Make Function able to hold capturing closures.

This commit "reverts" a change introduced in bee98999,
by making `Function` be simultaneously `Fn` and `Clone`.

src/function/mod.rs

@@ -4,6 +4,26 @@ use crate::{error::EvalexprResult, value::Value};
 
 pub(crate) mod builtin;
 
+/// A helper trait to enable cloning through `Fn` trait objects.
+trait ClonableFn
+where
+    Self: Fn(&Value) -> EvalexprResult<Value>,
+    Self: Send + Sync + 'static,
+{
+    fn dyn_clone(&self) -> Box<dyn ClonableFn>;
+}
+
+impl<F> ClonableFn for F
+where
+    F: Fn(&Value) -> EvalexprResult<Value>,
+    F: Send + Sync + 'static,
+    F: Clone,
+{
+    fn dyn_clone(&self) -> Box<dyn ClonableFn> {
+        Box::new(self.clone()) as _
+    }
+}
+
 /// A user-defined function.
 /// Functions can be used in expressions by storing them in a `Context`.
 ///
@@ -18,17 +38,31 @@ pub(crate) mod builtin;
 /// })).unwrap(); // Do proper error handling here
 /// assert_eq!(eval_with_context("id(4)", &context), Ok(Value::from(4)));
 /// ```
-#[derive(Clone)]
 pub struct Function {
-    function: fn(&Value) -> EvalexprResult<Value>,
+    function: Box<dyn ClonableFn>,
+}
+
+impl Clone for Function {
+    fn clone(&self) -> Self {
+        Self {
+            function: self.function.dyn_clone(),
+        }
+    }
 }
 
 impl Function {
     /// Creates a user-defined function.
     ///
-    /// The `function` is a boxed function that takes a `Value` and returns a `EvalexprResult<Value, Error>`.
-    pub fn new(function: fn(&Value) -> EvalexprResult<Value>) -> Self {
-        Self { function }
+    /// The `function` is boxed for storage.
+    pub fn new<F>(function: F) -> Self
+    where
+        F: Fn(&Value) -> EvalexprResult<Value>,
+        F: Send + Sync + 'static,
+        F: Clone,
+    {
+        Self {
+            function: Box::new(function) as _,
+        }
     }
 
     pub(crate) fn call(&self, argument: &Value) -> EvalexprResult<Value> {

tests/integration.rs

@@ -276,6 +276,36 @@ fn test_n_ary_functions() {
     );
 }
 
+#[test]
+fn test_capturing_functions() {
+    let mut context = HashMapContext::new();
+    // this variable is captured by the function
+    let three = 3;
+    context
+        .set_function(
+            "mult_3".into(),
+            Function::new(move |argument| {
+                if let Value::Int(int) = argument {
+                    Ok(Value::Int(int * three))
+                } else if let Value::Float(float) = argument {
+                    Ok(Value::Float(float * three as f64))
+                } else {
+                    Err(EvalexprError::expected_number(argument.clone()))
+                }
+            }),
+        )
+        .unwrap();
+
+    let four = 4;
+    context
+        .set_function("function_four".into(), Function::new(move |_| Ok(Value::Int(four))))
+        .unwrap();
+
+    assert_eq!(eval_with_context("mult_3 2", &context), Ok(Value::Int(6)));
+    assert_eq!(eval_with_context("mult_3(3)", &context), Ok(Value::Int(9)));
+    assert_eq!(eval_with_context("mult_3(function_four())", &context), Ok(Value::Int(12)));
+}
+
 #[test]
 fn test_builtin_functions() {
     // Log