error checking (it's a start anyway)

frustration.rs

@@ -15,7 +15,7 @@ enum State {
 enum Post {
     Nothing,
     EatWord,
-    EatLine,
+    WarmReset,
 }
 
 #[derive(Debug)]
@@ -171,13 +171,13 @@ fn tick(c: &mut Core) {
                 }
                 None => {
                     println!(" ' cannot find {}", name);
-                    c.post = Post::EatLine;
+                    c.post = Post::WarmReset;
                 }
             }
         }
         _ => {
             println!(" ' needs an argument");
-            c.post = Post::EatLine;
+            c.post = Post::WarmReset;
         }
     }
 }
@@ -214,8 +214,14 @@ fn push(c: &mut Core, val: u16) {
 }
 
 fn pop(c: &mut Core) -> u16 {
-    c.tds -= 1;
-    return c.dstack[c.tds];
+    if c.tds == 0 {
+        println!(" stack underflow");
+        c.post = Post::WarmReset; // note: could get overwritten later :(
+        return 0; // half-assed, should really return straight to interpreter
+    } else {
+        c.tds -= 1;
+        return c.dstack[c.tds];
+    }
 }
 
 fn dup(c: &mut Core) {
@@ -342,25 +348,25 @@ fn lit(c: &mut Core) {
 fn add(c: &mut Core) {
     let v1 = pop(c);
     let v2 = pop(c);
-    push(c, v1 + v2);
+    push(c, v1.wrapping_add(v2));
 }
 
 fn sub(c: &mut Core) {
     let v1 = pop(c);
     let v2 = pop(c);
-    push(c, v2 - v1);
+    push(c, v2.wrapping_sub(v1));
 }
 
 fn mul(c: &mut Core) {
     let v1 = pop(c);
     let v2 = pop(c);
-    push(c, v1 * v2);
+    push(c, v1.saturating_mul(v2));
 }
 
 fn div(c: &mut Core) {
     let v1 = pop(c);
     let v2 = pop(c);
-    push(c, v2 / v1);
+    push(c, v2.saturating_div(v1));
 }
 
 // --- Outer interpreter ---
@@ -406,7 +412,12 @@ fn outer(c: &mut Core, s: &str) {
                                     }
                                 }
                             }
-                            Err(_) => { if t != "" { println!("{}?", t) }}
+                            Err(_) => {
+                                if t != "" {
+                                    println!("{}?", t);
+                                    c.post = Post::WarmReset;
+                                }
+                            }
                         }
                     }
                 }
@@ -415,7 +426,11 @@ fn outer(c: &mut Core, s: &str) {
         }
         match c.post {
             Post::EatWord => { _ = tokens.next(); }
-            Post::EatLine => { break; }
+            Post::WarmReset => {
+                c.tds = 0;
+                c.trs = 0;
+                break; // discard rest of input line
+            }
             Post::Nothing => { }
         };
     }