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reader-error

reader-error Condition Type

Class Precedence List:

reader-error, parse-error, stream-error, error, serious-condition, condition, t

Description:

The type reader-error consists of error conditions that are related to tokenization and parsing done by the Lisp reader .

See Also:

read, stream-error-stream, Section 23.1 (Reader Concepts)

Expanded Reference: reader-error​

The reader-error condition type​

reader-error is the condition type for errors related to tokenization and parsing by the Lisp reader. It inherits from both parse-error and stream-error.

(subtypep 'reader-error 'parse-error)
=> T
=> T

(subtypep 'reader-error 'stream-error)
=> T
=> T

Catching reader errors​

You can use handler-case to catch reader-error conditions signaled by read or read-from-string.

(handler-case
    (read-from-string ")")
  (reader-error (c)
    (format nil "Caught reader error: ~A" (type-of c))))
=> "Caught reader error: SIMPLE-READER-ERROR"
;; The exact message is implementation-dependent

Accessing the stream from a reader-error​

Since reader-error is a subtype of stream-error, you can use stream-error-stream to retrieve the stream that was being read when the error occurred.

(handler-case
    (with-input-from-string (s ")")
      (read s))
  (stream-error (c)
    (not (null (stream-error-stream c)))))
=> T

Common causes of reader errors​

Reader errors are signaled for unbalanced parentheses, invalid tokens, undefined dispatch macro sub-characters, and other malformed input.

;; Unbalanced parentheses
(handler-case
    (read-from-string "(a b")
  (end-of-file () :got-eof))
=> :GOT-EOF

;; Invalid sharpsign notation
(handler-case
    (read-from-string "#Z")
  (reader-error () :got-reader-error))
=> :GOT-READER-ERROR