[Openmcl-devel] process-run-function and "dynamic vs lexical"
dlw at itasoftware.com
Thu Oct 22 20:26:16 UTC 2009
Taoufik Dachraoui wrote:
> ? (defvar x 1) ; x is special
> ? (let ((x 1)) ; without declaring x special it is special
> because of defvar (lost control)
> (let ((x 2)) ; so where in the specs that says (defvar name init)
> (+ x x))) ; must treat name as special everywhere even if it is
> 4 ; used in a let block, where let is supposed to create
> ; a new lexical variable binding (not special unless
> declared so)
No, that's not the Common Lisp language. If x is globally
declared special, and then you do a let naming x as a
variable, let does a dynamic binding operation.
> Again look at this example of a closure:
> (let ((x 1)) (defun foo (y) (+ x y)))
> It is obvious that if x is defvar'ed then the code will not behave as
> the programmer
That depends on what the programmer did, in fact, intend!
But if what you're saying is that this form does different
things depending on whether a global special declaration
has been established (as "defvar" does), that's right, it does.
> This closure definition is legitimate, but the programmer can use a symbol
> name that is defvar'ed without him knowing.
Yes, a programmer could be fooled by not being
aware that x has been declared special.
That's why the *x* convention is used.
Now, whether Lisp SHOULD work this way is a separate
discussion, but that's how Common Lisp is defined.
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