the OO metaphor

Vlad Dumitrescu <>
Tue Nov 28 23:46:03 CET 2000


The "thread thread" is very interesting, and talking about OO reminded me of 
an idea I got... I think it's not a very bad one, so I will share it with 
you, to see what you think.

The OO methodology can be easily applied to Erlang, once one gets to ffel 
the pulse of the language. There is however one thing that is missing, and 
that I think it could be useful. That thing is inheritance.

The natural way for inheritance to be implemented in Erlang isn't the C++ 
way, but the CLOS way, through some kind of generic functions. A better name 
than inheritance might be "shared implementation between modules". An 
example might be appropriate.

-module(base).

fun1(blip) -> foo;
fun1(A) -> {empty, A}.

==== and (the syntax is just a proposal)

-module(base_extended).
-extend({base, [fun1]}).

fun1({help}) -> hohoho;
-inherited(). %% need not be always present
fun1(_) -> false.

The meaning would be that the clauses of base:fun1 are imported into the 
ones of base_extended:fun1, allowing some of them or all to be overriden. 
The gain would be for example that when extending the lists module, one only 
needs to write implementations for the functions that are different.

There are problems with the above way of doing it, I know. One might want to 
reorder clauses, or to keep only part of the inherited ones... But it is the 
general idea that I'd like to discuss.

As someone said earlier (Ulf I think), this creates dependencies between 
modules. This isn't always a Good Thing. But on the other hand, when writing 
a gen_server, there is a dependency upon the gen_server module... When 
trying to implement a module based on another (and that can be used instead 
of the other), avoiding to write 50 stub functions that just delegate he job 
for the 2 or 3 that need rewriting is kind of a waste. Plus att the compiler 
might be able to optimize the calls.

It's almost midnight, and I'm not very sure I managed to explain properly.

I look forward to find out what a wonderful idea you all thin this is :-)

Or alternatively, to find out why it is such a bad idea!

regards,
Vlad
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