zero() might not be implemented correctly #25

Activity

1. ebfull commented on Oct 21, 2015

   

   ebfull

   on Oct 21, 2015

   Author

   More actions

   edit: actually, I understand succ now. My point above still stands.
2. ebfull commented on Oct 21, 2015

   

   ebfull

   on Oct 21, 2015

   Author

   More actions

   It occurs to me that you could actually combine these two operations into one:

   use std::marker::PhantomData;
   
   struct Foo<E, P>(PhantomData<(E, P)>);
   struct Var<N>(PhantomData<N>);
   struct S<N>(PhantomData<N>);
   struct Z;
   
   trait Pop<N> {
       type FinalE;
       type FinalP;
   }
   
   impl<A, B> Pop<Z> for (A, B) {
       type FinalE = B;
       type FinalP = A;
   }
   
   impl<A, B: Pop<N>, N> Pop<S<N>> for (A, B) {
       type FinalE = B::FinalE;
       type FinalP = B::FinalP;
   }
   
   impl<N, E: Pop<N>> Foo<E, Var<N>> {
       fn pop(self) -> Foo<E::FinalE, E::FinalP> {
           Foo(PhantomData)
       }
   }
   
   fn test1<A>(a: Foo<(A, ()), Var<Z>>) -> Foo<(), A> {
       a.pop()
   }
   
   fn test2<A, B>(a: Foo<(A, (B, ())), Var<S<Z>>>) -> Foo<(), B> {
       a.pop()
   }
   
   fn main() {}

   I can't imagine a situation where you wouldn't want .succ() until you had to .zero(), since you can't do anything else anyway.
3. Munksgaard commented on Oct 21, 2015

   

   Munksgaard

   on Oct 21, 2015

   Owner

   More actions

   Compare zero to succ. I believe zero should be restoring the original environment when it pops the protocol. That is, zero should return Chan<E, P>.

   That is definitely another possible design, but I think it has a couple of drawbacks: Mainly, the current design allows us to do looping without having to enter the protocol scope every time we recurse:

   fn foo(c: Chan<(), Rec<Var<Z>>) {
       let mut c = c.enter;
       loop {
           // Do some work
           c = c.zero();
       }
   }

   If we were to modify the library as you suggest, looping would instead look something like this:

   fn foo(mut c: Chan<(), Rec<Var<Z>>) {
       loop {
           let _c = c.enter()
           // Do some work
           c = _c.zero();
       }
   }

   Furthermore, to accommodate this, we'd have to change the signature of enter to the following:

   enter: Chan<E, Rec<P>>  -> Chan<(Rec<P>, E), P>
   

   I think that the current design is better, but that's just my opinion :)

   NB: Forgive me for any typos or syntax errors, I haven't tested the code above.

   It occurs to me that you could actually combine these two operations into one

   This is an interesting idea, but I'll have to consider it more in detail later today.
4. ebfull commented on Oct 21, 2015

   

   ebfull

   on Oct 21, 2015

   Author

   More actions

   If we were to modify the library as you suggest, looping would instead look something like this:

   Interesting! Thank you for the insight.

   I modified my "combine into one operation" idea to fit those semantics:

   use std::marker::PhantomData;
   
   struct Foo<E, P>(PhantomData<(E, P)>);
   struct Var<N>(PhantomData<N>);
   struct S<N>(PhantomData<N>);
   struct Z;
   
   trait Pop<N> {
       type FinalE;
       type FinalP;
   }
   
   impl<A, B> Pop<Z> for (A, B) {
       type FinalE = (A, B);
       type FinalP = A;
   }
   
   impl<A, B: Pop<N>, N> Pop<S<N>> for (A, B) {
       type FinalE = B::FinalE;
       type FinalP = B::FinalP;
   }
   
   impl<N, E: Pop<N>> Foo<E, Var<N>> {
       fn pop(self) -> Foo<E::FinalE, E::FinalP> {
           Foo(PhantomData)
       }
   }
   
   fn test1<A>(a: Foo<(A, ()), Var<Z>>) -> Foo<(A, ()), A> {
       a.pop()
   }
   
   fn test2<A, B>(a: Foo<(A, (B, ())), Var<S<Z>>>) -> Foo<(B, ()), B> {
       a.pop()
   }
   
   fn main() {}
5. Munksgaard commented on Oct 21, 2015

   

   Munksgaard

   on Oct 21, 2015

   Owner

   More actions

   Ah, yes, I see what you're doing! Combining succ and zero into one operation like this is a great idea, as far as I can tell, especially if it doesn't require any additional annotations from the user (which it doesn't look like this will). I'd be interested to see an actual PR for this, if you want to make one?
6. ebfull commented on Oct 21, 2015

   

   ebfull

   on Oct 21, 2015

   Author

   More actions

   Sure.