Fix calling functions without arguments

src/context/mod.rs

@@ -5,6 +5,7 @@
 //! 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::{
+    cell::Cell,
     collections::BTreeMap,
     fmt::Display,
     fs,
@@ -25,7 +26,7 @@ mod predefined;
 /// An immutable context.
 pub trait Context {
     /// Returns the value that is linked to the given identifier.
-    fn get_value(&self, identifier: &str) -> Option<&Value>;
+    fn get_value(&self, identifier: &str) -> EvalexprResult<Option<Value>>;
 
     /// Calls the function that is linked to the given identifier with the given argument.
     /// If no function with the given identifier is found, this method returns `EvalexprError::FunctionIdentifierNotFound`.
@@ -49,7 +50,7 @@ pub trait ContextWithMutableFunctions: Context {
 }
 
 /// A context that stores its mappings in hash maps.
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Debug)]
 pub struct VariableMap {
     variables: BTreeMap<String, Value>,
 }
@@ -87,25 +88,36 @@ impl PartialEq for VariableMap {
 impl VariableMap {
     /// Create a new instace.
     pub fn new() -> Self {
-        Default::default()
+        VariableMap {
+            variables: BTreeMap::new(),
+        }
     }
 }
 
 impl Context for VariableMap {
-    fn get_value(&self, identifier: &str) -> Option<&Value> {
+    fn get_value(&self, identifier: &str) -> EvalexprResult<Option<Value>> {
         let split = identifier.split_once(".");
-        if let Some((map_name, next_identifier)) = split {
-            let value = self.variables.get(map_name)?;
-            if let Value::Map(map) = value {
-                map.get_value(next_identifier)
+        if let Some((identifier, next_identifier)) = split {
+            if let Some(value) = self.variables.get(identifier) {
+                if let Value::Map(map) = value {
+                    map.get_value(next_identifier)
+                } else {
+                    Err(EvalexprError::ExpectedMap {
+                        actual: value.clone(),
+                    })
+                }
             } else {
-                None
+                Ok(None)
             }
         } else {
-            self.variables.get(identifier).or_else(|| {
-                self.call_function(identifier, &Value::Empty);
-                None
-            })
+            let value = self.variables.get(identifier);
+
+            if let Some(value) = value {
+                Ok(Some(value.clone()))
+            } else {
+                self.call_function(identifier, &Value::Empty)
+                    .map(|value| Some(value))
+            }
         }
     }
 
@@ -119,7 +131,9 @@ impl ContextWithMutableVariables for VariableMap {
         let split = identifier.split_once(".");
 
         if let Some((map_name, next_identifier)) = split {
-            if let Some(map_value) = self.variables.get_mut(map_name) {
+            let get_value = self.variables.get_mut(map_name);
+
+            if let Some(map_value) = get_value {
                 if let Value::Map(map) = map_value {
                     map.set_value(next_identifier, value)
                 } else {
@@ -128,11 +142,10 @@ impl ContextWithMutableVariables for VariableMap {
                     });
                 }
             } else {
-                let mut new_map = VariableMap {
-                    variables: BTreeMap::new(),
-                };
+                let mut new_map = VariableMap::new();
 
                 new_map.set_value(next_identifier, value)?;
+
                 self.variables
                     .insert(map_name.to_string(), Value::Map(new_map));
 

src/operator/mod.rs

@@ -446,7 +446,7 @@ impl Operator {
             VariableIdentifierRead { identifier } => {
                 expect_operator_argument_amount(arguments.len(), 0)?;
 
-                if let Some(value) = context.get_value(identifier).cloned() {
+                if let Some(value) = context.get_value(identifier)? {
                     Ok(value)
                 } else {
                     Err(EvalexprError::VariableIdentifierNotFound(