Merge pull request #136 from babbageclunk/23707

23707

Author: steveklabnik

src/23707.rs

@@ -0,0 +1,106 @@
+#![recursion_limit="2048"]
+
+use std::marker::PhantomData;
+use std::fmt;
+use std::fmt::Debug;
+
+pub struct Z( () );
+pub struct S<T> (PhantomData<T>);
+
+
+pub trait Nat {
+    fn sing() -> Self;
+    fn get(&self) -> usize;
+}
+
+impl Nat for Z {
+    fn sing() -> Z { Z( () ) }
+    #[inline(always)]
+    fn get(&self) -> usize {
+        0
+    }
+}
+
+impl<T : Nat> Nat for S<T> {
+    fn sing() -> S<T> { S::<T>( PhantomData::<T> ) }
+    #[inline(always)]
+    fn get(&self) -> usize {
+        let prd : T = Nat::sing();
+        1 + prd.get()
+    }
+}
+
+pub type N0 = Z;
+pub type N1 = S<N0>;
+pub type N2 = S<N1>;
+pub type N3 = S<N2>;
+pub type N4 = S<N3>;
+pub type N5 = S<N4>;
+
+
+pub struct Node<D : Nat>(usize,PhantomData<D>);
+
+impl<D:Nat> Node<D> {
+    pub fn push(&self, c : usize) -> Node<S<D>> {
+        let Node(i,_) = *self;
+        Node(10*i+c, PhantomData::<S<D>>)
+    }
+}
+
+impl<D:Nat> Node<S<D>> {
+    pub fn pop(&self) -> (Node<D>,usize) {
+        let Node(i,_) = *self;
+        (Node(i/10, PhantomData::<D>), i-10*(i/10))
+    }
+}
+
+impl<D:Nat> Debug for Node<D> {
+    fn fmt(&self, f : &mut fmt::Formatter) -> fmt::Result {
+        let s : D = Nat::sing();
+        write!(f, "Node<{}>: i= {}",
+               s.get(), self.0)
+    }
+}
+pub trait Step {
+    fn step(&self, usize) -> Self;
+}
+
+impl Step for Node<N0> {
+    #[inline(always)]
+    fn step(&self, n : usize) -> Node<N0> {
+        println!("base case");
+        Node(n,PhantomData::<N0>)
+    }
+}
+
+impl<D:Nat> Step for Node<S<D>>
+    where Node<D> : Step {
+        #[inline(always)]
+        fn step(&self, n : usize) -> Node<S<D>> {
+            println!("rec");
+            let (par,c) = self.pop();
+            let cnew = c+n;
+            par.step(c).push(cnew)
+        }
+
+}
+
+fn tst<D:Nat>(ref p : &Node<D>, c : usize) -> usize
+    where Node<D> : Step {
+        let Node(i,_) = p.step(c);
+        i
+}
+
+
+
+fn main() {
+    let nd : Node<N3> = Node(555,PhantomData::<N3>);
+
+    // overflow...core::marker::Size
+    let Node(g,_) = tst(nd,1);
+
+    // ok
+    //let Node(g,_) = nd.step(1);
+
+    println!("{:?}", g);
+}