Yes, that's basically right. One other point: the blame objects that a val-first projection gets have a #f instead of a negative party, and so you have to be careful to pass the #:missing-party argument to raise-blame-error when you discover a contract violation. And if you pass #f, the contract system builds a val-first projection for you.
FWIW, I think it is still as subject to change as it was when I wrote that note. :) The benefit this has is that more work can be done on the "server" side when putting a contract onto a value. With the standard-style projections, the blame argument can't be built until the client side, which means that, for some combinators, there is a bunch of work that gets duplicated on each different client side that could, in principle, be done only once, on the server side. In general, the only thing not known on the server side is the name of the client, so the val-first projections are set up to take that argument last. Of course, this works only for "first order" contracts. When a function takes a function as an argument that contract construction doesn't happen until much later and so we end up doing more work in that case. I had hoped that this would turn into a more significant savings for class/c than it did. I'm still hoping that I think of a better way to set things up so class/c can benefit more. Somehow. This is also related to the way that -> and friends have a special path. They can turn an N-ary function into an N+1-ary function where that extra argument is the negative party, making for a smoother path to get into a first-order function. As far as documentation goes, I hope to document and explain it more when I'm more sure it is a good idea. hth, Robby On Sat, Feb 7, 2015 at 11:05 PM, Alexis King <[email protected]> wrote: > The documentation for make-contract and similar functions includes an > optional #:val-first-projection keyword argument. What is this for? How does > it differ from the #:projection argument? > > The documentation does include this note: > > The precise details of the val-first-projection argument are subject to > change. > > > I don’t know how long that message has been there, but I somehow doubt that > the details are still “subject to change”. Could that argument be > documented? I’d at least like to understand what it’s for and how I’m > supposed to use it in new contract combinators. Also, what happens if I > leave it out or pass #f? > > As far as I can tell, the primary difference is that the procedure supplied > for val-first-projection takes an additional curried argument, the “negative > party”. This is handled specially for contract failures that fail on this > “first projection” by passing the argument to raise-blame-error via an > undocumented keyword argument, #:missing-party. What does this do, exactly? > > Thanks, > Alexis > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users > ____________________ Racket Users list: http://lists.racket-lang.org/users
