raku - Use of colon in method and function calls in Perl 6

Question

I'm wondering what colons have to do with method and function calls in Perl 6. For the record, I am using perl6 version 2015.05-55-gd84bbbc built on MoarVM version 2015.05.

I just saw the following in a Perl6 spec test (S32-io) (I added the comment):

$fh.print: "0123456789A";   # prints '0123456789A' to the file

As far as I can tell, this is equivalent to:

$fh.print("0123456789A");   # prints '0123456789A' to the file

Both of these seem to take multiple arguments and to flatten lists fine:

$fh.print: "012", "345", "6789A";   # prints '0123456789A' to the file
$fh.print("012", "345", "6789A");   # prints '0123456789A' to the file

my @a = <012 345 6789A>;

$fh.print(@a);   # prints '0123456789A' to the file
$fh.print: @a;   # prints '0123456789A' to the file

There must be some reason to have these two different syntaxes. Is there any reason to use one or the other syntax?

I also noticed that we have to use either : or () with print, when used as a method:

$fh.print(@a);   # Works
$fh.print: @a;   # Works!
$fh.print @a;    # ERROR!

There is also some interesting behavior when using a colon with the function print. In this case, : and () are not equivalent:

print @a;  # Prints '0123456789A' (no newline, just like Perl 5)
print(@a); # Ditto
print: @a; # Prints '012 345 6789A' followed by a newline (at least in REPL)

print  @a, @a; # Error (Two terms in a row)
print: @a, @a; # Prints '012 345 6789A 012 345 6789A' followed by a newline (in REPL) 

Then I tried using print in a script file. This works for prints to standard output:

print @a;

However, this does not print to standard output:

print: @a, @a;

But the method version works fine:

$fh.print: @a, @a; # Prints '0123456789A0123456789A' to the file

I feel like I almost understand this, but I can't put it into words. Could someone please explain these varieties of using print. Also, are these behaviors going to change due to the Great List Refactor?

Answer 1

One of the main reasons to use a colon instead of parens is that it can help declutter your code, by removing one set of parens. Otherwise they are exactly the same.

When you have print: @a, what you are really doing is putting a label on the line, and letting the @a fall-through. Which in the REPL will call say with the value.

If you don't use parens or a colon on a method call, then the method would get called with no arguments.

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You can swap the order of the method, and the invocant if you use a colon.

say $*ERR: 'hello world'; # $*ERR.say('hello world')