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

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Sat Aug 25 13:03:51 EDT 2012

It's the wrong way to go. Let's investigate more before we jump to conclusions. 

On Aug 25, 2012, at 12:54 PM, Neil Toronto wrote:

> 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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>>>  http://lists.racket-lang.org/dev
> _________________________
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