Improve repl and rhai_runner examples with error messages.

examples/repl.rs

@@ -1,25 +1,66 @@
-use rhai::{Engine, RegisterFn, Scope};
-use std::io::{stdin, stdout, Write};
-use std::process::exit;
+use rhai::{Engine, EvalAltResult, Scope};
+use std::{
+    io::{stdin, stdout, Write},
+    iter,
+};
 
-pub fn main() {
-    let mut engine = Engine::new();
-    let mut scope = Scope::new();
+fn print_error(input: &str, err: EvalAltResult) {
+    fn padding(pad: &str, len: usize) -> String {
+        iter::repeat(pad).take(len).collect::<String>()
+    }
 
-    engine.register_fn("exit", || exit(0));
+    let lines: Vec<_> = input.split("\n").collect();
 
-    loop {
-        print!("> ");
-
-        let mut input = String::new();
-        stdout().flush().expect("couldn't flush stdout");
-
-        if let Err(e) = stdin().read_line(&mut input) {
-            println!("input error: {}", e);
+    // Print error
+    match err.position() {
+        p if p.is_eof() => {
+            // EOF
+            let last = lines[lines.len() - 2];
+            println!("{}", last);
+            println!("{}^ {}", padding(" ", last.len() - 1), err);
         }
+        p if p.is_none() => {
+            // No position
+            println!("{}", err);
+        }
+        p => {
+            // Specific position
+            let pos_text = format!(
+                " (line {}, position {})",
+                p.line().unwrap(),
+                p.position().unwrap()
+            );
 
-        if let Err(e) = engine.consume_with_scope(&mut scope, &input) {
-            println!("error: {}", e);
+            println!("{}", lines[p.line().unwrap() - 1]);
+            println!(
+                "{}^ {}",
+                padding(" ", p.position().unwrap() - 1),
+                err.to_string().replace(&pos_text, "")
+            );
+        }
+    }
+}
+
+fn main() {
+    let mut engine = Engine::new();
+    let mut scope = Scope::new();
+
+    let mut input = String::new();
+
+    loop {
+        print!("rhai> ");
+        stdout().flush().expect("couldn't flush stdout");
+
+        input.clear();
+
+        if let Err(err) = stdin().read_line(&mut input) {
+            println!("input error: {}", err);
+        }
+
+        if let Err(err) = engine.consume_with_scope(&mut scope, true, &input) {
+            println!("");
+            print_error(&input, err);
+            println!("");
         }
     }
 }