Re: [racket-users] Basic macro question

[keccak384]

On Sunday, February 5, 2017 at 10:04:47 PM UTC-5, Philip McGrath wrote:
> You need to have the lexical context information of x, y, and z come from 
> stx: otherwise, they will be protected by macro expansion (as a matter of 
> hygiene).
> 
> 
> Here's one way to do it:
> (define-syntax (a stx)
>   (syntax-parse stx
>     [(a)
>      (with-syntax ([x (datum->syntax stx 'x)]
>                           [y (datum->syntax stx 'y)]
>                           [z (datum->syntax stx 'z)])
>        #`(begin
>            (define x 97)
>            (define y 98)
>            (define z 99)))]))
> 
> 
> You may also want to look at format-id.
> 
> 
> Also, I found Greg Hendershott's Fear of Macros 
> (http://www.greghendershott.com/fear-of-macros/index.html) a useful 
> supplement to the Racket Guide/Reference in understanding the macro system: 
> ch. 4 addresses some of these issues.
> 
> 
> 
> -Philip
> 
> 
> 
> On Sun, Feb 5, 2017 at 8:41 PM,   wrote:
> I must be missing something simple here.
> 
> 
> 
> 229> (define-syntax a (lambda (stx) (syntax-parse stx [(a) #`(begin (define x 
> 97) (define y 98) (define z 99))])))
> 
> 230>(a)
> 
> 231>y
> 
> 232; y:undefined;
> 
> 233; cannot reference undefined identifier
> 
> 234; [,bt for context]
> 
> 
> 
> If the macro is given these ids, like (a x y z), then it will work, but can't 
> I also pick standard names like this in advance, or is that somehow 
> fundamentally "unhygienic"? Perhaps I have to generate the names in a place 
> visible to both the definition and use or something...
> 
> 
> 
> 
> 
> ## Peter
> 
> 
> 
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Ahh... datum->syntax, I thought I had seen something like this before. It is 
treating "a", or a's form, as the scope for the new ids essentially, but I can 
pick standard names. This just presupposes I only care to use my "a" macro once 
in any given scope. Does this really make it an unhygenic solution? 

In truth, I want many of these names generated and only care to use it once, so 
this seems better than passing in every name at the one macro invocation. 

Thanks!

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[racket-users] Basic macro question

[keccak384]

I must be missing something simple here.

229> (define-syntax a (lambda (stx) (syntax-parse stx [(a) #`(begin (define x 
97) (define y 98) (define z 99))]))) 
230>(a)
231>y
232; y:undefined; 
233; cannot reference undefined identifier 
234; [,bt for context]

If the macro is given these ids, like (a x y z), then it will work, but can't I 
also pick standard names like this in advance, or is that somehow fundamentally 
"unhygienic"? Perhaps I have to generate the names in a place visible to both 
the definition and use or something...


## Peter 

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