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*readtable*

∗readtable∗ Variable

Value Type:

a readtable.

Initial Value:

A readtable that conforms to the description of Common Lisp syntax in Chapter 2 (Syntax).

Description:

The value of *readtable* is called the current readtable. It controls the parsing behavior of the Lisp reader , and can also influence the Lisp printer (e.g., see the function readtable-case).

Examples:

(readtablep \*readtable\*) → true 

(setq zvar 123) → 123 
(set-syntax-from-char #\z #\’ (setq table2 (copy-readtable))) → T 
zvar → 123 
(setq \*readtable\* table2) → #<READTABLE> 
zvar → VAR 
(setq \*readtable\* (copy-readtable nil)) → #<READTABLE> 
zvar → 123

Affected By:

compile-file, load

See Also:

compile-file, load, readtable, Section 2.1.1.1 (The Current Readtable)

Expanded Reference: *readtable*​

Inspecting the current readtable​

*readtable* holds the current readtable, which controls how the Lisp reader parses input.

(readtablep *readtable*)
=> T

(readtable-case *readtable*)
=> :UPCASE

Binding a modified readtable locally​

The standard idiom for making temporary readtable modifications is to bind *readtable* to a copy.

(let ((*readtable* (copy-readtable)))
  (setf (readtable-case *readtable*) :preserve)
  (symbol-name (read-from-string "Hello")))
=> "Hello"

;; Outside the LET, the original readtable is still in effect
(readtable-case *readtable*)
=> :UPCASE

Resetting to the standard readtable​

Assigning a copy of the standard readtable (via nil) restores standard syntax.

(let ((*readtable* (copy-readtable)))
  ;; Make a modification
  (set-syntax-from-char #\z #\' *readtable*)
  ;; Reset to standard
  (setf *readtable* (copy-readtable nil))
  ;; Now z is a normal constituent again
  (with-input-from-string (s "zebra")
    (read s)))
=> ZEBRA

Custom reader syntax via readtable​

All reader macros and syntax changes operate through the current readtable.

(let ((*readtable* (copy-readtable)))
  (set-macro-character #\@
    (lambda (stream char)
      (declare (ignore char))
      (list 'at-sign (read stream t nil t))))
  (with-input-from-string (s "@foo")
    (read s)))
=> (AT-SIGN FOO)