What's the difference these two function calling conventions?
As Raiph tells you above, say: is a label. So you didn't say anything (even though you thought you did) and -- outside use of the REPL -- the compiler will complain that your use of <a b c> was useless:
say: <a b c>; # OUTPUT: «WARNINGS for <tmp>:Useless use of constant value a b c in sink context (lines 1, 1, 1, 1, 1, 1)»
However, you often can use a : notation instead of parentheses in method calls. Consider the four routine calls below (two subroutine calls then two method calls):
my @numbers = (33, 77, 49, 11, 34);
say map *.is-prime, @numbers ; # simplest subroutine call syntax
say map( *.is-prime, @numbers ); # same meaning, but delimiting args
say @numbers.map( *.is-prime ) ; # similar, but using .map *method*
say @numbers.map: *.is-prime ; # same, but using : instead of parens
These sentences will all return the same (False False False True False).
In general, as you see above with map, you can use () in method calls wherever you would use :, but the opposite is not true; : can be used only in method calls.
Use () if the arguments need to be delimited precisely, as Raiph comments below.
This answer focuses on the basics. See Raiph's answer for more exhaustive coverage of the precise details of routine call syntax. (As an important example, the meaning of these calls normally changes if there's any spaces between the routine name and the colon (:) or opening parenthesis (()).
@jjmerelo's answer covers the basics. This complementary answer, which aims at being somewhat exhaustive but hopefully not exhausting, covers traps, rare cases, and advice.
foo: valuea, valueb, ...
Surprisingly perhaps, this is not a call of a sub or method called foo.
Instead it's a statement that begins with a label, foo:.
The say: line in your question won't work in an ordinary program:
say: <a b c>; # Useless use of constant value a b c ...
The "Useless use" warning means the <a b c> doesn't get used in a useful way. The say: isn't doing anything with the list of values. It's just a label that doesn't do anything.
Presumably you are using something like the Perl 6 REPL. The REPL automatically says the last value in a line if it isn't otherwise used, thus making the line appear to work without a warning.
.a-method:
If a postfix method call using the form .a-method has no arguments other than the invocant (the argument to the left of the ., or the current topic if there isn't an explicit invocant) then you can just write it in the form:
42.say ;
You can optionally append a colon:
42.say: ;
There's no good reason to, but it's consistent with:
.a-method: arg2, arg3, ...
If you want to supply one or more arguments (other than the invocant) to a postfix .a-method call, then you have to pick one of two ways to introduce them.
One way is to write a colon immediately after the method name, before the argument(s). There must be no space between the method name and opening parenthesis.1
For example, the following uses a colon before the Numeric argument in the following method call:
say <abc 2 def ghi> .first: Numeric ; # 2
In the above line the method call expression (.first: Numeric) ends at the statement terminator (;). If there's an enclosing sub-expression such as an array subscript then the method call expression ends at the end of that sub-expression:
say .[1 + .first: Numeric] given <abc 2 def ghi> ; # ghi
The argument list of a colon form method call is also brought to a close by a valid statement modifier like given:
say .first: Numeric given <abc 2 def ghi> ; # 2
a-sub arg1, arg2, ...
This is the corresponding form for subroutine calls. The only format differences are that the sub has no invocant or . before the sub name and you must omit the colon after the sub name.
.a-method( arg2, arg3, ... )
a-sub( arg1, arg2, ... )
The other common form used for both method and sub calls is to immediately follow the method or sub name with parens to delimit arguments. The opening paren must immediately follow, without any space between the routine name and (.
Here's parens used with the .first method:
say 1 + .first(Numeric) given <abc 2 def ghi> ; # 3
This has the advantage that it's arguably prettier than the alternative of using outer parens:
say 1 + (.first: Numeric) given <abc 2 def ghi> ; # 3
If you want to put a sub call directly inside a double quoted string, you need to prefix the sub name with an & sigil and use the postfix parens form:
my @array = <abc 2 def ghi> ;
say "first number is &first(Numeric,@array)" ; # first number is 2
To put in a method call, you again have to use the postfix parens form, and you must also provide an explicit invocant (you can't just write "Some text .a-method()"):
my @array = <abc 2 def ghi> ;
say "first number is @array.first(Numeric)" ; # first number is 2
If there are no arguments (other than the invocant for a method call) you still need to use this form with empty parens if you want to interpolate a sub or method call in a string:
my @array = <abc 2 def ghi> ;
say "no method call @array[3].uc" ; # no method call ghi.uc
say "with method call @array[3].uc()" ; # with method call GHI
say "&rand"; # &rand
say "&rand()"; # 0.929123203371282
.a-method ( arrgh, arrgh, ... ) ;
This won't work.
Because the .a-method isn't followed by a colon, the method call is considered complete.
That means the next thing must be either an expression/statement ender like ;, or a postfix operator that will operate on the result of the method call, or an infix operator that will operate on the result and some following argument.
But ( arrgh, arrgh, ... ) is none of these. So you get a "Two terms in a row" compilation error.
.a-method:( arrgh, arrgh, ... ) ;
.a-method: ( arrgh, arrgh, ... ) ;
In general, DO NOT MIX use of a : with use of parens around arguments as part of a method call. There is no good reason to do so because it will either not work, or work only by accident, or work but very likely confuse readers.
Doing so without a space between the colon and opening paren yields a cryptic compilation error:
This type (QAST::WVal) does not support positional operations
Leaving a space appears to work -- but typically only by luck:
say .first: (Numeric) given <abc 2 def ghi> ; # 2
The (Numeric) is a single value in parens which yields Numeric so this line is the same as:
say .first: Numeric given <abc 2 def ghi> ; # 2
But if there are two or more arguments in parens, things will go awry. Use one of these forms:
say .first: Numeric, :k given <abc 2 def ghi> ; # 1
say .first(Numeric, :k) given <abc 2 def ghi> ; # 1
which correctly yield the array index ("key") of the 2 element rather than:
say .first: (Numeric, :k) given <abc 2 def ghi> ; # Nil
which yields Nil because the .first method doesn't do anything useful with a single argument that's a list of the form (Numeric, :k).
Of course, you may occasionally want to pass a single argument that's a list of values in parens. But you can do so without using a colon. For the sake of clarity, it's my advice that you instead write this as:
invocant.a-method(( valuea, valueb, ... ));
a-sub ( arrgh1, arrgh2, ... ) ;
As just explained for method calls, this passes ONE argument to a-sub, namely the single list ( arrgh1, arrgh2, ... ) which will seldom be what the writer means.
Again, my advice is to instead write this as:
`a-sub( valuea, valueb, ... ) ;`
or:
`a-sub valuea, valueb, ... ;`
if you mean to pass multiple arguments, or if you wish to pass a list as a single argument, then:
`a-sub(( valuea, valueb, ... )) ;`
.a-method : arrgha, arrghb, ...
a-sub : arrgha, arrghb, ...
For the method form this will net you a "Confused" compilation error.
The same is true for the sub form if a-sub takes no arguments. If a-sub takes arguments you'll get a "Preceding context expects a term, but found infix : instead" compilation error.
.&a-sub
There's a call form which lets you call a routine declared as a sub -- but use the .method call syntax. The following feeds the "invocant" qux on the left of the dot as the first argument to a sub called a-sub:
qux.&a-sub
Use a : or parentheses as usual to pass additional arguments to a-sub:
sub a-sub ($a, $b) { $a == $b }
say 42.&a-sub(42), 42.&a-sub(43); # TrueFalse
say 42.&a-sub: 42; # True
(In my original version of this section I wrote that one can not pass additional arguments. I had tested this and thought one could not. But I must have just gotten confused by something. @Enheh's comment led me to retest and discover that one can pass additional arguments just as with ordinary method calls. Thank you @Enheh. :))
a-method( invocant: arg2, arg3, ... )
a-method invocant: arg2, arg3, ...
Called "Indirect object notation" in the design docs, these formats are an undocumented and very rarely seen form of method call in which the call mimics the method declaration -- the method name comes first and then the invocant followed by a colon:
say first <abc 2 def ghi>: Numeric ; # 2
Note that say is a sub call because the next token, first, isn't followed by a colon. In contrast first is a method call because the token after it is followed by a colon.
Footnotes
1 All comments about spaces/spacing in this answer ignore unspacing.