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Re: unhandled constant?

From: David Kastrup
Subject: Re: unhandled constant?
Date: Sat, 01 Feb 2020 15:23:12 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

Linus Björnstam <address@hidden> writes:

> On Sat, 1 Feb 2020, at 12:09, David Kastrup wrote:
>> Can you expand about the "expansion time and macro time separation"?
>> If we have
>> (define decl '())
>> (define (make-var n v) (list "var" n v))
>> (defmacro define-session (name value)
>>   (define (inner n v)
>>     (set! decl
>>         (cons
>>          (make-var n v)
>>          decl))
>>     )
>>   `(,inner ',name ,value))
>> (define-session foo 1)
>> (display decl)
>> (newline)
>> as stated, the local function "inner" is defined at macro time, but the
>> form
>> `(,inner ',name ,value)
>> does not export the _name_ inner but rather the defined function.  That
>> part naively appears to me like it should work; an "expansion time and
>> macro time" issue appears rather to be that inner calls make-var (and
>> accesses decl) which is only being defined at expansion time.
>> The error message, however, rather appears to complain about inner being
>> undefined rather than the definition of inner referring to undefined
>> entities.
> I am not sure what is really the problem. Either the inner function is
> not present at runtime due to separation of compile time and runtime,
> or it is a result of how defmacro re-introduces the result of the
> macro into the syntactic context of the macro usage (where inner is
> not visible). Either way, having the inner definition in the macro
> output will solve the problem of it not being visible.

This fails when byte-compiling, so it would appear that the equivalent

(defmacro ...
  (define local-fun ...
  `(,local-fun ...)))

is not able to represent the local (and thus ultimately anonymous)
function here.  Whether this is a general shortcoming or is conditioned
on local-fun calling functions not available at byte-compile time (in
which case those would need to get wrapped in eval-and-expand) I haven't
checked yet.

David Kastrup

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