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POHow to tell a lisp reader function to ignore errors during parsing
 

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  1. POHow to tell a lisp reader function to ignore errors during parsing
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    <p>I need a way to suppress any error messages raised when parsing code using <code>read-from-string</code> so that I can read from <code>Clojure</code> code using something like this:</p> <pre><code>(let* ((string-with-code " '(func UpperCase \"string\")") (brace-pos (position #\( string-with-code)) (end-of-token (+ brace-pos (position #\Space (subseq string-with-code brace-pos)))) (first-token (subseq string-with-code (incf brace-pos) end-of-token))) (format t "Found function: '~a'" (read-from-string first-token))) ;; ==&gt; Found function: 'func' </code></pre> <p>It basically prints the function name from the lisp code in the string. It works Ok until you try to use the dot operator(<code>.</code>) as the first item in the list. <code>Clojure</code> uses the <code>.</code> both to <code>cons</code> and to access classes in a Java package and hence valid code like: </p> <pre><code>(defmacro chain ([x form] `(. ~x ~form)) ([x form &amp; more] `(chain (. ~x ~form) ~@more))) </code></pre> <p>would cause an error: </p> <pre><code>*** - READ from #&lt;INPUT STRING-INPUT-STREAM&gt;: token "." not allowed here </code></pre> <p>if I were to walk it printing every function in the code. I want <strong>a way to ignore/suppress the error messages from <code>read-from-string</code></strong> for this code to work preferably without modifying the way the lisp reader works.</p> <p><strong>EDIT :</strong></p> <p>A complete program: </p> <pre><code>(defvar string-with-code "(defmacro chain ([x form] `(d ~x ~form)) ([x form &amp; more] `(chain (. ~x ~form) ~@more))) ") (defvar end-of-token 0) (defvar first-token 0) (defun functions-in-string (code) (let ((bpos (position #\( code))) (unless (null bpos) (setq end-of-token (+ bpos (position #\Space (subseq code bpos)))) (setq first-token (subseq code (incf bpos) end-of-token)) (ignore-errors (format t "Found function: '~(~A~)'~%" (read-from-string first-token))) (functions-in-string (subseq code end-of-token))))) ;; (ignore-errors ;; (functions-in-string 0 code)) (functions-in-string string-with-code) </code></pre> <p><strong>OUTPUT :</strong> </p> <pre><code>Found function: 'defmacro' Found function: '[x' Found function: 'd' Found function: '[x' Found function: 'chain' ;; You'll get the error below if ignore-errors wraps around the function call ;; *** - READ from #&lt;INPUT STRING-INPUT-STREAM&gt;: token "." not allowed here </code></pre>
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    1. COWhat does it mean 'ignore'? Should it do anything useful? Just return with no result?
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      1. POHow to tell a lisp reader function to ignore errors during parsing
      1. USRainer Joswig
    2. COLisp has notoriously simple syntax, but parsing it the way you do is... well, not really a good idea. I'd suggest you look into `cl-yacc`. This will make your life much easier, and, as a side effect, will also solve this one particular problem with dot character.
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    3. COIt's not really a good idea to parse Clojure code with a Common Lisp reader. One approach would be to reprogram the reader to accept things like dots as symbols and similar things. You don't need to write a Clojure parser, but one which can read Clojure s-expressions.
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      1. POHow to tell a lisp reader function to ignore errors during parsing
      1. USRainer Joswig
 

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