Reading Lisp objects from a string

Question

I am familiar with the basic template for collecting the Lisp objects from a file, as in:

(with-open-file (stream "filename.lisp")
    (loop for object = (read stream nil 'eof)
          until (eq object 'eof)
          collect object))

But I'm not sure how to translate this into collecting the objects from a string, for example using read-from-string. Do you then have to keep track of the index where you left off in the string? Also, how do you avoid a name conflict with eof or any other legitimate Lisp object like nil or t in the input?

WITH-INPUT-FROM-STRING

Barmar
– Barmar

2017-03-17 22:33:36 +00:00
Commented Mar 17, 2017 at 22:33 — Barmar
– Barmar, Commented Mar 17, 2017 at 22:33

Barmar · Accepted Answer · 2017-03-19 06:15:33Z

5

You can use with-input-from-string to read from a string.

To prevent a conflict with the eof symbol, you can use an uninterned symbol or some other object that's created dynamically.

(with-input-from-string (stream "this is a (list of 3 things)")
  (loop with eof-marker = '#:eof
        for object = (read stream nil eof-marker)
        until (eq object eof-marker)
        collect object))

edited Mar 19, 2017 at 6:15

answered Mar 17, 2017 at 22:37

Barmar

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4 Comments

coredump Over a year ago

In another answer, sds pointed out that some people typically use the stream object itself: (read stream nil stream).

davypough Over a year ago

I get an undefined variable #:eof. Will everything still work properly with (make-symbol "eof")?

davypough Over a year ago

What I don't quite understand yet is that since the eof-marker is created before the string is read, why there couldn't be a variable conflict while reading from the string, for example while reading from "#:eof". Is it because the eof-marker is not interned?

Barmar Over a year ago

Yes, it's because it's not interned. So it's not possible for the reader to return that same symbol -- every time it reads #:eof it creates a new, non-interned symbol.

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