[plt-scheme] Higher-order contracts for variable-arity functions

From: Carl Eastlund (carl.eastlund at gmail.com)
Date: Thu May 25 15:47:03 EDT 2006

On 5/25/06, Robby Findler <robby at cs.uchicago.edu> wrote:
> > > My point is that I don't care about the list.  I care about the
> > > function arguments.  I want to apply a function contract to each
> > > argument to my function.  If they get taken out of the list, or
> > > other
> > > values get put into the list, I don't care, because the list itself
> > > was only a convenient delivery mechanism for my many arguments.
> It is unsound in general to do that and in this specific case, the
> contract system can't tell that the list won't be mutated (if you
> follow the N negatives there) and so is being conservative.

I am not trying to apply a contract to a list.  I do not want to use
listof - my point is not to change listof, but to write an alternative
so that I may avoid using listof.  In my interpretation, the fact that
the arguments are delivered in a list is a byproduct.  Only the list's
elements are actually arguments to the function, so I want to apply a
contract to the arguments that are passed in.  Conceptually, I want to
consider the contract as coming before the "list" is built around the
rest arguments.  Pragmatically, this may mean the contract system has
to destructure the list, apply contracts to each element, and rebuild
the list.  This should be safe, though - the list is fresh anyway,
replacing it with a different one shouldn't change anything.

Carl Eastlund
"Cynical, but technically correct."

Posted on the users mailing list.