sub iter_to_seq {
    my ($iter) = @_;
    sub {
        my $val = $iter->();
        defined $val ? ($val) : ();
    };
}

sub seq_to_iter {
    my ($seq) = @_;
    sub {
        my @val = $seq->();
        @val ? [@val] : undef;
    };
}

enumerate_iter(
    seq_to_iter( iter_to_seq( iter( qw[a b c] ) ) )
);
#  0: (['a'])
#  1: (['b'])
#  2: (['c'])