my $compress = sub ($x) {
    state $previous;
    $x ne $previous ?? $previous = $x !! return;
}

##</code><code>##

compress :: Eq a => [a] -> [a]
compress = map head . group

##</code><code>##

my $compress = sub ($x) {
    state $previous;
    return $x ne $previous ?? $previous = $x !! ();
}