Re: Contracts macro example

[Maxime Devos]

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,
> > contract:.
>
> 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 
> https://srfi.schemers.org/srfi-197/srfi-197.html, or 
> https://srfi.schemers.org/srfi-26/srfi-26.html, 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?
Yes.
> 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)!


Greetings,
Maxime

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