RCWA package changes how free magma elements are printed (including group ring elements) #2020
Description
@cdwensley reported the following: With no packages loaded, one gets this:
gap> R := GroupRing( GF(2), Group( (1,2,3) ) );;
gap> z := Zero(R);
<zero> of ...
But if the RCWA package by @Stefan-Kohl is loaded, one gets this instead:
gap> R := GroupRing( GF(2), Group( (1,2,3) ) );;
gap> z := Zero(R);
0
The problem is caused by this method in RCWA, file lib/rcwamap.gi:3165:
InstallMethod( ViewObj,
"for elements of group rings of rcwa groups (RCWA)",
ReturnTrue, [ IsElementOfFreeMagmaRing ], 100,
function ( elm )
...The description string suggests this is "for elements of group rings of rcwa groups (RCWA)", but the type selector applies to arbitrary group rings, thus causing the breakage.
Actually, the function tries to restrict itself, via this check:
l := CoefficientsAndMagmaElements(elm);
grpelms := l{[1,3..Length(l)-1]};
coeffs := l{[2,4..Length(l)]};
if not ForAll(grpelms,IsRcwaMapping) then TryNextMethod(); fi;But this fails for the zero element, because for that l is empty.
One fix therefore would be to simply change RCWA to also print <zero> of ..., like the library.
Alternatively, one could always perform the check on the one element, like so:
if not IsRcwaMapping(CoefficientsAndMagmaElements(One(elm))[1]) then
TryNextMethod();
fi;