Problem with sorting inside a loop in GAP

Question

I'm very new to GAP, and I'm struggling with SortBy. Starting with a list of lists, I want to sort each list by AbsoluteValue in GAP. However, I can only get Sort or SortBy to work outside of a loop. The problem exists already in this tiny example:

gap> A:=[[-5,-3,-1,2,4,6],[-6,-4,-2,1,3,5]];
[ [ -5, -3, -1, 2, 4, 6 ], [ -6, -4, -2, 1, 3, 5 ] ]
gap> sorter:=function(X)
>       for i in [1..Length(X)] do
>       X[i]:=SortBy(X[i],AbsoluteValue);
>       od;
> end;;
gap> sorter(A);
Error, Function Calls: <func> must return a value in
  X[i] := SortBy( X[i], AbsoluteValue ); at *stdin*:24 called from
<function "sorter">( <arguments> )
 called from read-eval loop at line 27 of *stdin*
you can supply one by 'return <value>;'
brk>
gap> A[1];A[2];
[ -1, 2, -3, 4, -5, 6 ]
[ -6, -4, -2, 1, 3, 5 ]

It sorts the first list, but apparently doesn't like the second one. I think the error message says that AbsoluteValue doesn't return a value, which is false. What am I missing?

Answer 1

Just -- for future readers -- the reason for the initial problem:

SortBy changes a list (but does not return the sorted list), while e.g. Sorted would leave the list unchanged and return the sorted copy. The general language convention is that verbs do something to an object, while nouns create a new object with the desired characteristics.