Made an actual working example!

src/examples.rs

@@ -10,8 +10,6 @@ use crate::scratch;
 
 fn cube_thing() -> Rule {
 
-    let mesh = prim::cube();
-
     // Quarter-turn in radians:
     let qtr = std::f32::consts::FRAC_PI_2;
 
@@ -28,24 +26,42 @@ fn cube_thing() -> Rule {
         geometry::Rotation3::from_axis_angle(z, -qtr).to_homogeneous(),
     ];
 
-    let rec = || -> RuleEval {
+    let rec = move |self_: Rc<Rule>| -> RuleEval {
         let gen_rulestep = |rot: &Mat4| -> Child {
             let m: Mat4 = rot *
                 Matrix4::new_scaling(0.5) *
                 geometry::Translation3::new(6.0, 0.0, 0.0).to_homogeneous();
             Child {
-                rule: Rule { eval: Box::new(rec) },
+                rule: self_.clone(),
                 xf: m,
                 vmap: vec![],
             }
         };
 
         RuleEval {
-            geom: mesh.clone(),
+            geom: prim::cube(),
             final_geom: prim::empty_mesh(),
             children: turns.iter().map(gen_rulestep).collect(),
         }
     };
+    // I can't really do *mutual* recursion with the above, can I? I'd
+    // need actual functions for that.
+
+    // Also: 'turns' above is a problem.  I can't clone it *inside* the
+    // closure because it doesn't live long enough (it is out of scope
+    // when the closure runs).  I can't move it, or Fn becomes
+    // FnOnce. It doesn't implement Copy, and really can't, because it
+    // has vectors inside.
+    //
+    // (I guess I could use Rc<OpenMesh> instead if I want cheap
+    // support for Copy.  Or... just put prim::cube() right inside.)
+    //
+    // what did I learn from this? That "constants" outside the
+    // closure only work the way I think they should work if:
+    // - they're actually static
+    // - they implement Copy
+    // - the closure can move them
+    
     Rule { eval: Box::new(rec) }
 }
 
@@ -492,7 +508,16 @@ pub fn main() {
         println!("Writing {}...", fname);
         mesh.write_stl_file(&fname).unwrap();
     }
-    */
+     */
+    
+    fn run_test_iter(r: &Rc<Rule>, iters: usize, name: &str) {
+        println!("Running {}...", name);
+        let (mesh, nodes) = Rule::to_mesh_iter(r.clone(), iters);
+        println!("Evaluated {} rules", nodes);
+        let fname = format!("{}.stl", name);
+        println!("Writing {}...", fname);
+        mesh.write_stl_file(&fname).unwrap();
+    }
     
     /*
     run_test(CubeThing::init(), Rule { eval: CubeThing::rec }, 3, "cube_thing");
@@ -516,6 +541,11 @@ pub fn main() {
     // let f = 20;
     // run_test_iter(Twist::init(f as f32, 32), 100*f, "twist2");
 
+    let rule = Rc::new(cube_thing());
+
+    run_test_iter(&rule, 3, "cube_thing3");
+
+    if false
     {
         let a = vec![1,2,3];
         

src/rule.rs

@@ -1,5 +1,6 @@
 use crate::openmesh::{OpenMesh, Tag, Mat4};
 //use crate::prim;
+use std::rc::Rc;
 
 /// Definition of a rule.  In general, a `Rule`:
 ///
@@ -7,7 +8,7 @@ use crate::openmesh::{OpenMesh, Tag, Mat4};
 /// - tells what other rules to invoke, and what to do with their
 /// geometry
 pub struct Rule {
-    pub eval: Box<dyn Fn() -> RuleEval>,
+    pub eval: Box<dyn Fn(Rc<Rule>) -> RuleEval>,
 }
 // TODO: It may be possible to have just a 'static' rule that requires
 // no function call.
@@ -51,7 +52,7 @@ pub struct RuleEval {
 pub struct Child {
 
     /// Rule to evaluate to produce geometry
-    pub rule: Rule,
+    pub rule: Rc<Rule>,
 
     /// The transform to apply to all geometry produced by `rule`
     /// (including its own `geom` and `final_geom` if needed, as well
@@ -79,11 +80,11 @@ impl Rule {
     /// Convert this `Rule` to mesh data, recursively (depth first).
     /// `iters_left` sets the maximum recursion depth.  This returns
     /// (geometry, number of rule evaluations).
-    pub fn to_mesh(&self, iters_left: u32) -> (OpenMesh, usize) {
+    pub fn to_mesh(s: Rc<Self>, iters_left: u32) -> (OpenMesh, usize) {
 
         let mut evals = 1;
 
-        let rs: RuleEval = (self.eval)();
+        let rs: RuleEval = (s.eval)(s.clone());
         if iters_left <= 0 {
             return (rs.final_geom, 1);
             // TODO: This is probably wrong because of the way that
@@ -96,7 +97,7 @@ impl Rule {
 
         let subgeom: Vec<(OpenMesh, &Vec<usize>)> = rs.children.iter().map(|sub| {
             // Get sub-geometry (still un-transformed):
-            let (submesh, eval) = sub.rule.to_mesh(iters_left - 1);
+            let (submesh, eval) = Rule::to_mesh(sub.rule.clone(), iters_left - 1);
             // Tally up eval count:
             evals += eval;
             
@@ -112,7 +113,7 @@ impl Rule {
     /// This should be identical to to_mesh, but implemented
     /// iteratively with an explicit stack rather than with recursive
     /// function calls.
-    pub fn to_mesh_iter(&self, max_depth: usize) -> (OpenMesh, usize) {
+    pub fn to_mesh_iter(s: Rc<Self>, max_depth: usize) -> (OpenMesh, usize) {
 
         struct State {
             // The set of rules we're currently handling:
@@ -133,7 +134,7 @@ impl Rule {
         // (usually because they involve multiple rules).
         //
         // We evaluate our own rule to initialize the stack:
-        let eval = (self.eval)();
+        let eval = (s.eval)(s.clone());
         let mut stack: Vec<State> = vec![State {
             rules: eval.children,
             next: 0,
@@ -162,7 +163,7 @@ impl Rule {
             
             // Evaluate the rule:
             let child = &s.rules[s.next];
-            let mut eval = (child.rule.eval)();
+            let mut eval = (child.rule.eval)(child.rule.clone());
             eval_count += 1;
 
             // Make an updated world transform:

src/scratch.rs

@@ -118,3 +118,180 @@ impl Foo5 {
     }
 }
 
+// This is valid and I can recurse:
+struct W {
+    b: Box<dyn Fn() -> W>,
+}
+struct Foo6 {}
+impl Foo6 {
+    fn fn1(s: &Rc<Self>) -> W {
+        let s2 = Rc::clone(&s);
+        W { b: Box::new(move || Self::fn1(&s2)) }
+    }
+    fn fn2(s: &Rc<Self>) -> (W, W) {
+        let s2 = Rc::clone(&s);
+        let w2 = W { b: Box::new(move || Self::fn1(&s2)) };
+        let s3 = Rc::clone(&s);
+        let w3 = W { b: Box::new(move || Self::fn1(&s3)) };
+        (w2, w3)
+    }
+}
+
+fn foo6() {
+
+    // Whatever (note that it doesn't automatically do Copy):
+    struct State {
+        v: u32,
+    }
+
+    // Purposely put state somewhere it goes out of scope:
+    let s = {
+        let s_orig = State {
+            v: 105,
+        };
+        Rc::new(s_orig)
+    };
+    /*
+    let fn1 = |f: &dyn Fn(&dyn Fn() -> W) -> (&dyn Fn() -> W)| -> (&dyn Fn() -> W) {
+        &(|| -> W {
+            let s2 = Rc::clone(&s);
+            W { b: Box::new(move || f(f)) }
+        })
+    };
+
+    let f2 = fn1(fn1);
+    */
+}
+
+fn foo7(t: impl Clone) -> impl Clone {
+    t.clone()
+}
+
+fn foo7b<T: Clone>(t: T) -> T {
+    t.clone()
+}
+
+fn foo7c<T>(t: T) -> T where T: Clone {
+    t.clone()
+}
+
+// A simple implementation of the Y Combinator
+// λf.(λx.xx)(λx.f(xx))
+// <=> λf.(λx.f(xx))(λx.f(xx))
+ 
+// CREDITS: A better version of the previous code that was posted here, with detailed explanation.
+// See <y> and also <y_apply>.
+ 
+// A function type that takes its own type as an input is an infinite recursive type.
+// We introduce a trait that will allow us to have an input with the same type as self, and break the recursion.
+// The input is going to be a trait object that implements the desired function in the interface.
+// NOTE: We will be coercing a reference to a closure into this trait object.
+ 
+trait Apply<T, R> {
+    fn apply(&self, f: &dyn Apply<T, R>, t: T) -> R;
+}
+ 
+// In Rust, closures fall into three kinds: FnOnce, FnMut and Fn.
+// FnOnce assumed to be able to be called just once if it is not Clone. It is impossible to
+// write recursive FnOnce that is not Clone.
+// All FnMut are also FnOnce, although you can call them multiple times, they are not allow to
+// have a reference to themselves. So it is also not possible to write recursive FnMut closures
+// that is not Clone.
+// All Fn are also FnMut, and all closures of Fn are also Clone. However, programmers can create
+// Fn objects that are not Clone
+ 
+// This will work for all Fn objects, not just closures
+// And it is a little bit more efficient for Fn closures as it do not clone itself.
+impl<T, R, F> Apply<T, R> for F where F:
+  Fn(&dyn Apply<T, R>, T) -> R
+{
+  fn apply(&self, f: &dyn Apply<T, R>, t: T) -> R {
+    self(f, t)
+ 
+    // NOTE: Each letter is an individual symbol.
+    // (λx.(λy.xxy))(λx.(λy.f(λz.xxz)y))t
+    // => (λx.xx)(λx.f(xx))t
+    // => (Yf)t
+  }
+}
+ 
+// This works for all closures that is Clone, and those are Fn.
+// impl<T, R, F> Apply<T, R> for F where F: FnOnce( &Apply<T, R>, T ) -> R + Clone {
+//     fn apply( &self, f: &Apply<T, R>, t: T ) -> R {
+//         (self.clone())( f, t )
+ 
+//         // If we were to pass in self as f, we get -
+//         // NOTE: Each letter is an individual symbol.
+//         // λf.λt.sft
+//         // => λs.λt.sst [s/f]
+//         // => λs.ss
+//     }
+// }
+ 
+// Before 1.26 we have some limitations and so we need some workarounds. But now impl Trait is stable and we can
+// write the following:
+ 
+fn y<T,R>(f:impl Fn(&dyn Fn(T) -> R, T) -> R) -> impl Fn(T) -> R {
+  move |t| (
+    |x: &dyn Apply<T,R>, y| x.apply(x, y)
+  ) (
+    &|x: &dyn Apply<T,R>, y| f(
+      &|z| x.apply(x,z),
+      y
+    ),
+    t
+  )
+}
+ 
+// fn y<T,R>(f:impl FnOnce(&Fn(T) -> R, T) -> R + Clone) -> impl FnOnce(T) -> R {
+//    |t| (|x: &Apply<T,R>,y| x.apply(x,y))
+//        (&move |x:&Apply<T,R>,y| f(&|z| x.apply(x,z), y), t)
+ 
+//     // NOTE: Each letter is an individual symbol.
+//     // (λx.(λy.xxy))(λx.(λy.f(λz.xxz)y))t
+//     // => (λx.xx)(λx.f(xx))t
+//     // => (Yf)t
+// }
+ 
+// Previous version removed as they are just hacks when impl Trait is not available.
+ 
+fn fac(n: usize) -> usize {
+  let almost_fac = |f: &dyn Fn(usize) -> usize, x|
+    if x == 0 {
+      1
+    } else {
+      x * f(x - 1)
+    }
+  ;
+  let fac = y( almost_fac );
+  fac(n)
+}
+ 
+fn fib( n: usize ) -> usize {
+  let almost_fib = |f: &dyn Fn(usize) -> usize, x|
+    if x < 2 {
+      1
+    } else {
+      f(x - 2) + f(x - 1)
+    };
+  let fib = y(almost_fib);
+  fib(n)
+}
+ 
+fn optimal_fib( n: usize ) -> usize {
+  let almost_fib = |f: &dyn Fn((usize,usize,usize)) -> usize, (i0,i1,x)| 
+    match x {
+      0 => i0,
+      1 => i1,
+      x => f((i1,i0+i1, x-1))
+    }        
+  ;
+  let fib = |x| y(almost_fib)((1,1,x));
+  fib(n)
+}
+ 
+fn test_y() {
+  println!("{}", fac(10));
+  println!("{}", fib(10));
+  println!("{}", optimal_fib(10));
+}