oldk1331 wrote:
>
>
> > Sorry that I did not comment earlier: this kind of change is
> > very dangerous. Namely, it can work quite nice in testing
> > and then lead to error say 3 years later. The point is
> > that there is correspondence between FriCAS types and
> > Lisp representation. Part of this correspondence are
> > known to Spad compiler and (via declarations) transmited to
> > Lisp compiler. So Lisp compiler is told that effectively
> > Record never is NIL. Breaking this can lead to nasty
> > errors when valid optimization is breaking "working" code.
>
> Another way to look at this "use NIL to represent empty tree"
> problem:
>
> We disallow the existence of empty tree.
>
> 1. Empty tree is not required by the definition of tree.
> 2. You can not construct an empty tree from existing
> and future operations of Tree:
Well, in FriCAS tree is an aggregate. And empty aggregate
is always legal. In fact, empty aggregate is a generic
way to start building an aggregate. So disallowing it
does not look right.
--
Waldek Hebisch
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