Just for the heck of it, I am trying to match a junction against a regex with the m// operator in raku (search for Explicit topic match on that page).
In the perl6 REPL:
> any('a','b') ~~ m/./
False
Afterwards, no matter how I call m// I get an immutable-match complaint:
> 'x' ~~ m/./
Cannot modify an immutable Match (｢a｣)
in block <unit> at <unknown file> line 1
Question
What is happening behind the scenes here?
Discussion
The problem seems to stem from the fact that the $/ special variable is set to the junction
any(｢a｣, ｢b｣)
after the junction match, and it seems to be that ｢a｣ in the junction that's raising the complaint.
As soon as I do anything that changes $/ to something else, functionality is restored:
> $/=Any
(Any)
> 'x' ~~ m/./
｢x｣
or
> 'x' ~~ /./
｢x｣
> 'x' ~~ m/./
｢x｣
(so matching with // first, so as to change $/, and then match with m//).
Clarification
I am not trying to "achieve" anything beyond what the question's asking: I simply want to understand this behavior.
Edit
For cross-reference purposes, this is now also a rakudo github issue, as suggested by @jjmerelo.
The side issue of whether the match should return False or True is settled, I think, in the comment by @raiph.
On the other hand, the main problem of receiving the immutable Match error was, it seems, a bug, with a commit that at least on my system fixes it.
So the problem was (as per the commit message) that regex match objects were not expected to be junctions.