Course of Raku / Advanced / Subroutines

Calling with a colon 🆕

When you call a method or a subroutine, you usually put its arguments in parentheses: @a.grep(* > 5). Raku offers a second form — put a colon after the name, and everything after the colon becomes the argument list, with no parentheses at all:

say (1..10).grep: * %% 2; # (2 4 6 8 10)

The colon in grep: does the same job as the parentheses would: (1..10).grep: * %% 2 means exactly (1..10).grep(* %% 2). The two forms are interchangeable:

say <a b c>.join('-'); # a-b-c
say <a b c>.join: '-'; # a-b-c

The colon form is most pleasant with methods that take a block or a Whatever expression, because it removes a layer of nesting. Compare:

my @a = 3, 1, 2;

say @a.sort({ $^b <=> $^a }); # parentheses around the block
say @a.sort: { $^b <=> $^a }; # colon — no closing paren to match

Both print (3 2 1), but the colon version reads more cleanly, especially when the block is long.

This is the method-call sibling of the parenthesis-less call you already use with list operators like say 1, 2, 3 — there, say takes everything after it as its arguments. The one rule to remember is that the colon form consumes the rest of the statement as arguments, so it has to come at the end of a call chain. That is why ('a' .. 'z')».uc».ord.grep: 60 < * < 70 works: grep is the last call, and there is nothing to chain after it. If you need to keep calling more methods, use the parenthesised form instead or group the parts of the while expression to make it clear:

say (('a' .. 'z')».uc».ord.grep: 60 < * < 70)».chr # (A B C D E)

One thing to watch: do not combine the colon and parentheses as .grep:( … ). That spelling clashes with the signature-literal syntax :( … ) and fails to parse. Use either .grep( … ) or .grep: … — one or the other, not both.

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