In Common Lisp, how can one read & write symbolic expressions from/to streams? For example, I might want to write an anonymous function to file and then read and funcall it:
;;; sexp-io.lisp
;;; Try writing a sexp to file and reading it back in
(with-open-file (file "~/Documents/Lisp/Concurrency/sexp.lisp"
:direction :output :if-exists :supersede)
(print #'(lambda () (+ 1 1)) file))
(with-open-file (file "~/Documents/Lisp/Concurrency/sexp.lisp"
:direction :input)
(read file))
However, that code results in dubious output
#<Anonymous Function #x3020018F950F>
which does result in an error when I try reading it back in:
> Error: Reader error on #<BASIC-FILE-CHARACTER-INPUT-STREAM ("/Users/frank/Documents/Lisp/Concurrency/sexp.lisp"/7 UTF-8) #x3020018F559D>, near position 3, within "
> #<Anonymous ":
> "#<" encountered.
> While executing: CCL::SIGNAL-READER-ERROR, in process Listener(4).
You are doing TRT, except for #' which turns the list (lambda () (+ 1 1)) into a function object. Just replace the sharp-quote (which is read as function) with a simple quote (which is read as quote) and it should work.
Another change you might want to make is replacing print with write with argument :readably t:
(write my-object :stream out :readably t)
The benefit of :readably is that it fails if it cannot write in a way that will preserve print-read consistency.