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Re: Contracts macro example

From: Zelphir Kaltstahl
Subject: Re: Contracts macro example
Date: Sun, 24 Jul 2022 01:21:55 +0000

Hello Maxime!

I needed some time to implement the things you explained/mentioned, but I think I've got it now:

There is still one issue though: The placeholder of the ensure clauses is assumed to be at the top level. So I will have to rewrite the macro, which looks at the clauses and replaces the placeholder with the result of the function, so that it can replace the placeholder in an arbitrarily nested structure. Maybe it will be easy, maybe it will be difficult.

Thanks for all your ideas!

I don't understand the following thing or how it works yet:

(define-syntax require
   (syntax-error "'require' can only be used as part of a contract construct")))

Can you explain how it works?

Best regards,

On 7/20/22 10:55, Maxime Devos wrote:

On 20-07-2022 10:39, Zelphir Kaltstahl wrote:

It would also be nice to define a global 'require' and 'ensure' and
'<?>' somewhere (e.g.: (define-syntax require (identifier-syntax
(syntax-error "'require' can only be used as part of a contract
construct")))), that way, require / ensure / <?> can be renamed during
importing, so all contract things could be prefixed with, say,

I thought about implementing <?> for the insertion location of the result in a predicate, but initially wanted to keep it simple and get a simple version to work. I think I have seen this for pipelining in an SRFI before … *checks* … Maybe in, or, or maybe in some other repository.

Now that the basic version works, I can try to introduce the placeholder.

The idea is to define these globally in the module, so that they can be exported separately, so that they can be renamed upon import, correct?

How could a macro check, whether it is used inside something else? If the pattern matching only looks at the form of the macro itself, how can I get the "context", in which it was used and check, whether that is inside a `define-with-contract`? I think I have not yet unlocked that knowledge yet : )

That's one way to implement things (syntax-parameterize sounds useful here), but that sounds way more complicated than needs to be.  All you need to do is:

  * keep the original code
  * Add:
    (define <?> "consider define-syntax+identifier-syntax+syntax-error for
    better error messages but this will do for now)
  * Export <?> (at least, once your code is turned into a module, if the users
    of define-with-contract are in the same module as define-with-contract
    then exporting isn't required though harmless)
  * Likewise for 'require' and 'ensure'

By doing that, syntax-rules knows that its '<?>', 'require' and 'ensure' is not just the symbol '<?>' 'require' and 'ensure', but the _identifier_ (which keeps being the same identifier after renaming) '<?>', 'require' and 'ensure'.

(Note that as a consequence,  if you do that, (let ((require 0)) (define-with-contract foo (require) (ensure) (lambda _ 0))) will be a syntax error, because the 'require' in define-with-contract now refers to the variable 'require' from the let, not the identifier from your RnRS module).

That's all you need to do (untested)!



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