[Nickle] Funny situation

Keith Packard keithp at keithp.com
Sat Jun 28 12:28:59 PDT 2003

Around 0 o'clock on Jun 28, Bart Massey wrote:

> What am I doing wrong here?  Or is this a Nickle bug?

Recall that reference types are just syntactic sugar for pointer types:

      public typedef &elem list;
      public list empty = &nilcell;
  list l = empty;

Using the reference rules, we rewrite this as:

	public typedef *elem list;

	public list empty = &nilcell;

  list l = *empty;

As you can see, there is now a type mismatch -- reference types make 
naming the referenced value easy and naming the referring value hard.  You 
can "fix" this by using & here:

  list l = ∅

That's because initializers for reference variables are a bit mystic -- 
they compute the referring value rather than the referenced value.

To make this datatype work like you expect, you can either use pointers or 
you can move the union:

  namespace List {

      public typedef list;

      typedef elem;

      public typedef union {
          &elem cell;
          void nil;
      } list;

      typedef struct {
         poly first;
         list rest;
      } elem;

      public list cons(poly v, list l) {
          return (list) { cell = reference ((elem) { first = v, rest = l })};

      public poly car(list l) {
          return l.cell.first;

      public list cdr(list l) {
          return l.cell.rest;

      public bool null(list l) {
          union switch(l) {
              case cell c:
                  return false;
              case nil:
                  return true;

      public list empty = { nil = <> };


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