Re: [PATCH] local-eval, local-compile, and the-environment (v3)

[David Kastrup]

Mark H Weaver <address@hidden> writes:

> David Kastrup <address@hidden> writes:
>
>> Mark H Weaver <address@hidden> writes:
>>
>>> David Kastrup <address@hidden> writes:
>>>
>>>> I am not sure that the reasons for not permitting definition context in
>>>> local-eval are not of somewhat more theoretical than practical nature,
>>>
>>> There's at least one practical reason not to allow it, namely that it is
>>> _impossible_ to implement.  Consider this:
>>>
>>>   (let ((x 1))
>>>     (define (get-x) x)
>>>     (the-environment))
>>>
>>> If we allow (the-environment) to add definitions to the implicit
>>> `letrec',
>>
>> Then just let's not allow it.  Consider the body of local-eval to be
>> wrapped inside of an implicit (begin ...).
>
> I think you mean an implicit (let () ...).  If that's what you want,
> then you can do it yourself, and the result will be less likely to
> confuse.

What is confusing here?  "Obviously, definitions made in the scope of
local-eval can't retroactively affect the environment where
the-environment was called."  And now instead of continuing with the
unfriendly "For this reason, they are prohibited." we continue with "You
may consider the local-eval body as being wrapped inside of an implicit
(let () ...)."

I repeat: is there any valid construct under the currently proposed
local-eval code that would change its behavior?

>> Is there any currently valid construct that would change its
>> behavior?  If not, you _gain_ functionality, and the resulting
>> semantics are still straightforward to explain as far as I can see.
>
> I don't understand this at all.  Change what behavior?

The behavior of local-eval given arbitrary expressions.  Are there any
expressions currently valid that would change their behavior if the body
of local-eval were wrapped in an implicit (let () ...)?

> What functionality do you gain?

The ability to make definitions valid in the local-eval without having
to write (let () ...).  Again: any drawback that is more substantial
than "I just don't want you to do that"?

-- 
David Kastrup