50g System RPL list processing

[Claudio L.]

(10-31-2014 10:06 PM)Gilles Wrote:  Note that the 50G use sometimes a '?' value (something like unknown or undetermined algebraic... For examplle lim (x->+oo,SIN(X)) -> ?
Could be interesting in some cases. A kind of NIL for algebraic

I like this idea better than the nil and one and then trying to operate on them. If the result is undetermined, it becomes symbolic, so any attempt to operate on it will return a symbolic with the unevaluated expression. Of course, we could call symbolic 'nil' (or '?' as it already exists) and symbolic 'one', and then we don't need to define any other operations. I think this is more generic, but still doesn't cover all cases, since it won't work for objects that are not accepted inside symbolics, like strings.
'?'+"HELLO" will give an error.
So '?' would have to be a hybrid that behaves like a symbolic when possible and like the nil/one solution proposed above for other objects. Or, we define nil/one and convert them to symbolic for the cases we don't know how to resolve like one+one => 'one+one'.
There doesn't seem to be one consistent solution that covers all cases, other than just issue the error on empty lists.