How to generate a string out of an (error)object without actual printing?

Question

I want to retrieve the string, generated by write for further processing without doing any actual output, but write seems always to also output into the REPL

CL-USER>(let ((err-string (write (make-instance 'error) :stream nil)))
          (do-awesome-stuff-with-string err-string))
<ERROR> ;;this is the printing I want to get rid of
"awesome-result"

Why does write still outputs into the REPL, and how do I get rid of that?

Answer 1

You can use with-output-to-string for this. Here's an example:

(flet ((do-awesome-stuff-with-string (string)
         (concatenate 'string string " is awesome!")))
  (let ((err-string (with-output-to-string (s)
                      (write (make-instance 'error) :stream s))))
    (do-awesome-stuff-with-string err-string)))
;; => "#<ERROR {25813951}> is awesome!"

Here's here's the HyperSpec entry on with-output-to-string.

The reason (write (make-instance 'error) :stream nil) doesn't work is that the :stream argument to write is a stream designator and in that context nil is shorthand for *standard-output*. (The fact that format instead takes nil to mean that it should return a string is a common point of confusion).

Answer 2

Keep in mind that portably errors are made with MAKE-CONDITION. The standard does not say that errors are CLOS classes, so MAKE-INSTANCE might not work in some implementations.

There are two simple ways to get a string:

a) a textual description:

CL-USER 15 > (princ-to-string (make-condition 'error))
"The condition #<ERROR 4020311270> occurred"

b) the error object printed:

CL-USER 16 > (prin1-to-string (make-condition 'error))
"#<ERROR 402031158B>"