diff --git a/src/configuration/mod.rs b/src/configuration/mod.rs
index 5659161..048c190 100644
--- a/src/configuration/mod.rs
+++ b/src/configuration/mod.rs
@@ -1,8 +1,11 @@
 use crate::{error::Error, value::Value};
 use std::collections::HashMap;
+use function::Function;
 
 pub trait Configuration {
     fn get_value(&self, identifier: &str) -> Option<&Value>;
+
+    fn get_function(&self, identifier: &str) -> Option<&Function>;
 }
 
 pub struct EmptyConfiguration;
@@ -11,12 +14,40 @@ impl Configuration for EmptyConfiguration {
     fn get_value(&self, identifier: &str) -> Option<&Value> {
         None
     }
+
+    fn get_function(&self, identifier: &str) -> Option<&Function> {
+        None
+    }
 }
 
-pub type HashMapConfiguration = HashMap<String, Value>;
+pub struct HashMapConfiguration {
+    variables: HashMap<String, Value>,
+    functions: HashMap<String, Function>,
+}
+
+impl HashMapConfiguration {
+    pub fn new() -> Self {
+        Self {
+            variables: Default::default(),
+            functions: Default::default(),
+        }
+    }
+
+    pub fn insert_variable(&mut self, identifier: String, value: Value) {
+        self.variables.insert(identifier, value);
+    }
+
+    pub fn insert_function(&mut self, identifier: String, function: Function) {
+        self.functions.insert(identifier, function);
+    }
+}
 
 impl Configuration for HashMapConfiguration {
     fn get_value(&self, identifier: &str) -> Option<&Value> {
-        self.get(identifier)
+        self.variables.get(identifier)
+    }
+
+    fn get_function(&self, identifier: &str) -> Option<&Function> {
+        self.functions.get(identifier)
     }
 }
\ No newline at end of file
diff --git a/src/function.rs b/src/function.rs
new file mode 100644
index 0000000..8af8061
--- /dev/null
+++ b/src/function.rs
@@ -0,0 +1,7 @@
+use error::Error;
+use value::Value;
+
+pub struct Function {
+    parameter_amount: usize,
+    function: fn() -> Result<Value, Error>, // TODO continue type
+}
\ No newline at end of file
diff --git a/src/lib.rs b/src/lib.rs
index 9410c6a..523c636 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -8,6 +8,7 @@ mod operator;
 mod token;
 mod tree;
 mod value;
+mod function;
 
 pub fn eval(string: &str) -> Result<Value, Error> {
     tree::tokens_to_operator_tree(token::tokenize(string)?)?.eval(&EmptyConfiguration)
@@ -109,12 +110,12 @@ mod test {
     #[test]
     fn test_with_configuration() {
         let mut configuration = HashMapConfiguration::new();
-        configuration.insert("tr".to_string(), Value::Boolean(true));
-        configuration.insert("fa".to_string(), Value::Boolean(false));
-        configuration.insert("five".to_string(), Value::Int(5));
-        configuration.insert("six".to_string(), Value::Int(6));
-        configuration.insert("half".to_string(), Value::Float(0.5));
-        configuration.insert("zero".to_string(), Value::Int(0));
+        configuration.insert_variable("tr".to_string(), Value::Boolean(true));
+        configuration.insert_variable("fa".to_string(), Value::Boolean(false));
+        configuration.insert_variable("five".to_string(), Value::Int(5));
+        configuration.insert_variable("six".to_string(), Value::Int(6));
+        configuration.insert_variable("half".to_string(), Value::Float(0.5));
+        configuration.insert_variable("zero".to_string(), Value::Int(0));
 
         assert_eq!(eval_with_configuration("tr", &configuration), Ok(Value::Boolean(true)));
         assert_eq!(eval_with_configuration("fa", &configuration), Ok(Value::Boolean(false)));
diff --git a/src/operator/mod.rs b/src/operator/mod.rs
index e5e774c..bc70c9e 100644
--- a/src/operator/mod.rs
+++ b/src/operator/mod.rs
@@ -477,12 +477,14 @@ impl Operator for Identifier {
     }
 
     fn eval(&self, arguments: &[Value], configuration: &Configuration) -> Result<Value, Error> {
-        expect_argument_amount(arguments.len(), 0)?;
-
-        if let Some(value) = configuration.get_value(&self.identifier).cloned() {
-            Ok(value)
+        if arguments.len() == 0 {
+            if let Some(value) = configuration.get_value(&self.identifier).cloned() {
+                Ok(value)
+            } else {
+                Err(Error::IdentifierNotFound(self.identifier.clone()))
+            }
         } else {
-            Err(Error::IdentifierNotFound(self.identifier.clone()))
+            unimplemented!()
         }
     }
 }