Proper behavior of slot-initforms in defstruct?

[Kenneth Tilton]

On Mon, Aug 3, 2015 at 10:12 PM, Peter Stirling <peter at pjstirling.plus.com>
wrote:

> My read of the spec is that either behaviour is allowable.
>
> DEFSTRUCT[1] says:
>
>     "If a slot is not initialized in this way, it is initialized by
> evaluating *slot-initform* in the slot description at the time the
> constructor function is called."
>
> Which makes no reference to the environment in which the form should be
> evaluated in.
>
> Later on it says:
>
>     "It is as if the *slot-initforms* were used as *initialization forms*
> <http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_i.htm#initialization_form>
> for the *keyword parameters*
> <http://www.lispworks.com/documentation/HyperSpec/Body/26_glo_k.htm#keyword_parameter>
> of the constructor function"
>

Later later on it says:

 "The slot default init forms are evaluated in the lexical environment in
which the defstruct form itself appears and in the dynamic environment in
which the call to the constructor function appears."

So the defstruct sees the local/lexical serial-no but a call to make-bar
outside does not.

In between we get more clues: "The symbols which name the slots must not be
used by the implementation as the names for the lambda variables in the
constructor function, since one or more of thosesymbols might have been
proclaimed special or might be defined as the name of a constant variable. "

Indeed, if one compiles the example provided, when one *compiles *(ACL has
both an interpreter and compiler, and they vary) make-bar one learns:

   Warning: Free reference to undeclared variable serial-no assumed special.

I checked the spec on included structs but did not notice anything about
included slot init-form evaluation, but I guess it makes sense that
included slots be assembled and evaluated as if coded with the including
struct.

-hk



>
> The page describing keyword arguments in ordinary lambda lists[2] also
> makes no reference to which environment should be used for initforms.
>
> I guess they didn't think about people using closures for initforms when
> they were drawing it up?
>
> To side-step the issue I would invoke a closure from the initform instead
> (if that's how you want to do it).
>
> [1] http://www.lispworks.com/documentation/HyperSpec/Body/m_defstr.htm
> [2] http://www.lispworks.com/documentation/HyperSpec/Body/03_dad.htm
>
>
> On 03/08/15 23:04, Peter Stirling wrote:
>
> That makes no sense, the bindings in the let are independent.
>
>
> On 03/08/15 22:42, Laughing Water wrote:
>
> It looks like you need a LET* to guarantee the order of evaluation within
> your LET. Otherwise, it’s undefined.
>
> Laughing Water
>
> On Aug 3, 2015, at 3:27 PM, Jean-Claude Beaudoin
> <jean.claude.beaudoin at gmail.com> <jean.claude.beaudoin at gmail.com> wrote:
>
>
> Please consider the following code:
>
> (defparameter init-a 1)
>
> (let ((init-a 42) (serial-no 0))
>    (defstruct foo (a init-a) (b (incf serial-no)))
>    (defun get-foo-serial-no () serial-no))
>
> (defstruct (bar (:include foo)) (c 33) d)
>
> When one loads the above and then try to call #'make-bar the result
> varies widely from one lisp implementation (clisp) to another.
>
> clisp: (make-bar) --> #S(BAR :A 1 :B 1 :C 33 :D NIL)
> ccl: (make-bar) --> <enter the debugger saying: "Unbound variable:
> SERIAL-NO">
>
> lispworks, allegro and sbcl also behave more or less like ccl.
>
> What is the proper ANSI-CL behavior in this case here?
> Is clisp right in evaluating the slot initform in its "proper" lexical
> context?
> Or is the correct behavior to replicate the slot initform verbatim
> in the sub-structure constructor regardless of its original lexical
> context
> like the others do?
>
> I guess that this question has probably been asked before, in a somewhat
> distant past, but my google skills have not been sharp enough to find it,
> sorry.
>
>
>
>
>
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-- 
Kenneth Tilton
54 Isle of Venice Dr
Fort Lauderdale, FL 33301

ken at tiltontec.com
http://tiltontec.com
@tiltonsalgebra

646-269-1077

"In a class by itself." *-Macworld*
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