Re: Spotting an empty array as argument

[GrandFather]

Perl "flattens" arrays and hashes passed as parameters to a sub into a list of scalar values (see the perlsub Description section, paragraph two. In your case it is not possible for the sub to tell the difference between a call with no parameters and a call containing one or more empty array and empty hash parameters.

Perhaps a solution is to use a "sayArray" sub that does the multi-line thing and use Perl's "say" in other cases? That has the advantage that it clearly signals to the user that something special is happening in the array case.

Perl's built in "say" doesn't need any magic for handling a "no argument" case. It just does what print does (in that case it prints nothing) then prints a newline, which is what it always does.

[Chuma]

Right, I suspected there might not be any nice way of doing it. I mean, "say" does seem to do some magic in the "no argument" case, just like "print", since it treats it differently from the "empty array" case. But maybe that's just a special rule for built-in functions.

It's weird though, because there are so many built-in functions that have the same behaviour, defaulting to $_ if there are no arguments, and it's clearly useful, so it's surprising if there's no way of recreating that for your own functions.

[choroba]

The prototype (_) can be used for the last argument. Some builtin functions can't be simulated, though, you can detect them by prototype('CORE::func') returning undef.

For example, you can easily replicate the behaviour of uc, as prototype('CORE::uc') returns _. Similarly, pack has the prototype of $_ (the underscore must be the last one). But print or chomp return undef, so their behaviour is more complex and prototypes can't express it. So there is a special rule, but only for some of the built-in functions.

[ikegami]

While prototype would normally be a useful thing to try, say can't be prototyped because it supports say FH ..., so a definitive answer can't be obtained from prototype here (assuming the lack of file handle support is acceptable).

[LanX]

> (the underscore must be the last one)

FWIW, the last of the mandatory ones!

I.e. you can define following prototypes as long as they are marked optional with a semicolon.

Trouble here is the scalar context!

  DB<237> sub tst (_;@) { dd \@_ }

  DB<238> $_=666; @a='a'..'c'; $a='A'

  DB<239> tst
[666]

  DB<240> tst $a,1,2,3
["A", 1, 2, 3]

  DB<241> tst $a,@a
["A", "a", "b", "c"]

  DB<242> tst @a                         # OOPS
[3]