added prog and prog2 examples

Author: BartMassey

README.md

@@ -29,3 +29,4 @@ in a controlled repository now.
 * `phantom.rs`: phantom type example
 * `deref.rs`: deref trait example
 * `from.rs`: from / try_from trait example
+* `prog.rs`, `prog2.rs`: state machine examples using closures

prog.rs

@@ -0,0 +1,34 @@
+struct State {
+    r: [i32;2],
+    pc: usize,
+}
+
+type Op = fn(State) -> State;
+
+impl State {
+    fn run(mut self, prog: &[Op]) -> i32 {
+        while self.pc < prog.len() {
+            let pc = self.pc;
+            self = prog[pc](self);
+            self.pc += 1;
+        }
+        self.r[0]
+    }
+}
+
+fn add(mut state: State) -> State {
+    state.r[0] += state.r[1];
+    state
+}
+
+fn sub(mut state: State) -> State {
+    state.r[0] -= state.r[1];
+    state
+}
+
+
+fn main() {
+    let state = State { r: [0, 1], pc: 0 };
+    let prog = [add, add, sub, add];
+    println!("{}", state.run(&prog));
+}

prog2.rs

@@ -0,0 +1,40 @@
+struct State {
+    r: [i32;4],
+    pc: usize,
+}
+
+type Op = Box<dyn Fn(State) -> State>;
+
+impl State {
+    fn run(mut self, prog: &[Op]) -> i32 {
+        while self.pc < prog.len() {
+            let pc = self.pc;
+            self = prog[pc](self);
+            self.pc += 1;
+        }
+        self.r[0]
+    }
+}
+
+fn add(mut state: State) -> State {
+    state.r[0] += state.r[1];
+    state
+}
+
+fn sub(mut state: State) -> State {
+    state.r[0] -= state.r[1];
+    state
+}
+
+fn mov(from: usize, to: usize) -> Op {
+    Box::new(move |mut state: State| -> State {state.r[to] = state.r[from]; state})
+}
+
+
+fn main() {
+    let state = State { r: [0, 1, 0, 0], pc: 0 };
+    let add = || Box::new(add);
+    let sub = || Box::new(sub);
+    let prog = [add(), mov(0, 1), add(), mov(0, 1), add(), mov(3,1), sub()];
+    println!("{}", state.run(&prog));
+}