[racket-dev] When is it safe to not rename a runtime value in a macro?

From: Neil Toronto (neil.toronto at gmail.com)
Date: Sat Aug 25 12:54:08 EDT 2012

Then I suppose the obvious thing to do is move the optimizations up to 
the macro level? :)

It's also for debugging, by the way. Having to mentally or manually 
substitute bindings to understand fully expanded code annoys me and 
wastes time.

Neil ⊥

On 08/25/2012 10:24 AM, Robby Findler wrote:
> This is definitely a macro writer bill of rights situation. Too bad
> that TR's optimizer cannot take advantage of all these kinds of things
> that are already happening in the optimizer one level down.
> Robby
> On Sat, Aug 25, 2012 at 11:19 AM, Neil Toronto <neil.toronto at gmail.com> wrote:
>> A lot of macros start by renaming some syntax that's assumed to eventually
>> represent a runtime value, like the `or' macro does:
>>> (syntax->datum (expand-syntax #'(or #t #f)))
>> '(let-values (((or-part) '#t)) (if or-part or-part '#f))
>> But it's not always a good thing, particularly when the output will be
>> processed by another macro, and parts will be tested using an `identifier=?'
>> function. For example, my `inline-sort' takes `<' as an argument. So
>>    (inline-sort < a b)
>> expands to
>>    (let ([lt? <] [temp1 a] [temp2 b])
>>      (if (lt? temp1 temp2) (values temp1 temp2) (values temp2 temp1)))
>> Well, it did at first. When I discovered that Typed Racket's optimizer would
>> never detect that `lt?' is actually `<' and replace it with `unsafe-fl<', I
>> altered the macro to detect when the `<' is an identifier, and not rename it
>> with a `let'.
>> I know that I should assert my rights under the Macro Bill. But I like to
>> program defensively. So what's safe to not rename? I think the following
>> are:
>> symbol
>> number
>> string
>> bytes
>> null
>> character
>> regexp
>> prefab struct key
>> Also, should I ever have to intern syntax that's one of these kinds of
>> things?
>> Neil ⊥
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