TL;DR

What’s the Raku equivalent of shift || 5 in Perl? The Raku community answered.

In previous post Brute forcing “The monkey and the coconuts” I did an observation where I preferred the Perl way of doing one thing:

Getting the input number of sailors from the command line is somehow worse though:

my $sailors = @*ARGS ?? @*ARGS[0].Int !! 5;

I like Perl better in this case:

my $sailors = shift || 5;

(Of course I hope someone will point out how to express this in Raku 😋)

My wish was kindkly fulfilled by gfldex:

 my $sailors = shift || 5;

literally translates to:

 my Int() $sailors = @*ARGS.shift // 5;

The coercer can coerce to many things ofc, as long as they can be coerced from a string. Like the following:

 my Date() $when = @*ARGS.shift // now;

That works because infix:<//> does a boolean check and then skips over the Failure when @*ARGS is already empty. In Raku we do have types and exceptions. So we need the tools to handle them.

Well, certainly thanks!

And, of course, something more to study.

I was intrigued by the usage of Int() where I would have expected Int actually. It turns out that they’re not the same and… Int() is correct, at least in this case.

Inputs in @*ARGS are strings (type/class Str), so we need to coerce them into integers (type/class Int). If we omit the parentheses we get an error:

my Int $integer = '12';
# OUTPUT: Type check failed in assignment to $integer; expected Int but
#         got Str ("12")

This is because Int tells Raku that we demand to receive an Int, and whatever is not compatible will have to be thrown away. (IntStr would still be OK, but it’s not what ends up in @*ARGs, sorry!).

On the other hand, it seems that the version with the parentheses asks Raku to do the coercion, if possible:

my Int() $integer = '12';
put $integer.^name;
# OUTPUT: Int

while still complaining when it’s not possible:

my Int() $integer = 'galook';
# OUTPUT: Cannot convert string to number: ...

Finding documentation about the usage of Int() proved surprisingly difficult for me, although I later understood why.

Again, gfldex came to the rescue, kindly sharing further details:

• Type coercion in variable declation follows the same rules as in Signature;
• a detailed description of how to provide coercers can be found here;
• coercion is very useful, as gfldex’s blog shows.

So much stuff to read now!!!