Editing Scheme (programming language) (section)

===Redefinition of standard procedures===
In Scheme, procedures are bound to variables. At R5RS the language standard formally mandated that programs may change the variable bindings of built-in procedures, effectively redefining them. (R5RS "Language changes")<ref name="r5rs"/>  For example, <code>+</code> can be extended to accept strings as well as numbers by redefining it:

<syntaxhighlight lang="Scheme">
(set! +
      (let ((original+ +))
        (lambda args
          (apply (if (or (null? args) (string? (car args)))
                     string-append
                     original+)
                 args))))
(+ 1 2 3)
===> 6
(+ "1" "2" "3")
===> "123"
</syntaxhighlight>

In R6RS every binding, including the standard ones, belongs to some library, and all exported bindings are immutable. (R6RS sec 7.1)<ref name="r6rs"/>  Because of this, redefinition of standard procedures by mutation is forbidden. Instead, it is possible to import a different procedure under the name of a standard one, which in effect is similar to redefinition.