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Asked: 2026-05-11T13:06:18+00:00

Does REMOVE ever return the same sequence in any real implementations of Common Lisp?

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Does REMOVE ever return the same sequence in any real implementations of Common Lisp? The spec suggests that it is allowed:

The result of remove may share with sequence; the result may be identical to the input sequence if no elements need to be removed.

SBCL does not seem to do this, for example, but I only did a crude (and possibly insufficient) test, and I’m wondering what other implementations do.

 CL-USER> (defparameter *str* 'bbb') *STR* CL-USER> *str* 'bbb' CL-USER> (defparameter *str2* (remove #\a *str*)) *STR2* CL-USER> (eq *str* *str2*) NIL CL-USER> *str*  'bbb' CL-USER> *str2* 'bbb'
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1 Answer

  1. Returning the original string could be useful. In case no element of a string gets removed, returning the original sequence prevents allocation of a new sequence. Even if a new sequence has been allocated internally, this new sequence could be turned into garbage as soon as possible.

    CLISP for example returns the original string.

    [1]> (let ((a 'abc')) (eq a (remove #\d a)))  T
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