Increment (++) and decrement (--) strings in Perl

Question

With perl -e \'$string=\"a\";print ++$string;\' we get b,
but with perl -e \'$string=\"b\";print --$string;\' we get -1

Answer 1

perlop(1) explains that this is true, but doesn't give a rationale:

The auto-increment operator has a little extra builtin magic to it. [If applicable, and subject to certain constraints,] the increment is done as a string, preserving each character within its range, with carry[...]

The auto-decrement operator is not magical.

The reason you get -1 is because when interpreted as a number, "b" turns into 0 since it has no leading digits (Contrarily, "4b" turns into 4).

Answer 2

There are at least three reasons:

1. because there isn't any great need for it
2. the magic of auto-incrementing has been seen to be faulty, and there is no reason implement auto-decrementing in the same faulty way
3. the magic of auto-incrementing cannot be fixed because p5p doesn't like to break backwards compatibility

Raku (née Perl 6) on the other hand does not suffer from a need for backwards compatibility, and therefore has better behavior for auto-incrementing strings and has auto-decrementing as well. The ++ and -- operators work by calling the succ and pred methods on the object they are operating on.

Answer 3

Perl give us lots of facilities, why not this one?

Because it is not intuitive what values should precede the "lowest" character in range. It may make sense that "A" + 1 should be "B", and that "B" + 1 should be "C". And therefore "B" - 1 should be "A". But what should "A" - 1 be?