Store GAP REPL representation of expression in string

Question

I have noticed that the representation that is displayed by the GAP REPL (read-evaluate-print loop) when an expression is evaluated sometimes differs from the result of printing that same expression, e.g.:

gap> x:=PartialPerm([1,2],[3,4]);
[1,3][2,4]
gap> Print(x);
PartialPerm( [1, 2], [3, 4] );

Is there some general way to obtain the former string representation outside of an interactive GAP session which will work for any expression? (Or, failing that, a way that works in the special case of partial permutations?)

Answer 1

GAP has operations DisplayString, ViewSrting and PrintString documented here. Usually, displaying an object produces a human-readable relatively complete and verbose output, viewing produces a short and concise output, and printing produces an output in a complete form which is GAP readable (if at all possible), such that reading the output into GAP produces an object which is equal to the original one.

Depending on the type of the object, they may delegate to each other in the order specified here.

In the example in question, we have

gap> PrintString(x);
"PartialPerm( \>[ 1, 2 ], \<\>[ 3, 4 ]\<\> )\<"
gap> ViewString(x);
"\>[\>1\<,\>3\<\<]\>[\>2\<,\>4\<\<]"
gap> DisplayString(x);
"<object>\n"

As you see, the output have additional control characters \< (ASCII 1) and \> (ASCII 2) that allow proper line breaks. The function StripLineBreakCharacters (see here) may be used to remove them:

gap> StripLineBreakCharacters(PrintString(x));
"PartialPerm( [ 1, 2 ], [ 3, 4 ] )"
gap> StripLineBreakCharacters(ViewString(x));
"[1,3][2,4]"
gap> StripLineBreakCharacters(DisplayString(x));
"<object>\n"

There is also String (see here) which should approximate as closely as possible the character sequence you see if you print an object:

gap> String(x);
"PartialPerm( [ 1, 2 ], [ 3, 4 ] )"

Perhaps for partial permutations that's the best option.