Re: [Ocaml-lib-devel] Next Step

[Brian H. <bh...@sp...>]

On 6 Oct 2003, skaller wrote:

> On Sun, 2003-10-05 at 19:59, Nicolas Cannasse wrote:
> > Hi people !
> > 
> > Right now, SF downloads are showing that there is (only) around 60 peoples
> > that downloaded the ExtLib (no counts on how many are actually using it :-).
> 
> Well, I'm concerned still with the semantics of Enum,
> and since Enum-extending other data structures is a key
> feature ..

I agree.  My laundry list of things I don't like about the current enum 
implementation:

1) Creating new enums is clumsy.

2) The next function returns 'a and throws an execption to indicate the 
end of elements in one place, and returns 'a option and returns None to 
indicate the end of elements in another.

3) Next both returns the "current" element and moves the enumeration 
forward to the next element.  These two different operations should be 
seperate functions.

4) Imperitive/side effect driven.  Given a choice, I prefer functional 
semantics.  Note that you can still do enumerations of imperitive 
functions (like reading a file).  What you do is cache the next object, 
making a "pseudo-functional" enumeration object.  If it's already returned 
a next enum, you return the cached value.  If it hasn't, it creates a next 
element, caches it, and returns it.  Effectively, the enumerations become 
a singly linked list.

I came up with a brilliant solution to all the of above, based upon 
objects and inheritance (IMHO an enumeration is an object).  
Unfortunately, I needed the following class to be typeable in Ocaml:

class virtual ['a] enum =
	object
		method virtual map: ('a -> 'b) -> 'b enum
	end;;

Unfortunately, this doesn't work (in 3.06 or 3.07).  Not being able to do 
this, my next best solution meant every time you called step (to move the 
enumeration forward) on an enumeration that had been mapped n times, you 
needed to create n+1 new objects, and calling current (to return the 
current element) you needed to create n option variables.  Which is why I 
haven't posted the code to the list :-).

Brian