Add support for custom type indexers.

README.md

@@ -15,7 +15,7 @@ Rhai's current features set:
 
 * Easy-to-use language similar to JS+Rust
 * Easy integration with Rust [native functions](#working-with-functions) and [types](#custom-types-and-methods),
-  including [getter/setter](#getters-and-setters)/[methods](#members-and-methods)
+  including [getters/setters](#getters-and-setters), [methods](#members-and-methods) and [indexers](#indexers)
 * Easily [call a script-defined function](#calling-rhai-functions-from-rust) from Rust
 * Freely pass variables/constants into a script via an external [`Scope`]
 * Fairly efficient (1 million iterations in 0.75 sec on my 5 year old laptop)
@@ -923,6 +923,41 @@ let result = engine.eval::<i64>("let a = new_ts(); a.xyz = 42; a.xyz")?;
 println!("Answer: {}", result);                     // prints 42
 ```
 
+Indexers
+--------
+
+Custom types can also expose an _indexer_ by registering an indexer function.
+A custom with an indexer function defined can use the bracket '`[]`' notation to get a property value
+(but not update it - indexers are read-only).
+
+```rust
+#[derive(Clone)]
+struct TestStruct {
+    fields: Vec<i64>
+}
+
+impl TestStruct {
+    fn get_field(&mut self, index: i64) -> i64 {
+        self.field
+    }
+
+    fn new() -> Self {
+        TestStruct { field: vec![1, 2, 42, 4, 5] }
+    }
+}
+
+let engine = Engine::new();
+
+engine.register_type::<TestStruct>();
+
+engine.register_fn("new_ts", TestStruct::new);
+engine.register_indexer(TestStruct::get_field);
+
+let result = engine.eval::<i64>("let a = new_ts(); a[2]")?;
+
+println!("Answer: {}", result);                     // prints 42
+```
+
 Needless to say, `register_type`, `register_type_with_name`, `register_get`, `register_set` and `register_get_set`
 are not available when the [`no_object`] feature is turned on.
 

src/api.rs

@@ -1,7 +1,7 @@
 //! Module that defines the extern API of `Engine`.
 
 use crate::any::{Dynamic, Variant};
-use crate::engine::{make_getter, make_setter, Engine, Map, State};
+use crate::engine::{make_getter, make_setter, Engine, Map, State, FUNC_INDEXER};
 use crate::error::ParseError;
 use crate::fn_call::FuncArgs;
 use crate::fn_register::RegisterFn;
@@ -42,6 +42,16 @@ pub trait ObjectSetCallback<T, U>: Fn(&mut T, U) + 'static {}
 #[cfg(not(feature = "sync"))]
 impl<F: Fn(&mut T, U) + 'static, T, U> ObjectSetCallback<T, U> for F {}
 
+#[cfg(feature = "sync")]
+pub trait ObjectIndexerCallback<T, X, U>: Fn(&mut T, X) -> U + Send + Sync + 'static {}
+#[cfg(feature = "sync")]
+impl<F: Fn(&mut T, X) -> U + Send + Sync + 'static, T, X, U> ObjectIndexerCallback<T, X, U> for F {}
+
+#[cfg(not(feature = "sync"))]
+pub trait ObjectIndexerCallback<T, X, U>: Fn(&mut T, X) -> U + 'static {}
+#[cfg(not(feature = "sync"))]
+impl<F: Fn(&mut T, X) -> U + 'static, T, X, U> ObjectIndexerCallback<T, X, U> for F {}
+
 #[cfg(feature = "sync")]
 pub trait IteratorCallback:
     Fn(Dynamic) -> Box<dyn Iterator<Item = Dynamic>> + Send + Sync + 'static
@@ -299,6 +309,54 @@ impl Engine {
         self.register_set(name, set_fn);
     }
 
+    /// Register an indexer function for a registered type with the `Engine`.
+    ///
+    /// The function signature must start with `&mut self` and not `&self`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// #[derive(Clone)]
+    /// struct TestStruct {
+    ///     fields: Vec<i64>
+    /// }
+    ///
+    /// impl TestStruct {
+    ///     fn new() -> Self                { TestStruct { fields: vec![1, 2, 3, 4, 5] } }
+    ///
+    ///     // Even a getter must start with `&mut self` and not `&self`.
+    ///     fn get_field(&mut self, index: i64) -> i64 { self.fields[index as usize] }
+    /// }
+    ///
+    /// # fn main() -> Result<(), Box<rhai::EvalAltResult>> {
+    /// use rhai::{Engine, RegisterFn};
+    ///
+    /// let mut engine = Engine::new();
+    ///
+    /// // Register the custom type.
+    /// engine.register_type::<TestStruct>();
+    ///
+    /// engine.register_fn("new_ts", TestStruct::new);
+    ///
+    /// // Register an indexer.
+    /// engine.register_indexer(TestStruct::get_field);
+    ///
+    /// assert_eq!(engine.eval::<i64>("let a = new_ts(); a[2]")?, 3);
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[cfg(not(feature = "no_object"))]
+    #[cfg(not(feature = "no_index"))]
+    pub fn register_indexer<T, X, U, F>(&mut self, callback: F)
+    where
+        T: Variant + Clone,
+        U: Variant + Clone,
+        X: Variant + Clone,
+        F: ObjectIndexerCallback<T, X, U>,
+    {
+        self.register_fn(FUNC_INDEXER, callback);
+    }
+
     /// Compile a string into an `AST`, which can be used later for evaluation.
     ///
     /// # Example

src/engine.rs

@@ -74,6 +74,7 @@ pub const KEYWORD_EVAL: &str = "eval";
 pub const FUNC_TO_STRING: &str = "to_string";
 pub const FUNC_GETTER: &str = "get$";
 pub const FUNC_SETTER: &str = "set$";
+pub const FUNC_INDEXER: &str = "$index$";
 
 /// A type that encapsulates a mutation target for an expression with side effects.
 enum Target<'a> {
@@ -577,6 +578,11 @@ impl Engine {
             });
         }
 
+        // Return default value (if any)
+        if let Some(val) = def_val {
+            return Ok(val.clone());
+        }
+
         // Getter function not found?
         if let Some(prop) = extract_prop_from_getter(fn_name) {
             return Err(Box::new(EvalAltResult::ErrorDotExpr(
@@ -593,17 +599,20 @@ impl Engine {
             )));
         }
 
-        // Return default value (if any)
-        if let Some(val) = def_val {
-            return Ok(val.clone());
-        }
-
-        // Raise error
         let types_list: Vec<_> = args
             .iter()
             .map(|name| self.map_type_name(name.type_name()))
             .collect();
 
+        // Getter function not found?
+        if fn_name == FUNC_INDEXER {
+            return Err(Box::new(EvalAltResult::ErrorFunctionNotFound(
+                format!("[]({})", types_list.join(", ")),
+                pos,
+            )));
+        }
+
+        // Raise error
         Err(Box::new(EvalAltResult::ErrorFunctionNotFound(
             format!("{} ({})", fn_name, types_list.join(", ")),
             pos,
@@ -787,20 +796,20 @@ impl Engine {
                 Expr::Index(idx, idx_rhs, pos) => {
                     let is_index = matches!(rhs, Expr::Index(_,_,_));
 
-                    let indexed_val = self.get_indexed_mut(obj, idx_val, idx.position(), op_pos, false)?;
+                    let indexed_val = self.get_indexed_mut(fn_lib, obj, idx_val, idx.position(), op_pos, false)?;
                     self.eval_dot_index_chain_helper(
                         fn_lib, indexed_val, idx_rhs.as_ref(), idx_values, is_index, *pos, level, new_val
                     )
                 }
                 // xxx[rhs] = new_val
                 _ if new_val.is_some() => {
-                    let mut indexed_val = self.get_indexed_mut(obj, idx_val, rhs.position(), op_pos, true)?;
+                    let mut indexed_val = self.get_indexed_mut(fn_lib, obj, idx_val, rhs.position(), op_pos, true)?;
                     indexed_val.set_value(new_val.unwrap(), rhs.position())?;
                     Ok((Default::default(), true))
                 }
                 // xxx[rhs]
                 _ => self
-                    .get_indexed_mut(obj, idx_val, rhs.position(), op_pos, false)
+                    .get_indexed_mut(fn_lib, obj, idx_val, rhs.position(), op_pos, false)
                     .map(|v| (v.clone_into_dynamic(), false))
             }
         } else {
@@ -818,14 +827,14 @@ impl Engine {
                 // {xxx:map}.id = ???
                 Expr::Property(id, pos) if obj.is::<Map>() && new_val.is_some() => {
                     let mut indexed_val =
-                        self.get_indexed_mut(obj, id.to_string().into(), *pos, op_pos, true)?;
+                        self.get_indexed_mut(fn_lib, obj, id.to_string().into(), *pos, op_pos, true)?;
                     indexed_val.set_value(new_val.unwrap(), rhs.position())?;
                     Ok((Default::default(), true))
                 }
                 // {xxx:map}.id
                 Expr::Property(id, pos) if obj.is::<Map>() => {
                     let indexed_val =
-                        self.get_indexed_mut(obj, id.to_string().into(), *pos, op_pos, false)?;
+                        self.get_indexed_mut(fn_lib, obj, id.to_string().into(), *pos, op_pos, false)?;
                     Ok((indexed_val.clone_into_dynamic(), false))
                 }
                 // xxx.id = ???
@@ -847,7 +856,7 @@ impl Engine {
                     let is_index = matches!(rhs, Expr::Index(_,_,_));
 
                     let indexed_val = if let Expr::Property(id, pos) = dot_lhs.as_ref() {
-                        self.get_indexed_mut(obj, id.to_string().into(), *pos, op_pos, false)?
+                        self.get_indexed_mut(fn_lib, obj, id.to_string().into(), *pos, op_pos, false)?
                     } else {
                         // Syntax error
                         return Err(Box::new(EvalAltResult::ErrorDotExpr(
@@ -1010,8 +1019,9 @@ impl Engine {
     /// Get the value at the indexed position of a base type
     fn get_indexed_mut<'a>(
         &self,
+        fn_lib: &FunctionsLib,
         val: &'a mut Dynamic,
-        idx: Dynamic,
+        mut idx: Dynamic,
         idx_pos: Position,
         op_pos: Position,
         create: bool,
@@ -1084,11 +1094,18 @@ impl Engine {
                 }
             }
 
-            // Error - cannot be indexed
+            _ => {
-            _ => Err(Box::new(EvalAltResult::ErrorIndexingType(
+                let args = &mut [val, &mut idx];
-                type_name.to_string(),
+                self.exec_fn_call(fn_lib, FUNC_INDEXER, args, None, op_pos, 0)
-                op_pos,
+                    .map(|v| v.into())
-            ))),
+                    .map_err(|_| {
+                        Box::new(EvalAltResult::ErrorIndexingType(
+                            // Error - cannot be indexed
+                            type_name.to_string(),
+                            op_pos,
+                        ))
+                    })
+            }
         }
     }
 

src/result.rs

@@ -47,7 +47,7 @@ pub enum EvalAltResult {
     /// String indexing out-of-bounds.
     /// Wrapped values are the current number of characters in the string and the index number.
     ErrorStringBounds(usize, INT, Position),
-    /// Trying to index into a type that is not an array, an object map, or a string.
+    /// Trying to index into a type that is not an array, an object map, or a string, and has no indexer function defined.
     ErrorIndexingType(String, Position),
     /// Trying to index into an array or string with an index that is not `i64`.
     ErrorNumericIndexExpr(Position),
@@ -104,7 +104,7 @@ impl EvalAltResult {
             }
             Self::ErrorStringIndexExpr(_) => "Indexing into an object map expects a string index",
             Self::ErrorIndexingType(_, _) => {
-                "Indexing can only be performed on an array, an object map, or a string"
+                "Indexing can only be performed on an array, an object map, a string, or a type with an indexer function defined"
             }
             Self::ErrorArrayBounds(_, index, _) if *index < 0 => {
                 "Array access expects non-negative index"

tests/get_set.rs

@@ -8,6 +8,7 @@ fn test_get_set() -> Result<(), Box<EvalAltResult>> {
     struct TestStruct {
         x: INT,
         y: INT,
+        array: Vec<INT>,
     }
 
     impl TestStruct {
@@ -24,7 +25,11 @@ fn test_get_set() -> Result<(), Box<EvalAltResult>> {
         }
 
         fn new() -> Self {
-            TestStruct { x: 1, y: 0 }
+            TestStruct {
+                x: 1,
+                y: 0,
+                array: vec![1, 2, 3, 4, 5],
+            }
         }
     }
 
@@ -37,10 +42,16 @@ fn test_get_set() -> Result<(), Box<EvalAltResult>> {
     engine.register_fn("add", |value: &mut INT| *value += 41);
     engine.register_fn("new_ts", TestStruct::new);
 
+    #[cfg(not(feature = "no_index"))]
+    engine.register_indexer(|value: &mut TestStruct, index: INT| value.array[index as usize]);
+
     assert_eq!(engine.eval::<INT>("let a = new_ts(); a.x = 500; a.x")?, 500);
     assert_eq!(engine.eval::<INT>("let a = new_ts(); a.x.add(); a.x")?, 42);
     assert_eq!(engine.eval::<INT>("let a = new_ts(); a.y.add(); a.y")?, 0);
 
+    #[cfg(not(feature = "no_index"))]
+    assert_eq!(engine.eval::<INT>("let a = new_ts(); a[3]")?, 4);
+
     Ok(())
 }