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

--- Robby Findler <robby at cs.uchicago.edu> wrote:

> At Thu, 25 May 2006 12:35:27 -0700 (PDT), Ryan Culpepper wrote:
> > --- Carl Eastlund <carl.eastlund at gmail.com> wrote:
> > 
> > > On 5/25/06, Robby Findler <robby at cs.uchicago.edu> wrote:
> > > > listof cannot accept higher-order contracts, due to the
> presence
> > > of
> > > > mutation. The tricky case is that someone hands you the list
> and,
> > > at
> > > > that point, the contract is check and the functions inside
> the
> > > list are
> > > > wrapped. Now, imagine that someone mutates the list -- what
> > > happens?
> > > > How do the contracts get onto the new functions in the list?
> (We
> > > could
> > > > change mutation operator, but we haven't taken that step ...
> yet)
> > > >
> > > > Note that if you could use immutable lists, that would work
> fine.
> > > >
> > > > Maybe time to petition any nearby R6RS committee members? ;)
> > > 
> > > 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.
> 
> > You can apply a contract to the rest args of a function using the
> > ->* contract constructor.
> > 
> > For the rest args, since you presumably don't plan to mutate the
> > list, how about using list-immutableof to construct the contract
> > for it? 
> 
> > Does list-immutableof applied to a mutable list return an
> > immutable copy?
> 
> It does not. That would be unsound (since removing the contract
> checking from a program that doesn't signal a contract violation
> might cause the program's result to change).

I see. It's unfortunate that rest arg lists are mutable, then.

Ryan