[plt-dev] syntax-local-lift-expressions

From: Jay McCarthy (jay.mccarthy at gmail.com)
Date: Thu Jun 4 13:03:28 EDT 2009

I'm trying to use syntax-local-lift to lift the definition of a
structure to the top level.

syntax-local-lift-module-end-declaration doesn't work because it is
not guaranteed to be in an expression context, so I can't have a

syntax-local-lift-expression doesn't work because it only works for
establishing a single binding, whereas a structure definition requires
3+2n bindings.

I thought that I might be able to simulate this:

#lang scheme
(require (for-syntax scheme/list))

(define-for-syntax (syntax-local-lift-expression/values how-many stx)
  (define the-values-id
    (syntax-local-lift-expression (quasisyntax/loc stx
(call-with-values #,stx (lambda the-values the-values)))))
   (lambda (which-value)
     (syntax-local-lift-expression (quasisyntax/loc stx (list-ref
#,the-values-id #,which-value))))))

(define-syntax (testit stx)
  (syntax-case stx ()
     (let ([the-values
              (syntax-local-lift-expression/values 3 (syntax/loc stx
(values 1 2 3)))])
          (quasisyntax/loc stx
            (begin #,@(map (lambda (value-id)
                             (quasisyntax/loc stx
                               (printf "~S~n" #,value-id)))
     (syntax/loc stx (printf "Lifted~n"))]))


But I cannot, because syntax-local-lift-expression puts things in the
reverse order of the calls. This is expanded in the macro stepper to:

(define-values (lifted.6:84) (#%app:85 list-ref:86 lifted.0:87 '2))
   (define-values (lifted.4:88) (#%app:89 list-ref:86 lifted.0:87 '1))
   (define-values (lifted.2:90) (#%app:91 list-ref:86 lifted.0:87 '0))
   (define-values (lifted.0:87)
      (#%app:93 values:86 (quote 1) (quote 2) (quote 3))
      (lambda:94 the-values:86 the-values:86)))
   (#%app:96 call-with-values:96 (lambda:96 () (#%app:95 printf:86
(quote "Lifted~n"))) print-values:96)
   (#%app:97 printf:86 (quote "~S~n") lifted.2:90)
   (#%app:98 printf:86 (quote "~S~n") lifted.4:88)
   (#%app:99 printf:86 (quote "~S~n") lifted.6:84)

So... can we get either of these changed? Either a native
syntax-local-lift-expressions or put the syntax-local-lift-expression
calls in the order they are given, rather than the reverse?


Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University

"The glory of God is Intelligence" - D&C 93

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