stdin*:4
[2] test( n + 1 );
@ *stdin*:4
[3] test( n + 1 );
@ *stdin*:4
*[4] test( n + 1 );
@ *stdin*:4
<function "test">( <arguments> )
called from read-eval loop at *errin*:8
brk> DownEnv( -2 );
Expand Down Expand Up @@ -894,10 +907,13 @@ gap> fun := function() local x; x := 3; bar(); end;
function( ) ... end
gap> fun();
Error, !!
called from
bar( ); called from
<function>( <arguments> ) called from read-eval-loop
Entering break read-eval-print loop ...
Stack trace:
*[1] Error( "!!\n" );
@ *stdin*:4
[2] bar( );
@ *stdin*:7
<function "fun">( <arguments> )
called from read-eval loop at *stdin*:8
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> x;
Expand Down
10 changes: 5 additions & 5 deletions doc/tut/group.xml
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Expand Up @@ -927,11 +927,12 @@ gap> Size( Image( hom, DerivedSubgroup(s4) ) );
<P/>
<Log><![CDATA[
gap> PreImage( hom, (1,2,3) );
Error, <map> must be an inj. and surj. mapping called from
Error, <map> must be an injective and surjective mapping
*[1] ErrorNoReturn( "<map> must be an injective and surjective ", "mapping" );
@ GAPROOT/lib/mapping.gi:262
<function "PreImage">( <arguments> )
called from read-eval loop at line 4 of *stdin*
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
called from read-eval loop at *stdin*:7
type 'quit;' to quit to outer loop
brk> quit;
]]></Log>
<P/>
Expand Down Expand Up @@ -1258,4 +1259,3 @@ Chapter&nbsp;<Ref Chap="Group Homomorphisms" BookName="ref"/>.
<!-- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% -->
<!-- %% -->
<!-- %E -->

4 changes: 2 additions & 2 deletions doc/tut/introduc.xml
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Expand Up @@ -253,8 +253,8 @@ We will no longer mention the <B>Return</B> key from now on.
<Index>break loops</Index>
Sometimes a syntax error will cause &GAP; to enter a <E>break loop</E>. This
is indicated by the special prompt <C>brk></C>. If another syntax error occurs
while &GAP; is in a break loop, the prompt will change to <C>brk_02></C>,
<C>brk_03></C> and so on. You can leave the current break loop and exit to the
while &GAP; is in a break loop, the prompt will change to <C>brk_2></C>,
<C>brk_3></C> and so on. You can leave the current break loop and exit to the
next outer one by either typing <C>quit;</C> or by hitting <B>Ctrl-D</B>.
Eventually &GAP; will return to its normal state and show its normal
prompt <C>gap></C> again.
Expand Down
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