Course of Raku / Essentials / More about functions

Mind the space

Using functions in Raku is mostly intuitive and causes no problems, but you must be warned about the following aspect.

TL;DR

Do not add a space between the name of the function and the parentheses with its arguments. But use a space after for and if.

Long story

Let us take a simple function:

sub f($x, $y) {
    return $x + $y;
}

First of all, note the absence of spaces between the name of the function and the opening parentheses that keeps the parameters. Some coding styles in other languages may require a space there. Raku is fine with that, but this is not a practice to follow:

sub f ($x, $y) {
    return $x + $y;
}

On the calling side, the absence of space is important. If you call the function and omit the parentheses, then the space after the name of the function is unavoidable:

f 5, 6;

But if you add parentheses, you cannot have a space before them. This is a correct call:

f(5, 6);

And this call is wrong:

f (5, 6);

Such code causes an exception:

Too few positionals passed; expected 2 arguments but got 1
  sub f at t.raku line 1
  in block <unit> at t.raku line 5

Raku sees the call with a space f (5, 6) as an attempt to pass a single argument (5, 6) to the function. You can easily check the type of this argument if you rewrite the function to accept only one argument:

sub f($param) {
    say $param.WHAT;
}

f (5, 6); # (List)

So, calling f (5, 6) means that we are passing a list to the function. The list as a single entity.

When space is needed

In some cases, the space character is needed. That happens when you use parentheses with one of the keywords, such as if or for or loop. The following code is wrong:

if(True != False) {
    say 'OK';
}

The error message explains what’s wrong:

===SORRY!===
The word 'if' is interpreted as a 'if()' function call.  Please use
whitespace instead of parentheses.
at /Users/ash/raku-course/t.raku:1
------> if⏏(True != False) {
Unexpected block in infix position (two terms in a row)
at /Users/ash/raku-course/t.raku:1
------> if(True != False)⏏ {

With the case of if, parentheses are not required at all, so the problem have no chances to appear if you do not use them. But in some cases, as, for example, with loop, you need to have them, and the space becomes mandatory:

loop(my $c = 0; $c != 10; $c++) { say $c }

In this code, the same thing happens: loop(...) is understood by the compiler as a function call. Add a space to resolve the issue. Or better rewrite the fragment using for.

Conclusion

The bottom line: to avoid such issues, do not add a space between the name of the function and the parentheses with the arguments of the function and try to avoid parentheses if that’s possible with other language constructions such as control flow.