What does #+#. mean in lisp?

Question

It is almost impossible to google, hence my understanding is limited to contextual clues from reading through the slime source code: perhaps it is part of the object system in c

Answer 1

That's pretty rare to see.

#+clim clim:+red+ #-clim mygraphics:+red+

above means that the reader returns either red symbol and it depends whether there is a symbol with the name CLIM is on the list of features *features*. That's a built-in mechanism in Common Lisp.

#.(cl:if (cl:zerop (cl:random 2)) :high :low)

Above also is a mechanism of the reader. It allows to do computations at read time. Which btw. is a security problem and in Lisp applications it should be disabled - see the variable *read-eval* for controlling this. At read time the reader using READ will return either :HIGH or :LOW, randomly.

The combination #+#.(FOO) BAR means that the function foo returns a symbol at read time and this symbol then is checked by the reader if there is a symbol with this name on the feature list *features* and if that is the case, then the next item in input is read, otherwise the next item is skipped over.

Trivial example, IF always returns :CAPI in this example:

In LispWorks (where CAPI is on the features list):

CL-USER 41 > (read-from-string "#+#.(cl:if cl:t :capi :clim) a b")
A
31

In SBCL

* (read-from-string "#+#.(cl:if cl:t :capi :clim) a b")

B
32