Re: New lisp library -- cell.el

[Zhu Zihao <all_but_last@163.com>]

> Why not use nil for "none"?  That would simplify your code and most
> likely would make for much more natural/idiomatic code in the clients of
> your library.
> [ Traditionally in Lisp when you need something like a "option` (aka
>   `Maybe`) type, you use `(cons X nil)` for `some X` and `nil` for `none`.  ]

I tend to keep None variant a singleton of cell-option. nil is over-overloaded
in Lisp programming. For example, if we want to store a nullable value in the
slot of cl-struct and we use nil as none. It's hard to differentiate "Slot is
unintialized" and "Slot is initialized with None".

> Why bother with those indirections?

Docstring generated by `cl-defstruct` may not suitable for us. An alias to make
a meaningful doc.

Or should we use (put 'cell-box-inner 'function-documentation ".....")?

> That makes every call to `cell-weak-get` allocate a new symbol only to
> throw it away.  I'd recommend you define a top-level constant

Or use '(nil)? map.el use this. But IMO your solution (return nil for GCed weak ref) is better.