if exists, select; if not, create then select... ??

okay. i seem to recall some discussion on pl/pgsql wherein a
function called via SELECT can't do an INSERT.

is there a way to do something like

    select get_or_create(...);

which runs some pl/pgsql such as

    select * from tbl where...;
    if not found
        insert into tbl values (...);
        select * from tbl where...;
    end if;
    return found stuff, even if we had to create it...

or not?

i'm looking for results similar to perl code like

    $something ||= &make_new();
    # which translates to (more or less, for this application)
    if ( ! defined( $something ) ) {
        $something = &make_new();
    }
    return $something;

i've tried conceiving of a rule that might intervene properly,
but i'm stymied...

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