Set intersection using hash slices

my @colours = qw/red green yellow orange white mauve blue ochre
                 pink purple gold silver grey brown steel/;
my @find = qw/red blue green black/;
my %colours;
@colours{@colours} = @colours;
my @found = grep { defined } @colours{@find};
print join( ', ', @found );

This prints "red, blue, green". Adapted from a Perl.sig Perl Tip on slicing arrays and hashes.

Lambdaish Perl

Here's a snippet I wrote to test code references.

my $f = sub {
  my $a = shift;
  &$a();
};
&$f( sub { print "hej\n"; } );

Type cast in Perl DBI

Problem: A seemingly innocuous SQL can turn out to be very strenuous for a database if it needs to do type conversion. Worst case scenario is a full table scan just because the WHERE clause contains mismatching types in a comparison. Imagine, for example, that the column id in the table customers below is a VARCHAR. If we're talking to an Oracle database and send it an integer, the resulting execution plan could be near disastrous.

SELECT *
FROM customers
WHERE id = ?

In Perl, an untyped language where strings and integers are interchangeable, how can I ensure that I'm sending a string via DBI to the database if the string only contains numerals?

Solution #1: Wrap the value in a string:

$sth->execute( "$indata" );

Perl is more of a "semi-typed" language than a totally untyped language, and keeps track of whether a scalar is a string or an integer. The only reason the values are interchangeable is because of the transparent type conversion.

Solution #2: Use the explicit bind_param method and specify the data type:

use DBI qw(:sql_types);
[...]
$sth->bind_param( 1, $indata, SQL_VARCHAR );        # index, value and type
$sth->execute();

Not to be confused with nomología

This blog is my personal programming blog. It's not about the study of law or jurisprudence. As evidenced by the subtitle, I am using the second meaning of the word "nomology". I want a space where I can talk specifically about source code and programming languages, so I'll only be talking about the physical and logical laws as evident in and applied to computer programming.