[Openmcl-devel] multiple read-time conditionals, puzzlement

[Tim McNerney]

Nice investigating. If you want my educated opinion, I think you found a misleading example (#+f #+f x y), explored it to a reasonable extent, and discovered a CCL reader bug:  

     (setf *features* '(:a :b :c)) followed by '(#+a #-b #+c a b c) reading as (C)

--Tim

> On Mar 13, 2023, at 06:06, Arthur Cater <arthur.cater at ucd.ie> wrote:
> 
> Reading a bit of ccl’s sources I came across something that surprised me, and I thought it was neat. It was of the form
> 
>   #+feat #+feat
>  :keyword argument
> 
> which I thought nicer than the sort of thing I’d been doing, namely  #+feat :keyword #+feat argument
> 
> 
> But it set me thinking, and experimenting, and now I’m puzzled, and I wonder if anyone can explain this to me.
> 
> 
> 
> ? 
> NIL
> ? (setf *features* '(:a :b :c))
> (:A :B :C)
> 
> ? '(#+a #+a a b c)
> (A B C)
> 
> ? '(#+a #-b a b c)
> (B C)
> 
> ? '(#+a #+b #-c a b c)
> (B C)
> 
> ? '(#+a #-b #+c a b c)
> (C)
> ? 
> 
> 
> I thought I understood what was going on until this last one. Why does #-b cause both A and B to be skipped?
> 
> Arthur