From 6d06141751f158b3d1a2699f6e4bc5ddb635e19c Mon Sep 17 00:00:00 2001 From: Greg Hendershott Date: Wed, 24 Oct 2012 20:23:13 -0400 Subject: [PATCH] Fix problem with syntax-parameterize. Need to change order of examples so that `it` is still undefined in the evaluator. (Is there a way to "reset" the evaluator, for situation like this?) --- index.html | 6 ++++-- main.rkt | 17 +++++++++-------- 2 files changed, 13 insertions(+), 10 deletions(-) diff --git a/index.html b/index.html index c29adfe..3ce6c7b 100644 --- a/index.html +++ b/index.html @@ -187,8 +187,10 @@ audience I’m writing for, you don’t know them yet. You can skip learning them now. (Someday if you want to understand someone else’s older macros, you can learn about them then.)

The syntax variation of them works similarly. The idea is, we’ll define it to mean an error by default. Only inside of our -aif will it have a meaningful value:

> (require racket/stxparam)
> (define-syntax-parameter it
    (lambda (stx)
      (raise-syntax-error (syntax-e stx) "can only be used inside aif")))
> (define-syntax-rule (aif condition true-expr false-expr)
    (let ([tmp condition])
      (if tmp
          (syntax-parameterize ([it (make-rename-transformer #'tmp)])
            true-expr)
          false-expr)))
> (aif 10 (displayln it) (void))

10

> (aif #f (displayln it) (void))

We can still use it as a normal variable name:

> (define it 10)
> it

10

If we try to use it outside of an aif form, and -it isn’t otherwise defined, we get an error like we want:

> (displayln it)

10

Perfect.

6 Robust macros: syntax-parse

TO-DO.

7 Other questions

Hopefully I will answer these in the course of the other sections. But +aif will it have a meaningful value:

> (require racket/stxparam)
> (define-syntax-parameter it
    (lambda (stx)
      (raise-syntax-error (syntax-e stx) "can only be used inside aif")))
> (define-syntax-rule (aif condition true-expr false-expr)
    (let ([tmp condition])
      (if tmp
          (syntax-parameterize ([it (make-rename-transformer #'tmp)])
            true-expr)
          false-expr)))
> (aif 10 (displayln it) (void))

10

> (aif #f (displayln it) (void))

If we try to use it outside of an aif form, and +it isn’t otherwise defined, we get an error like we want:

> (displayln it)

it: can only be used inside aif

But we can still define it as a normal variable:

> (define it 10)
> it

10

6 Robust macros: syntax-parse

TO-DO. +TO-DO. +TO-DO.

7 Other questions

Hopefully I will answer these in the course of the other sections. But just in case:

7.1 What’s the point of with-syntax?

Done.

7.2 What’s the point of begin-for-syntax?

TO-DO.

7.3 What’s the point of racket/splicing?

TO-DO.

8 References/Acknowledgments

Eli Barzliay wrote a blog post, Writing ‘syntax-case’ Macros, which explains many key details. However it’s written diff --git a/main.rkt b/main.rkt index 2cc1b02..5ec7231 100644 --- a/main.rkt +++ b/main.rkt @@ -724,13 +724,6 @@ define @racket[it] to mean an error by default. Only inside of our (aif #f (displayln it) (void)) ] -We can still use @racket[it] as a normal variable name: - -@i[ -(define it 10) -it -] - If we try to use @racket[it] outside of an @racket[aif] form, and @racket[it] isn't otherwise defined, we get an error like we want: @@ -738,12 +731,20 @@ If we try to use @racket[it] outside of an @racket[aif] form, and (displayln it) ] -Perfect. +But we can still define @racket[it] as a normal variable: + +@i[ +(define it 10) +it +] + @; ---------------------------------------------------------------------------- @section{Robust macros: syntax-parse} +TO-DO. +TO-DO. TO-DO. @; ----------------------------------------------------------------------------