This and other RFCs are available on the web at http://dev.perl.org/rfc/ =head1 TITLE True Polymorphic Objects =head1 VERSION Maintainer: Nathan Wiger <[EMAIL PROTECTED]> Date: 25 Aug 2000 Mailing List: [EMAIL PROTECTED] Version: 1 Number: 159 Status: Developing =head1 ABSTRACT Currently, using objects in numeric and string contexts is not very useful or easy: $r = new CGI; $z = $r + $x; # oops print "$r\n"; # double-oops You can use facilities such as C<tie> to help fix this issue, but C<tie> is limited and slow. You can also overload operators, but this is not flexible enough for many applications since it applies to a package (and not individual objects). This RFC proposes the concept of B<true polymorphic objects>, which are objects that can morph into numbers, strings, booleans, and much more on-demand. As such, objects can be freely passed around and manipulated without having to care what they contain (or even that they're objects). =head1 DESCRIPTION =head1 Overview The top-level syntax remains the same. As such, transition to Perl 6 is very smooth for most people, and in fact most users don't have to care about any of the following details. To them, this script will "just work": $y = Math->data(7); $x = 3; $name = getname("Nate"); if ( $x < 5 ) { $y += $x; if ( ! $name ) { $name = "The math whiz"; } } print "$name got $y"; # "Nate got 10" However, under the hood things might work drastically differently. In fact, C<$y> and C<$name> might well be polymorphic objects: $y = Math->data(7); # $y->CREATE, $y->STORE(7) $x = 3; # $x = 3 $name = getname("Nate"); # $name->CREATE, $name->STORE("Nate") if ( $x < 5 ) { # $x < 5 $y += $x; # $y->STORE($y->NUMBER->PLUS($x)) if ( ! $name ) { # $name || $name->BOOLEAN $name = "The math whiz";# $name->STORE("...") } } print "$name got $y"; # $name->STRING , $y->STRING Here, C<$y> and C<$name> are objects, but we don't have to care. These objects have a key property: I<context sensitivity>. They have numerous different methods which are each called only in specific instances. So, being called in a numeric context calls C<NUMBER>, whereas being called in a string context would call C<STRING>. Plus, operators are overloadable as well. This means that we might decide to overload C<+> to become a Java-like concatenation operator on our objects: $string = $name + "Wiger"; # $name->STRING->PLUS("Wiger") Yuck. :-) But it can be done, and that's pretty cool. =head2 Polymorphic Methods The following are the proposed methods for Perl 6 objects. Note that these methods are completely I<optional> for a class to define. If they are not defined, the object would retain its current behavior. The hooks are in Perl if you want them, otherwise they don't get in the way. Note that C<STRING>, C<NUMBER>, and C<BOOLEAN> are specialized forms of C<FETCH>. If you define them, they are used instead of C<FETCH> in the given context, otherwise C<FETCH> is used. Also note that the operators, when overloaded, behave similarly to 'use overload', but on an I<object-by-object basis>, rather than package-wide. Data Conversion and Access ------------------------------------------------- STRING Called in a string context NUMBER Called in a numeric context BOOLEAN Called in a boolean context Operator Overloading ------------------------------------------------- PLUS Called in + context MINUS Called in - context TIMES Called in * context DIVIDED Called in / context MODULUS Called in % context NUMCMP Called in <=> context NUMEQ Called in == context NUMNE Called in != context NUMLT Called in < context NUMGT Called in > context NUMLE Called in <= context NUMGE Called in >= context STRCMP Called in cmp context STREQ Called in eq context STRNE Called in ne context STRLT Called in lt context STRGT Called in gt context STRLE Called in le context STRGE Called in ge context BITAND Called in & context BITOR Called in | context BITXOR Called in ^ context BITNOT Called in ~ context Assignment and Existence ------------------------------------------------- CREATE Called in object creation STORE Called in an lvalue = context FETCH Called in an rvalue = context READLINE Called in a <> context PRINT Called in a print context PRINTF Called in a printf context DESTROY Called in object destruction There are undoubtedly other functions that should be added. However, the purpose of this RFC is to propose an idea and a basic interface. [1] Others proposed an alternative set of names for these functions, such as "OP_*" and "OP_-". However, I strongly disagree with these names. First, they're not English words or phrases, so are bad for humans. Second, they're not legal \w+ names, which is something we should not circumvent with special cases. Third, they're not consistent with already-existing Perl functions like C<FETCH> and C<STORE>. Finally, there is actually a good amount of ambiguity. For example, does "OP_*" refer to a unary or binary * context? With the word "TIMES" there is no such ambiguity. Now, whether the word "TIMES" is better than "MULTIPLY" is open for debate. I was trying to get the English verb context right: $x * $y # $x->NUMBER->TIMES($y) Which I think makes the function names intuitive and easy to remember. I left out any potential _'s because these are a pain to type, and no other Perl builtins have them. Also, note the C<CREATE> method is actually quite useful. This allows you to have multiple methods for creating objects, and C<CREATE> is always called first (so you can do your basic blessing in there). This is just like a C<BEGIN> for objects. Again, you don't have to use it, but the hooks are there. =head1 IMPLEMENTATION Let's not get ahead of ourselves...yet... =head1 MIGRATION This introduces new functionality, however p52p6 would have to catch any subs defined with the names listed above and warn the user that this sub name is now reserved for Perl. =head1 NOTES [1] Abiding by the KISS property (Keep It Simple, Stupid) =head1 REFERENCES RFC 49: Objects should have builtin stringifying STRING method RFC 73: All Perl core functions should return objects Thanks to brian d foy and Damian Conway for their input Thanks to Uri Guttman for suggesting CREATE on a different topic
