How to define a function that can be used both in syntax transformers and ordinary code?

[Yushuo Xiao]

I am using syntax transformers to define macros in Racket. I want to create some helper functions to help me manipulate the syntax. However, the functions I defined outside the syntax transformer are not available inside the syntax transformer. For example, in the following code

(define (my-function x) (+ x 1))

(define-syntax my-macro

  (lambda (stx)

    (datum->syntax stx (my-function (cadr (syntax->datum stx))))))

I got the error "my-function: reference to an unbound identifier at phase: 1; the transformer environment".

After some searching, I am able to write the following code so that `my-function` is available inside the syntax transformer.

(begin-for-syntax

  (define (my-function x) (+ x 1)))

(provide (for-syntax my-function))

(define-syntax my-macro

  (lambda (stx)

    (datum->syntax stx (my-function (cadr (syntax->datum stx))))))

But the problem is, `my-function` is not available outside the syntax transformer this time. Sometimes I want to check those helper functions in ordinary code, so I need to be able to call it from both inside and outside the syntax transformer, just like the function `cadr`. How can I achieve that?

I know my question has something to do with Racket's syntax model, in particular the concept of "phase level", but I never really understand it. If you could provide some easy-to-follow tutorials explaining it I would even be more grateful.

(Also, I have asked the same question on StackOverflow before I am aware of this Google group. Is this place more suitable for asking Racket questions like this than StackOverflow? Should I delete the question on StackOverflow?)

[Jacob Jozef]

You can put your function in a module and require it both normally and for syntax.

#lang racket

 

(module my-function racket

(provide my-function)

(define (my-function x) (+ x 1)))

 

(require 'my-function (for-syntax 'my-function))

 

(define-syntax my-macro

  (lambda (stx)

    (datum->syntax stx (my-function (cadr (syntax->datum stx))))))

 

(my-macro 5)

(my-function 7)

 

See also cross phase persistent if you want one instantiation only.

Jos

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[Jacob Jozef]

Another solution:

 

#lang racket

 

(define-syntax (def-both-phases stx)

(syntax-case stx ()

  ((_ rest ...)

#'(begin

    (define rest ...)

     (define-for-syntax rest ...)))))

 

(def-both-phases (my-function x) (+ x 1))

 

(define-syntax my-macro

  (lambda (stx)

    (datum->syntax stx (my-function (cadr (syntax->datum stx))))))

 

(my-macro 5)

(my-function 7)

 

Jos

 

From: Yushuo Xiao
Sent: domingo, 9 de mayo de 2021 10:00
To: Racket Users
Subject: [racket-users] How to define a function that can be used both in syntax transformers and ordinary code?

 

I am using syntax transformers to define macros in Racket. I want to create some helper functions to help me manipulate the syntax. However, the functions I defined outside the syntax transformer are not available inside the syntax transformer. For example, in the following code

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