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copy-readtable (Function)

— Function: copy-readtable &optional from-readtable to-readtable readtable
Arguments and Values:

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

to-readtable—a readtable or nil. The default is nil.

readtable—the to-readtable if it is non-nil, or else a fresh readtable.


copy-readtable copies from-readtable.

If to-readtable is nil, a new readtable is created and returned. Otherwise the readtable specified by to-readtable is modified and returned.

copy-readtable copies the setting of readtable-case.

 (setq zvar 123)  123
 (set-syntax-from-char #\z #\' (setq table2 (copy-readtable)))  T
 zvar  123
 (copy-readtable table2 *readtable*)  #<READTABLE 614000277>
 zvar  VAR
 (setq *readtable* (copy-readtable))  #<READTABLE 46210223>
 zvar  VAR
 (setq *readtable* (copy-readtable nil))  #<READTABLE 46302670>
 zvar  123
See Also:

readtable, *readtable*

(setq *readtable* (copy-readtable nil))

restores the input syntax to standard Common Lisp syntax, even if the initial readtable has been clobbered (assuming it is not so badly clobbered that you cannot type in the above expression).

On the other hand,

(setq *readtable* (copy-readtable))

replaces the current readtable with a copy of itself. This is useful if you want to save a copy of a readtable for later use, protected from alteration in the meantime. It is also useful if you want to locally bind the readtable to a copy of itself, as in:

(let ((*readtable* (copy-readtable))) ...)