[Openmcl-devel] Where is the source of LET* macro

Shannon Spires svs at bearlanding.com
Fri Apr 14 14:33:28 PDT 2023

Tim's example is very interesting. Let's look at two forms seem like 
they should do the same thing, i.e. just funcall the c value.

  (let (c)
    (let* ((x 1)
           (x (progn (setf c (lambda () x)) 2)))
      (declare (special x))
      (values c (lambda () x)))))

  (let (c)
    (let ((x 1))
      (let ((x (progn (setf c (lambda () x)) 2)))
        (declare (special x))
        (values c (lambda () x))))))

Form B is just Form A reexpressed as nested lets rather than a single let*.

What's interesting is that CCL and Lispworks throw an error (unbound 
variable x) on Form A but return 1 on Form B.
SBCL returns 1 on both, without any error.

I don't know which implementation should be considered "correct."


On 4/14/23 3:01 PM, Tim Bradshaw wrote:
> I think this is surprisingly underspecified.  I think the only sane answer is that declarations should apply to all the bindings of a given name in a given let* (or other sequential-binding) form.
> It is possible to check this :
> (let (c)
>    (let ((x 1)
>          (x (progn (setf c (lambda () x)) 2))
>      (declare (special x))
>      (values c (lambda () x))))
> (sorry for paren/indentation errors, I'm typing this on a phone).  If the special declaration applies to both bindings then calling either function returned will be an error.  If it applies to only one: which, and why, and why does nothing say?  Either the spec simply omits this crucial information which would be a horrible omission, or the declarations apply to all the bindings.  Or, quite possibly I've just missed the place where it *does* say...
> --tim
>> On 14 Apr 2023, at 21:17, Arthur Cater <arthur.cater at ucd.ie> wrote:
>> I can only find a define-compiler-macro, I want to see how LET* handle declarations.
>> It surprises me that it is apparently legal to say
>> ? (let* ((it 7) (it (list it it)) (it (length it))) it)
>> 2
>> ?
>> and I wondered how declarations (if present) are treated - but I can’t find the source code.
>> TIA for any hep

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