On Tue, 2007-08-14 at 21:44 +0100, Jon Harrop wrote:

> You can also parameterize the type over metadata:
> 
>   type 'a expr =
>     | Int of int
>     | Var of string
>     | Seq of 'a seq
>   and 'a seq = {h: 'a expr; t: 'a expr array; meta: 'a};;
> 
> Now you can insert arbitrary metadata into Seq nodes.

This is not a precise description. You cannot put arbitrary
meta-data into the Seq nodes with this design, for a given
data structure, you can choose just one type of meta-data: this
is the same as for example a Hashtbl.

If you wish to *dynamically* vary the meta data, which is often
the case when you have a tool like a compiler that performs
phased analysis, a property list based on polymorphic variants
can be useful.

This is one of three ways I can think of to vary the data 
type dynamically across compilation unit boundaries.

A second way is to use mutable field of a class type
and change the object to a different implementation
of the same abstraction.

The third way is quite tricky and equivalent to the first:
use a function unit -> unit as the type, and execute it
to throw an exception. This is basically dynamic typing.

-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net