Add Context::get_value_cow()

src/context/mod.rs

@@ -4,7 +4,7 @@
 //! This crate implements two basic variants, the `EmptyContext`, that returns `None` for each identifier and cannot be manipulated, and the `HashMapContext`, that stores its mappings in hash maps.
 //! The HashMapContext is type-safe and returns an error if the user tries to assign a value of a different type than before to an identifier.
 
-use std::collections::HashMap;
+use std::{borrow::Cow, collections::HashMap};
 
 use crate::{
     function::Function,
@@ -16,6 +16,11 @@ mod predefined;
 
 /// An immutable context.
 pub trait Context {
+    /// Returns the value that is linked to the given identifier.
+    fn get_value_cow(&self, identifier: &str) -> Option<Cow<'_, Value>> {
+        self.get_value(identifier).map(Cow::Borrowed)
+    }
+
     /// Returns the value that is linked to the given identifier.
     fn get_value(&self, identifier: &str) -> Option<&Value>;
 

src/operator/mod.rs

@@ -425,8 +425,8 @@ impl Operator {
             VariableIdentifier { identifier } => {
                 expect_operator_argument_amount(arguments.len(), 0)?;
 
-                if let Some(value) = context.get_value(identifier).cloned() {
-                    Ok(value)
+                if let Some(value) = context.get_value_cow(identifier) {
+                    Ok(value.into_owned())
                 } else {
                     Err(EvalexprError::VariableIdentifierNotFound(
                         identifier.clone(),