🚀 go-pugleaf

In article <paula.658247771@shum>, paula@SHUM.HUJI.AC.IL (Paula Ross) writes:
> Is there any way in ML that you can create a data type which accepts
> sequences of objects of different types (when the number and types of the
> objects are not known in advance) ?

When they *are* known in advance, then you can use a datatype as a variant
record (essentially):

   datatype Object = INTEGER of int
                   | STRING of string
                   | ...

If you don't know how many cases you need in advance, then ML is (in
theory) capable of expressing open-ended datatypes that you can add to.
This is currently only available for the built-in type "exn" (the type
of exceptions), so:

   exception A and B of int;
   val list = [A, B 4];
   exception C of string;
   val list = (C "Hello") :: list;


   (A is a constructor of type exn, B has type int -> exn).

But, since you have to introduce a new constructor for each new type you
wish to work with, you have to retain the constructor in order to take
a value apart for that type.

-- 
Nick Rothwell,	Laboratory for Foundations of Computer Science, Edinburgh.
		nick@lfcs.ed.ac.uk    <Atlantic Ocean>!mcsun!ukc!lfcs!nick
~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~ ~~
 "Now remember - and this is most important - you must think in Russian."