Actually, postwalk expansion (if that is in fact what you were describing)
would ignore any binding forms created by the outer macro. This means that
something simple like:
(defmacro with-db [db & body]
`(with-open [~db (create-db)]
~@body))
would be expanded without any knowledge of the 'db' local variable, since
that would only get turned into a let form later. Prewalk is pretty much
the only way this works.
On Wed, Sep 4, 2013 at 9:50 AM, Ben Wolfson <wolf...@gmail.com> wrote:
> Postwalk expansion would break macros that inspect their argument forms
> for e.g. writing special-purpose queries, if they *also* adopt the symbols
> "and" and "or" for conjunction or disjunction. Korma's "where", for
> instance, does this; one can write
>
> (select my-table (where (and (...) (...))))
>
> And the "where" detects the "and".
>
> Arguably this is wrongheaded behavior from the get-go (it can be somewhat
> confusing and makes it necessary to use something like clojure.core/and
> within "where" if you want normal clojure-land "and" semantics), but it's a
> style of non-anaphoric macro that relies on receiving an unexpanded form.
>
>
> On Wed, Sep 4, 2013 at 9:27 AM, Zach Tellman <ztell...@gmail.com> wrote:
>
>> So "complete recursive expansion" is postwalk macroexpansion? It seems
>> like that could break anaphoric macros, and likely others. A macro has the
>> option of calling macroexpand-all on its own contents if it wants only
>> special forms, but it shouldn't be forced to take only special forms.
>>
>> Also, here's a sketch of how you could do symbol macros using Riddley:
>> https://gist.github.com/ztellman/6439318. Please let me know if I'm
>> missing something w.r.t. how symbol macros are done in tools.macros.
>>
>> Zach
>>
>>
>> On Wed, Sep 4, 2013 at 2:16 AM, Konrad Hinsen <
>> googlegro...@khinsen.fastmail.net> wrote:
>>
>>> **
>>> On Wed, Sep 4, 2013, at 09:25 AM, Zach Tellman wrote:
>>>
>>> I'm not sure what you mean by "complete recursive expansion". Could
>>> you expand
>>> on that?
>>>
>>> Completely ;-)
>>>
>>> By complete recursive expansion I mean that you get a form that is fully
>>> reduced to
>>> the core language, i.e. it contains no more macro applications at any
>>> level.
>>>
>>> If you leave macro expansion to the compiler, it does it when it arrives
>>> at the
>>> macro during evaluation. Then it does a plain non-recursive macroexpand
>>> and goes on
>>> evaluating. Any macro thus has access to the unexpanded contents of its
>>> form, but
>>> not to what it eventually expands to. For many applications that's just
>>> fine, which
>>> is why this approach has been the default in the Lisp world for a long
>>> time.
>>>
>>> As for replicating the behavior of the compiler, I'd assert that
>>> unless &env is
>>> precisely what it would be without ahead of time macroexpansion, the
>>> compiler's
>>> behavior isn't being replicated.
>>>
>>> I agree. tools.macro predates &env, which is why it is not supported.
>>> Since I have
>>> never need &env support and nobody ever asked for it (before now), it's
>>> not there.
>>> I don't see any reason why it couldn't be supported.
>>>
>>> Konrad.
>>>
>>>
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>
>
>
> --
> Ben Wolfson
> "Human kind has used its intelligence to vary the flavour of drinks, which
> may be sweet, aromatic, fermented or spirit-based. ... Family and social
> life also offer numerous other occasions to consume drinks for pleasure."
> [Larousse, "Drink" entry]
>
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