Improve repl and rhai_runner examples with error messages.

README.md

@@ -55,7 +55,7 @@ Other cool projects to check out:
 Examples
 --------
 
-The repository contains several examples in the `examples` folder:
+A number of examples can be found in the `examples` folder:
 
 | Example                    | Description                                                               |
 | -------------------------- | ------------------------------------------------------------------------- |
@@ -65,7 +65,7 @@ The repository contains several examples in the `examples` folder:
 | `reuse_scope`              | evaluates two pieces of code in separate runs, but using a common scope   |
 | `rhai_runner`              | runs each filename passed to it as a Rhai script                          |
 | `simple_fn`                | shows how to register a Rust function to a Rhai engine                    |
-| `repl`                     | a simple REPL, see source code for what it can do at the moment           |
+| `repl`                     | a simple REPL, interactively evaluate statements from stdin               |
 
 Examples can be run with the following command:
 
@@ -73,10 +73,13 @@ Examples can be run with the following command:
 cargo run --example name
 ```
 
+The `repl` example is a particularly good one as it allows you to interactively try out Rhai's
+language features in a standard REPL (**R**ead-**E**val-**P**rint **L**oop).
+
 Example Scripts
 ---------------
 
-We also have a few examples scripts that showcase Rhai's features, all stored in the `scripts` folder:
+There are also a number of examples scripts that showcase Rhai's features, all in the `scripts` folder:
 
 | Script                | Description                                                   |
 | --------------------- | ------------------------------------------------------------- |
@@ -96,8 +99,7 @@ We also have a few examples scripts that showcase Rhai's features, all stored in
 | `string.rhai`         | string operations                                             |
 | `while.rhai`          | while loop                                                    |
 
-To run the scripts, you can either make your own tiny program, or make use of the `rhai_runner`
+To run the scripts, either make a tiny program or use of the `rhai_runner` example:
-example program:
 
 ```bash
 cargo run --example rhai_runner scripts/any_script.rhai
@@ -106,12 +108,13 @@ cargo run --example rhai_runner scripts/any_script.rhai
 Hello world
 -----------
 
-To get going with Rhai, you create an instance of the scripting engine and then run eval.
+To get going with Rhai, create an instance of the scripting engine and then call `eval`:
 
 ```rust
 use rhai::{Engine, EvalAltResult};
 
-fn main() -> Result<(), EvalAltResult> {
+fn main() -> Result<(), EvalAltResult>
+{
     let mut engine = Engine::new();
 
     let result = engine.eval::<i64>("40 + 2")?;
@@ -232,7 +235,8 @@ fn get_an_any() -> Dynamic {
     Box::new(42_i64)
 }
 
-fn main() -> Result<(), EvalAltResult> {
+fn main() -> Result<(), EvalAltResult>
+{
     let mut engine = Engine::new();
 
     engine.register_fn("add", add);
@@ -278,7 +282,8 @@ fn showit<T: Display>(x: &mut T) -> () {
     println!("{}", x)
 }
 
-fn main() {
+fn main()
+{
     let mut engine = Engine::new();
 
     engine.register_fn("print", showit as fn(x: &mut i64)->());
@@ -310,7 +315,8 @@ fn safe_divide(x: i64, y: i64) -> Result<i64, EvalAltResult> {
     }
 }
 
-fn main() {
+fn main()
+{
     let mut engine = Engine::new();
 
     // Fallible functions that return Result values must use register_result_fn()
@@ -360,7 +366,8 @@ impl TestStruct {
     }
 }
 
-fn main() -> Result<(), EvalAltResult> {
+fn main() -> Result<(), EvalAltResult>
+{
     let mut engine = Engine::new();
 
     engine.register_type::<TestStruct>();
@@ -492,7 +499,8 @@ In this example, we first create a state with a few initialized variables, then
 ```rust
 use rhai::{Engine, Scope, EvalAltResult};
 
-fn main() -> Result<(), EvalAltResult> {
+fn main() -> Result<(), EvalAltResult>
+{
     let mut engine = Engine::new();
 
     // First create the state
@@ -505,13 +513,15 @@ fn main() -> Result<(), EvalAltResult> {
     scope.push("z".into(), 999_i64);
 
     // First invocation
-    engine.eval_with_scope::<()>(&mut scope, r"
+    // (the second boolean argument indicates that we don't need to retain function definitions
+    // because we didn't declare any!)
+    engine.eval_with_scope::<()>(&mut scope, false, r"
         let x = 4 + 5 - y + z;
         y = 1;
     ")?;
 
     // Second invocation using the same state
-    let result = engine.eval_with_scope::<i64>(&mut scope, "x")?;
+    let result = engine.eval_with_scope::<i64>(&mut scope, false, "x")?;
 
     println!("result: {}", result);  // should print 966
 
@@ -545,7 +555,7 @@ let /* intruder comment */ name = "Bob";
 Variables
 ---------
 
-Variables in `Rhai` follow normal naming rules (i.e. must contain only ASCII letters, digits and '`_`' underscores).
+Variables in Rhai follow normal naming rules (i.e. must contain only ASCII letters, digits and '`_`' underscores).
 
 ```rust
 let x = 3;

examples/repl.rs

@@ -1,25 +1,66 @@
-use rhai::{Engine, RegisterFn, Scope};
+use rhai::{Engine, EvalAltResult, Scope};
-use std::io::{stdin, stdout, Write};
+use std::{
-use std::process::exit;
+    io::{stdin, stdout, Write},
+    iter,
+};
 
-pub fn main() {
+fn print_error(input: &str, err: EvalAltResult) {
+    fn padding(pad: &str, len: usize) -> String {
+        iter::repeat(pad).take(len).collect::<String>()
+    }
+
+    let lines: Vec<_> = input.split("\n").collect();
+
+    // 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()
+            );
+
+            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();
 
-    engine.register_fn("exit", || exit(0));
+    let mut input = String::new();
 
     loop {
-        print!("> ");
+        print!("rhai> ");
-
-        let mut input = String::new();
         stdout().flush().expect("couldn't flush stdout");
 
-        if let Err(e) = stdin().read_line(&mut input) {
+        input.clear();
-            println!("input error: {}", e);
+
+        if let Err(err) = stdin().read_line(&mut input) {
+            println!("input error: {}", err);
         }
 
-        if let Err(e) = engine.consume_with_scope(&mut scope, &input) {
+        if let Err(err) = engine.consume_with_scope(&mut scope, true, &input) {
-            println!("error: {}", e);
+            println!("");
+            print_error(&input, err);
+            println!("");
         }
     }
 }

examples/rhai_runner.rs

@@ -1,27 +1,79 @@
-use rhai::{Engine, RegisterFn};
+use rhai::{Engine, EvalAltResult};
-use std::env;
+use std::{env, fs::File, io::Read, iter, process::exit};
-use std::fmt::Display;
 
-fn showit<T: Display>(x: &mut T) -> () {
+fn padding(pad: &str, len: usize) -> String {
-    println!("{}", x)
+    iter::repeat(pad).take(len).collect::<String>()
+}
+
+fn eprint_error(input: &str, err: EvalAltResult) {
+    fn eprint_line(lines: &Vec<&str>, line: usize, pos: usize, err: &str) {
+        let line_no = format!("{}: ", line);
+        let pos_text = format!(" (line {}, position {})", line, pos);
+
+        eprintln!("{}{}", line_no, lines[line - 1]);
+        eprintln!(
+            "{}^ {}",
+            padding(" ", line_no.len() + pos - 1),
+            err.replace(&pos_text, "")
+        );
+        eprintln!("");
+    }
+
+    let lines: Vec<_> = input.split("\n").collect();
+
+    // Print error
+    match err.position() {
+        p if p.is_eof() => {
+            // EOF
+            let line = lines.len() - 1;
+            let pos = lines[line - 1].len();
+            eprint_line(&lines, line, pos, &err.to_string());
+        }
+        p if p.is_none() => {
+            // No position
+            eprintln!("{}", err);
+        }
+        p => {
+            // Specific position
+            eprint_line(
+                &lines,
+                p.line().unwrap(),
+                p.position().unwrap(),
+                &err.to_string(),
+            )
+        }
+    }
 }
 
 fn main() {
-    for fname in env::args().skip(1) {
+    for filename in env::args().skip(1) {
         let mut engine = Engine::new();
 
-        engine.register_fn("print", showit as fn(x: &mut i32) -> ());
+        let mut f = match File::open(&filename) {
-        engine.register_fn("print", showit as fn(x: &mut i64) -> ());
+            Err(err) => {
-        engine.register_fn("print", showit as fn(x: &mut u32) -> ());
+                eprintln!("Error reading script file: {}\n{}", filename, err);
-        engine.register_fn("print", showit as fn(x: &mut u64) -> ());
+                exit(1);
-        engine.register_fn("print", showit as fn(x: &mut f32) -> ());
+            }
-        engine.register_fn("print", showit as fn(x: &mut f64) -> ());
+            Ok(f) => f,
-        engine.register_fn("print", showit as fn(x: &mut bool) -> ());
+        };
-        engine.register_fn("print", showit as fn(x: &mut String) -> ());
 
-        match engine.eval_file::<()>(&fname) {
+        let mut contents = String::new();
-            Ok(_) => (),
+
-            Err(e) => println!("Error: {}", e),
+        match f.read_to_string(&mut contents) {
+            Err(err) => {
+                eprintln!("Error reading script file: {}\n{}", filename, err);
+                exit(1);
+            }
+            _ => (),
+        }
+
+        if let Err(err) = engine.consume(&contents) {
+            eprintln!("{}", padding("=", filename.len()));
+            eprintln!("{}", filename);
+            eprintln!("{}", padding("=", filename.len()));
+            eprintln!("");
+
+            eprint_error(&contents, err);
         }
     }
 }