[Nickle]Recursive datatypes (was: Tonight's topics)

[Carl Worth]

On Jul 24, Bart Massey wrote:
 > 3) There should be no way to create a type with no obviously
 >    well-defined values. 

Agreed.

 > In the absence of partial <undef>
 >    this means that
 >      typedef x;
 >      typedef struct {
 >         x y;
 >      } x;
 >    has to go, and also its singly and mutually recursive
 >    cousins.  I sincerely doubt anyone will miss them.

While I could probably live without this, what's the fundamental
difference between that struct definition and the following:

typedef struct {
  poly y;
} x;

where I can still assign y a value of type x? It seems the first
definition would actually be better for this purpose as it provides
type checking.

Maybe the difference is that there is a defined "zero of type poly"
but no "zero of type structured value"? If so, might it make sense to
create a new zero?

-Carl

-- 
Carl Worth                                        
USC Information Sciences Institute                 cworth@east.isi.edu
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