“hash-table” objects vs. message-receiving objects
(Thanks to Julik for inspiring to think on the subject.)
In Io, JavaScript and Python there’s a model of “hash-table” objects. The object contains some slots, which you access and then decide what to do with them. If the slot appears to be a function, you can call it. In JS there’s a bit of magic: interpreter knows where the function just came from, so it can specify reasonable “this” pointer for function call. In Io there’s less smart decision: upon slot access interpreter checks “activatable” property of the object (not the slot entry, but the object this slot refers to!) and activates the object if it happens to be a method. However, in Io x := y is being a message setSlot(‘x’, y), so that you can hook in.
On the other hand, Ruby has a much stronger notion of message passing: you never ever can modify inner object data (that is, @ivars) without message send. The most simple @foo update happens through the accessor method called foo=.
However, from the implementation point of view, Ruby has two kinds of hash tables per object: method table and ivars table.