50g System RPL list processing

[Gilles]

Few thoughts after Claudio's explanations

1/

{6} ΣLIST --> 6
{"E"} ΣLIST --> "E"
{'a'} ΣLIST --> 'a'

idea is : 0+Value(s) -> Value (or ""+value(s) for string)

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{6} ΠLIST returns 6
{'a' } ΠLIST returns 'a'

idea is : 1*Value(s) -> value

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{} ΣLIST returns "Error :invalid dimension"
{} ΠLIST returns "Error :invalid dimension" (or '?' ,see below, because applies only on number/algebraic)
{} ΔLIST returns "Error :invalid dimension" (or '?', because applies only on number/algebraic)

Idea is :Sum, product, delta of nothing is invalid / undetermined


2/

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

50 ? *
gives ?

? SIN
gives ?
etc.

"Résult=" '?' +
gives
"Résult='?'"

3/

You have also the NOVAL wich is a type 19 object. But any attemps to calculate with NOVAL result in an error. By the way "XX=" NOVAL + returns "XX=NOVAL"


4/

Another big problem in RPL list processing is that DOSUBS or DOLIST command can return ... nothing ! whereas an empty list would be far more logical... I think it will be a real progress to change this....