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Re: Contracts macro example

From: Maxime Devos
Subject: Re: Contracts macro example
Date: Wed, 20 Jul 2022 10:55:53 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0

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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