[racket-dev] Class contracts: opaque or transparent?

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Fri Apr 27 18:01:30 EDT 2012

On Apr 27, 2012, at 5:55 PM, Asumu Takikawa wrote:

> You can write that, but the following isn't very useful:
>  (and/c (class/c #:opaque [m (->m number? number?)])
>         (class/c #:opaque [n (->m number? number?)]))
> Since the two class contracts both reject classes that the other would
> accept (unless `and/c` somehow merged the first-order checks of the
> two rather than checking both separately).


> It works if the two contracts mention exactly the same members.

That's the useless case. 

So I guess we're really giving up something with opaque contracts. 
Since you're the one who created most and you don't seem to mind 
copying contracts, I am fine with opaqueness. I won't use it myself, 
however, when I write contracts for classes. I have found the above
pattern too useful. 

-- Matthias

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