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Tue, 12 May 2009

Subroutines vs. Methods - Differences and Commonalities


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This is mostly a response to s1n's post about subs and methods in Perl 6. He maintains that since methods are very common in classes, a sub declaration inside a class should automatically get the semantics of a method. The reason was a deep confusion about what the difference is, and if they are different at all (especially since class methods and subs look very similar at first).

I want to stress that both are quite different in some ways, and indeed deserve different keywords. I won't discuss multi subs and multi methods, because they add nothing new to the points I want to make.

Methods

A method is a piece of code that is associated with an object, which we call the invocant (at least in Perl land). It can never be called without the invocant.

The lookup behaviour of a method is independent of the calling code's scope ore namespace; in the abstractest sense an object passes deals with method calls by passing that method call to its "meta" object (ie that things that manages the object under the hood).

The default meta object looks for that method in the actual class of the object, and if it can't find the method there it'll fall back the parent classes.

Note that this mechanism might be rather different for calls to foreign languages; if you call a method on a Lisp object, then the dispatch will happen with Lisp semantics.

In Perl 6, all methods automatically get access to the invocant through the self keyword.

Note that "class" or "static" methods are still ordinary objects, just the invocant is the class itself (or the type object), not an instance of that class.

Subroutines

Subroutines, short subs, are pieces of code that don't have an invocant, and live in a symbol table, a lexical scope or an ordinary variable holding it.

Calling a sub (that's not stored in an ordinary variable) involves looking down the lexical scopes, and falling back to the symbol table.

Subroutines in classes

Usually subroutines live in modules, but they can also exist in classes. However they don't have access to any attributes of that class, just to outer lexicals (as any piece of code in Perl 6 has).

Or to phrase it differently, it's like placing a subroutine in the namespace that the class happens to introduce anyway.

The "different things should look different" paradigm

In Perl 5, often things that do quite different things look very much the same. One example is reverse, which does string reversal in scalar context, and list reversal in list context. That implied that print reverse "abc"; actually prints abc, which is very confusing to the beginner, and still annoying to everybody else.

It also uses the same keyword for subs and methods, because in Perl 5 there's no difference between the two, only the invocation makes the difference. That proofed rather difficult for introspection and static code analysis.

The proposal to automatically promote subs to methods inside classes brings us back to the dangerous part of having different things looking similar; it also raises the question how you declare subroutines (not methods) inside a class. We'd have to invent a new syntax for that.

Then when somebody asks how do I write a sub in Perl?, the answer would be you simply write sub name { ... }, except in classes, where you write really_a_sub name { ... }. I don't want that kind of answers in our Perl 6 FAQs.

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