Oha wrote:

I was thinking how to abuse the p6 grammar to simplify the interface to SQL DBs.

- First i wanted to have a SELECT which works like a map.
- Then i wanted to use a pointy block to bind fields to variables.
- And i wanted also to make it lazy.

Ah, you have touched on a matter that I care greatly about and have been working on for awhile, albeit more comprehensively with my Muldis database projects.

First off, see Anton Berezin's DBIx::Perlish Perl 5 module on CPAN, for some functional prior art of what it looks like you're trying to do.

Next, design-wise, you should be able to have much tighter and more natural syntactic integration than your example shows, such that you make the database access code look just like normal Perl code, wherein the database is just a Perl variable, as if you had something like this (I apologize if this isn't correct Perl 6, and I'm not sure how to specify a hash slice that returns pairs rather than just values):

  my %db is Database = FooDBMS.new(...);

  my ($state, $dept) = <active storage>;

  my @fetched_users := (%db<users> where {
      $_{'state'} eq $state and $_{'dept'} eq $dept
    })<username age salary>;

  for @fetched_users -> $user {
    my ($username, $age, $salary) = $user<username age salary>;
    say "fetched '$username'";

... or since your filter is just anded equality tests, you could alternately say for the main query (the "matching" takes a Hash second argument, matches by common key/field names):

  my @fetched_users := (%db<users>
    matching {state => $state, dept => $dept}
    )<username age salary>;

A relational database is structurally equivalent to a Hash of Set of Hash (or if you have multiple schemas, a Hash of Hash of Set of Hash) and you should be able to syntactically treat it like one in Perl. The various things you can do in a SQL SELECT can easily have Perl-defined routine/operators doing the same. A WHERE is just a "grep", picking a subset of fields is just a hash slice keeping pairs or "map", ORDER BY is just "sort". Joins and groups are a bit more complicated.

See also my functional Set::Relation Perl 5 module on CPAN, which demonstrates 

As for your "(select a.foo, b.bar)" question, don't do that; instead, rename (with "AS" in SQL") any same-named fields to have distinct names, and then the field names in your rowset can be a properly flat namespace.

On a side note, you shouldn't name things all uppercase since those are reserved for use by Perl itself; just say "select".

-- Darren Duncan

I come up to the following code:

    my $db = FakeDBI.new;
    my ($state, $dept) = <active storage>;

    my @out := $db.SELECT:
                    -> $username, $age, $salary {
                            say "fetched '$username'";
                            "$username => $salary, $age";
}, 'from users where state = ? and dept = ?', ($state, $dept);

    say "2nd: ", @out[1];
    say "1st: ", @out[0];
    say "4th: ", @out[3];

the fake module which make the above code to work (but do not really connect to a db):

    class FakeDBI {

            method SELECT(&block, $stm, *...@binds) {
                    my $sig = &block.signature;
                    my @fields = map {
$_.name ~~ /\$(\w+)/ or die "invalid signature params: "~$_.perl;
                    }, $sig.params;

                    my @stm = ("SELECT");
                    if @fields ~~ ('_') { push @stm, '*'; }
                    else { push @stm, join ', ', @fields; }
                    push @stm, $stm;

                    say 'DBG> prepare "', join ' ', @stm, '"';

                    gather {
                            say 'DBG> fake execute, fetching rows...';
                            for <john paul sara> -> $row {
my @data = map { $row ~ "_$_" }, @fields;
                                    take &block.(|@data);
                            say 'DBG> no more rows...';

and the output:

DBG> prepare "SELECT username, age, salary from users where state = ? and dept = ? "
    DBG> fake execute, fetching rows...
    fetched 'john_username'
    fetched 'paul_username'
    2nd: paul_username => paul_salary, paul_age
    1st: john_username => john_salary, john_age
    fetched 'sara_username'
    DBG> no more rows...
    4th: Any()

I found the usage very nice, and the lazyness may be removed by assigning instead of binding,
so i thought i should share this little useless example, HTH!

TODO: i would like to find a way to use the Signature for table names (select a.foo, b.bar),
but i don't want to make it too complex, any suggestion?


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