How can I idiomatically ignore undefined array elements (or avoid assigning them to an array in the first place)?

Question

If you assign an undefined value to an array, it will contain that undefined value, which complicates using loops if you don't want to iterate over undefined values:

my @bar = 1, 2, Any;

for @bar -> $baz {
    put $baz;
}

This gives the following output, including a warning for the undefined values:

1
2
Use of uninitialized value $baz of type Any in string context.
Methods .^name, .perl, .gist, or .say can be used to stringify it to something meaningful.
  in block  at for.p6 line 4

I know that I can deal with this explicitly in a number of way, for example:

for @bar -> $baz {
    next unless $baz; # Skip this iteration
    put $baz;
}

for @bar.grep: *.defined {    # Just iterate over the defined values
    put $baz;
}

# Reassign @bar to only contain defined values
@bar.=grep(*.defined);
for @bar -> $baz {
    put $baz;
}

But, isn't there a more idiomatic way to avoid @foo receiving undefined values or to avoid iterating over undefined values?

---

For the time being, when assigning to an array, I'm going to be using the following defined-flat function to avoid these situations:

multi defined-flat (@array) {
    return @array.grep(*.defined).flat;
}

# Return single defined element (or empty list for single undefined value)
multi defined-flat ($element) {
    return $element // ();
}

Answer 1

You also can try to use map to solve the problem:

my @a = [0, 1, 2, 3, Any].map({ $_ if $_.defined });

Or with race to enable multi-threads:

my @a = [0, 1, 2, 3, Any].race.map({ $_ if $_.defined });

Or >>.:

my @a = [0, 1, 2, 3, Any]>>.&({ $_ if $_.defined });

I prefer the last one :p

Answer 2

You could use duckmap

my @bar = 1, 2, Any;

@bar.duckmap: -> Mu:D $baz {
    put $baz;
}

duckmap -> Mu:D $baz {
    put $baz;
}, @bar;

# the “:” makes this a method call
duckmap @bar: -> Mu:D $baz {
    put $baz;
}

Answer 3

Also sometimes handy is the with statement, which both topicalizes and tests for definedness:

my @bar = 1, 2, Any;

for @bar -> $baz {
    put $_ with $baz
}

Or:

my @bar = 1, 2, Any;

for @bar {
    with $_ -> $baz {
        put $baz;
    }
}