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set-syntax-from-char

set-syntax-from-char Function

Syntax:

set-syntax-from-char to-char from-char &optional to-readtable from-readtable → t

Arguments and Values:

to-char—a character .

from-char—a character .

to-readtable—a readtable. The default is the current readtable.

from-readtable—a readtable designator . The default is the standard readtable.

Description:

set-syntax-from-char makes the syntax of to-char in to-readtable be the same as the syntax of from-char in from-readtable.

set-syntax-from-char copies the syntax types of from-char. If from-char is a macro character , its reader macro function is copied also. If the character is a dispatching macro character , its entire dispatch table of reader macro functions is copied. The constituent traits of from-char are not copied.

A macro definition from a character such as " can be copied to another character; the standard definition for " looks for another character that is the same as the character that invoked it. The definition of ( can not be meaningfully copied to {, on the other hand. The result is that lists are of the form {a b c), not {a b c}, because the definition always looks for a closing parenthesis, not a closing brace.

Examples:

(set-syntax-from-char #\7 #\;) → T 
123579 → 1235

Side Effects:

The to-readtable is modified.

Affected By:

The existing values in the from-readtable.

See Also:

set-macro-character, make-dispatch-macro-character, Section 2.1.4 (Character Syntax Types)

Notes:

The constituent traits of a character are “hard wired” into the parser for extended tokens. For example, if the definition of S is copied to *, then * will become a constituent that is alphabetic2 but that cannot be used as a short float exponent marker . For further information, see Section 2.1.4.2 (Constituent Traits).

Expanded Reference: set-syntax-from-char​

Copying syntax from one character to another​

set-syntax-from-char makes the syntax of to-char be the same as the syntax of from-char. This includes the syntax type and, if from-char is a macro character, its reader macro function.

;; Give z the syntax of the single-quote character
(let ((*readtable* (copy-readtable)))
  (set-syntax-from-char #\z #\')
  (with-input-from-string (s "zhello")
    (read s)))
=> (QUOTE HELLO)

Making a character behave as a comment​

You can copy the syntax of ; (semicolon) to another character to make it act as a line comment.

(let ((*readtable* (copy-readtable)))
  (set-syntax-from-char #\% #\;)
  (with-input-from-string (s "hello % this is a comment
world")
    (list (read s) (read s))))
=> (HELLO WORLD)

Making a digit a comment character​

A more dramatic example: making a digit act as a comment character. The rest of the line after it is ignored.

(let ((*readtable* (copy-readtable)))
  (set-syntax-from-char #\7 #\;)
  (with-input-from-string (s "12357 this is ignored
89")
    (list (read s) (read s))))
=> (1235 89)

Copying from a specific readtable​

The fourth argument from-readtable specifies which readtable to copy the syntax from. By default it is the standard readtable (nil).

;; Copy the standard syntax of ( to a custom readtable
(let ((rt1 (copy-readtable))
      (rt2 (copy-readtable)))
  ;; Modify ( in rt1 somehow, then restore it from the standard readtable
  (set-syntax-from-char #\( #\( rt1 nil)
  ;; ( in rt1 now has standard behavior
  (not (null (get-macro-character #\( rt1))))
=> T

Caveat: macro definitions look for the original character​

When copying the macro definition from a character like ", the copied function still looks for the same terminating character. Copying " to | would make | start a string that ends at the next ", not at the next |.

;; Copying " to another character: the terminator behavior
;; is implementation-dependent
(let ((*readtable* (copy-readtable)))
  (set-syntax-from-char #\^ #\")
  (with-input-from-string (s "^hello^")
    (read s)))
=> "hello"