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Re: return macro

From: Taylan Kammer
Subject: Re: return macro
Date: Mon, 28 Jun 2021 03:15:34 +0200
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

On 28.06.2021 01:10, Damien Mattei wrote:
> hi,
> i wanted to create a macro that is used like a function definition and
> allow return using call/cc:
> (define-syntax def
> (syntax-rules (return)
> ((_ (name args ...) body body* ...)
> (define name (lambda (args ...)
> (call/cc (lambda (return) body body* ...)))))
> ((_ name expr) (define name expr))))
> unfortunaly i got error:
> scheme@(guile-user)> (def (test x) (cond ((= x 3) 7) ((= x 2) (return 5))
> (else 3)))
> ;;; <stdin>:2:42: warning: possibly unbound variable `return'
> scheme@(guile-user)> (test 2)
> ice-9/boot-9.scm:1685:16: In procedure raise-exception:
> Unbound variable: return
> any idea?

Hi Damien,

This is because of the "hygiene" rule of Scheme, where the notion of "lexical
scope" is taken very seriously: for an identifier to be bound, it must be
visible in the lexical (textual) surroundings where it has been bound.

So for instance, in the following example code:

  (def (test x)
     ((= x 3) (return 7))
     ((= x 2) (return 5))))

We can't see a binding for "return" anywhere in the text, therefore it cannot
be bound.

This is good "default" behavior because it makes code more flexible and easier
to understand.

An easy way of overcoming this issue is to let the user explicitly name the
return identifier however they like:

  (define-syntax def
    (syntax-rules ()
      ((_ (name ret arg ...) body body* ...)
       (define (name arg ...)
         (call/cc (lambda (ret) body body* ...))))))

Now you could define:

  (def (test return x)
     ((= x 3) (return 7))
     ((= x 2) (return 5))))

Or for instance:

  (def (test blubba x)
     ((= x 3) (blubba 7))
     ((= x 2) (blubba 5))))

However, sometimes you're sure that you want to make an implicit binding for
an identifier, and for those cases you need to write an "unhygienic" macro
which can be achieved with the more complex macro system "syntax-case".

Here's how your desired macro could be defined.  I will use identifiers like
"<foo>" just for easier readability; they don't have any special meaning:

  (define-syntax def
    (lambda (stx)
      (syntax-case stx ()
        ((_ (<name> <arg> ...) <body> <body>* ...)
         (let ((ret-id (datum->syntax stx 'return)))
           #`(define (<name> <arg> ...)
               (call/cc (lambda (#,ret-id) <body> <body>* ...))))))))

There's a few things here to take note of:

- Unlike with syntax-rules, the syntax-case is contained in a lambda which
  takes a single argument: a "syntax object" which is passed to syntax-case

- Unlike with syntax-rules, the "body" of the macro (where it begins with a
  'let') is not immediately part of the generated code; that 'let' is actually
  executed during compile-time.  The body of the macro must result in an
  object of the type "syntax object" that represents the generated code.

- You see that I define a variable called "ret-id" which I bind to the result
  of the expression:

    (datum->syntax stx 'return)

  which means "create a syntax object in the same lexical environment as stx,
  and is represented by the symbol 'return'."

- The actual code generation begins within the #`(...) which is a shorthand
  for (quasisyntax (...)) just like '(...) is short for (quote (...)).  The
  result of a quasisyntax expression is a syntax object.  Basically, it's the
  most convenient way of creating a syntax object, but like syntax-rules it's
  also hygienic by default and you need to insert "unhygienic" syntax objects
  into it explicitly.

- Within the quasisyntax, I use #,ret-id which is short for (unsyntax ret-id)
  to inject the unhygienic syntax object that holds the symbol 'return' into
  the generated code.

For someone used to macros in the Common Lisp or Elisp style, this may seem
over-complicated.  It's the cost of the "hygienic by default" behavior.

By the way I assume that you're just toying around with the language to learn.
If you were thinking of using a 'def' macro like this in real code, I would
discourage it because there's already a built-in mechanism that allows the
programmer something very similar, called 'let/ec':

  (import (ice-9 control))

  (define (test x)
    (let/ec return
        ((= x 3) (return 7))
        ((= x 2) (return 5)))))

As you see it allows you to define a "return" keyword anywhere, and it
doesn't need to be part of a function definition, it can appear anywhere.


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