Why am I only getting a single element from this "for" statement?

Question

I am trying to generate FASTQ files containing 10 random sequences with random quality scores. I originally used the following code, which worked fine:

my @seq  = (rand_fa_seq() for ^10);
my @qual = (rand_qual()   for ^10);

@seq.perl.say;
@qual.perl.say;

sub rand_fa_seq
{
    return join("", roll(20,"ACGT".comb));

}

sub rand_qual
{
    return join("", roll(20,"EFGHIJ".comb))
}

However, wanting to simplify it even more, I thought that perhaps I could remove the parentheses from around the right hand statement. When I did so, I only got a single element in @seq and @qual.

my @seq  = rand_fa_seq() for ^10;
my @qual = rand_qual()   for ^10;

@seq.perl.say;
@qual.perl.say;

sub rand_fa_seq
{
    return join("", roll(20,"ACGT".comb));

}

sub rand_qual
{
    return join("", roll(20,"EFGHIJ".comb))
}

Is this a bug or is this the way it should behave? Without the parentheses is this a scalar context? Will the Great List Refactor change this behavior?

---

Versions of Perl 6 interpreters in which I have seen this behavior:

MoarVM:

perl6 version 2015.03-204-g8578022 built on MoarVM version 2015.03-60-g36d56f7

JVM:

perl6 version 2015.03-305-ga95107d built on JVM java version "1.7.0_79"
OpenJDK Runtime Environment (rhel-2.5.5.1.el7_1-x86_64 u79-b14)
OpenJDK 64-Bit Server VM (build 24.79-b02, mixed mode)

Answer 1

Statement modifiers are one of the hallmarks of Perl syntax (cf Perl 5's perldoc), and they survived the transition to version 6.

This means your statements are equivalent to

my @seq;
for ^10 { @seq = rand_fa_seq() }

my @qual;
for ^10 { @qual = rand_qual() }

ie you're assigning a fresh value 10 times in a row (and only the last one survives).

Note that this could also be written more succintly as

my @seq  = rand_fa_seq() xx 10;
my @qual = rand_qual() xx 10;

Also note that by default, the last statement in a sub provides the return value.

Assuming you might need sequences of length other than 20, you could parametrize that value, ending up with the following:

sub rand-fa-seq($n = 20) { <A C G T>.roll($n).join }
sub rand-qual($n = 20)   { <E F G H I J>.roll($n).join }

my @seq  = rand-fa-seq() xx 10;
my @qual = rand-qual() xx 10;

where I've used quote words instead of splitting a string.

Answer 2

The second example is like the classical perl5 control statements at the end of the line.

Like

say "true" if something();
say "yeah" for ^10;

if you would put a print statement within rand_qual() you would notice that it's still being executed 10 times.