diff --git a/Cargo.toml b/Cargo.toml
index 3ec85b3..6b79a15 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "evalexpr"
-version = "1.2.1-alpha.0"
+version = "2.0.0-alpha.0"
 description = "A powerful arithmetic and boolean expression evaluator"
 keywords = ["expression", "evaluate", "evaluator", "arithmetic", "boolean"]
 categories = ["parsing", "game-engines"]
diff --git a/src/configuration/mod.rs b/src/configuration/mod.rs
deleted file mode 100644
index b4ed6b3..0000000
--- a/src/configuration/mod.rs
+++ /dev/null
@@ -1,70 +0,0 @@
-use crate::value::Value;
-use function::Function;
-use std::collections::HashMap;
-
-/// A configuration for an expression tree.
-///
-/// A configuration defines methods to retrieve values and functions for literals in an expression tree.
-/// This crate implements two basic variants, the `EmptyConfiguration`, that returns `None` for each identifier, and the `HashMapConfiguration`, that stores its mappings in hash maps.
-pub trait Configuration {
-    /// Returns the value that is linked to the given identifier.
-    fn get_value(&self, identifier: &str) -> Option<&Value>;
-
-    /// Returns the function that is linked to the given identifier.
-    fn get_function(&self, identifier: &str) -> Option<&Function>;
-}
-
-/// A configuration that returns `None` for each identifier.
-pub struct EmptyConfiguration;
-
-impl Configuration for EmptyConfiguration {
-    fn get_value(&self, _identifier: &str) -> Option<&Value> {
-        None
-    }
-
-    fn get_function(&self, _identifier: &str) -> Option<&Function> {
-        None
-    }
-}
-
-/// A configuration that stores its mappings in hash maps.
-///
-/// *Value and function mappings are stored independently, meaning that there can be a function and a value with the same identifier.*
-pub struct HashMapConfiguration {
-    variables: HashMap<String, Value>,
-    functions: HashMap<String, Function>,
-}
-
-impl HashMapConfiguration {
-    /// Constructs a `HashMapConfiguration` with no mappings.
-    pub fn new() -> Self {
-        Self {
-            variables: Default::default(),
-            functions: Default::default(),
-        }
-    }
-
-    /// Adds a variable mapping to the configuration.
-    ///
-    /// *Value and function mappings are stored independently, meaning that there can be a function and a variable with the same identifier.*
-    pub fn insert_variable<S: Into<String>, V: Into<Value>>(&mut self, identifier: S, value: V) {
-        self.variables.insert(identifier.into(), value.into());
-    }
-
-    /// Adds a function mappign to the configuration.
-    ///
-    /// *Value and function mappings are stored independently, meaning that there can be a function and a variable with the same identifier.*
-    pub fn insert_function<S: Into<String>>(&mut self, identifier: S, function: Function) {
-        self.functions.insert(identifier.into(), function);
-    }
-}
-
-impl Configuration for HashMapConfiguration {
-    fn get_value(&self, identifier: &str) -> Option<&Value> {
-        self.variables.get(identifier)
-    }
-
-    fn get_function(&self, identifier: &str) -> Option<&Function> {
-        self.functions.get(identifier)
-    }
-}
diff --git a/src/context/mod.rs b/src/context/mod.rs
new file mode 100644
index 0000000..9af07cf
--- /dev/null
+++ b/src/context/mod.rs
@@ -0,0 +1,110 @@
+use std::collections::HashMap;
+
+use EvalexprError;
+use EvalexprResult;
+use function::Function;
+use value::value_type::ValueType;
+
+use crate::value::Value;
+
+/// A configuration for an expression tree.
+///
+/// A configuration defines methods to retrieve values and functions for literals in an expression tree.
+/// This crate implements two basic variants, the `EmptyContext`, that returns `None` for each identifier, and the `HashMapContext`, that stores its mappings in hash maps.
+pub trait Configuration {
+    /// Returns the value that is linked to the given identifier.
+    fn get_value(&self, identifier: &str) -> Option<&Value>;
+
+    /// Returns the function that is linked to the given identifier.
+    fn get_function(&self, identifier: &str) -> Option<&Function>;
+}
+
+/// A context for an expression tree.
+///
+/// In addition to all functionality of a `Configuration`, a context also allows the manipulation of values and functions.
+/// This crate implements two basic variants, the `EmptyContext`, that returns `Err` for each identifier, 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.
+pub trait Context: Configuration {
+    /// Links the given value to the given identifier.
+    fn set_value<S: Into<String>, V: Into<Value>>(&mut self, identifier: S, value: V) -> EvalexprResult<()>;
+
+    /// Links the given function to the given identifier.
+    fn set_function<S: Into<String>>(&mut self, identifier: S, function: Function) -> EvalexprResult<()>;
+}
+
+/// A configuration that returns `None` for each identifier.
+pub struct EmptyContext;
+
+impl Configuration for EmptyContext {
+    fn get_value(&self, _identifier: &str) -> Option<&Value> {
+        None
+    }
+
+    fn get_function(&self, _identifier: &str) -> Option<&Function> {
+        None
+    }
+}
+
+impl Context for EmptyContext {
+    fn set_value<S: Into<String>, V: Into<Value>>(&mut self, _identifier: S, _value: V) -> EvalexprResult<()> {
+        Err(EvalexprError::ContextNotManipulable)
+    }
+
+    fn set_function<S: Into<String>>(&mut self, _identifier: S, _function: Function) -> EvalexprResult<()> {
+        Err(EvalexprError::ContextNotManipulable)
+    }
+}
+
+/// A context that stores its mappings in hash maps.
+///
+/// *Value and function mappings are stored independently, meaning that there can be a function and a value with the same identifier.*
+///
+/// This context is type-safe, meaning that an identifier that is assigned a value of some type once cannot be assigned a value of another type.
+pub struct HashMapContext {
+    variables: HashMap<String, Value>,
+    functions: HashMap<String, Function>,
+}
+
+impl HashMapContext {
+    /// Constructs a `HashMapContext` with no mappings.
+    pub fn new() -> Self {
+        Self {
+            variables: Default::default(),
+            functions: Default::default(),
+        }
+    }
+}
+
+impl Configuration for HashMapContext {
+    fn get_value(&self, identifier: &str) -> Option<&Value> {
+        self.variables.get(identifier)
+    }
+
+    fn get_function(&self, identifier: &str) -> Option<&Function> {
+        self.functions.get(identifier)
+    }
+}
+
+impl Context for HashMapContext {
+    fn set_value<S: Into<String>, V: Into<Value>>(&mut self, identifier: S, value: V) -> Result<(), EvalexprError> {
+        let identifier = identifier.into();
+        let value = value.into();
+        if let Some(existing_value) = self.variables.get_mut(&identifier) {
+            if ValueType::from(&existing_value) == ValueType::from(&value) {
+                *existing_value = value;
+                return Ok(());
+            } else {
+                return Err(EvalexprError::expected_type(existing_value, value));
+            }
+        }
+
+        // Implicit else, because `self.variables` and `identifier` are not unborrowed in else
+        self.variables.insert(identifier, value);
+        Ok(())
+    }
+
+    fn set_function<S: Into<String>>(&mut self, identifier: S, function: Function) -> Result<(), EvalexprError> {
+        self.functions.insert(identifier.into(), function);
+        Ok(())
+    }
+}
\ No newline at end of file
diff --git a/src/error/display.rs b/src/error/display.rs
index aaadc7d..22682da 100644
--- a/src/error/display.rs
+++ b/src/error/display.rs
@@ -85,6 +85,7 @@ impl fmt::Display for EvalexprError {
             ModulationError { dividend, divisor } => {
                 write!(f, "Error modulating {} % {}", dividend, divisor)
             },
+            ContextNotManipulable => write!(f, "Cannot manipulate context"),
             CustomMessage(message) => write!(f, "Error: {}", message),
         }
     }
diff --git a/src/error/mod.rs b/src/error/mod.rs
index 7c08f9a..b4b4101 100644
--- a/src/error/mod.rs
+++ b/src/error/mod.rs
@@ -7,6 +7,7 @@
 
 use token::PartialToken;
 use value::TupleType;
+use value::value_type::ValueType;
 
 use crate::value::Value;
 
@@ -76,6 +77,8 @@ pub enum EvalexprError {
     AppendedToLeafNode,
 
     /// Tried to append a child to a node such that the precedence of the child is not higher.
+    /// This error should never occur.
+    /// If it does, please file a bug report.
     PrecedenceViolation,
 
     /// A `VariableIdentifier` operation did not find its value in the configuration.
@@ -155,6 +158,9 @@ pub enum EvalexprError {
         divisor: Value,
     },
 
+    /// A `set_`-function was called on a `Context` that does not allow modifications.
+    ContextNotManipulable,
+
     /// A custom error explained by its message.
     CustomMessage(String),
 }
@@ -203,6 +209,17 @@ impl EvalexprError {
         EvalexprError::ExpectedTuple { actual }
     }
 
+    /// Constructs an error that expresses that the type of `expected` was expected, but `actual` was found.
+    pub(crate) fn expected_type(expected: &Value, actual: Value) -> Self {
+        match ValueType::from(expected) {
+            ValueType::String => Self::expected_string(actual),
+            ValueType::Int => Self::expected_int(actual),
+            ValueType::Float => Self::expected_float(actual),
+            ValueType::Boolean => Self::expected_boolean(actual),
+            ValueType::Tuple => Self::expected_tuple(actual),
+        }
+    }
+
     pub(crate) fn unmatched_partial_token(
         first: PartialToken,
         second: Option<PartialToken>,
@@ -242,20 +259,30 @@ impl EvalexprError {
 }
 
 /// Returns `Ok(())` if the actual and expected parameters are equal, and `Err(Error::WrongOperatorArgumentAmount)` otherwise.
-pub(crate) fn expect_operator_argument_amount(actual: usize, expected: usize) -> Result<(), EvalexprError> {
+pub(crate) fn expect_operator_argument_amount(
+    actual: usize,
+    expected: usize,
+) -> Result<(), EvalexprError> {
     if actual == expected {
         Ok(())
     } else {
-        Err(EvalexprError::wrong_operator_argument_amount(actual, expected))
+        Err(EvalexprError::wrong_operator_argument_amount(
+            actual, expected,
+        ))
     }
 }
 
 /// Returns `Ok(())` if the actual and expected parameters are equal, and `Err(Error::WrongFunctionArgumentAmount)` otherwise.
-pub(crate) fn expect_function_argument_amount(actual: usize, expected: usize) -> Result<(), EvalexprError> {
+pub(crate) fn expect_function_argument_amount(
+    actual: usize,
+    expected: usize,
+) -> Result<(), EvalexprError> {
     if actual == expected {
         Ok(())
     } else {
-        Err(EvalexprError::wrong_function_argument_amount(actual, expected))
+        Err(EvalexprError::wrong_function_argument_amount(
+            actual, expected,
+        ))
     }
 }
 
@@ -279,4 +306,5 @@ pub fn expect_boolean(actual: &Value) -> Result<bool, EvalexprError> {
 
 impl std::error::Error for EvalexprError {}
 
-pub type EvalexprResult<T> = Result<T, EvalexprError>;
\ No newline at end of file
+/// Standard result type used by this crate.
+pub type EvalexprResult<T> = Result<T, EvalexprError>;
diff --git a/src/interface/mod.rs b/src/interface/mod.rs
index e8ef2ba..a7b96e0 100644
--- a/src/interface/mod.rs
+++ b/src/interface/mod.rs
@@ -1,5 +1,5 @@
 use Configuration;
-use EmptyConfiguration;
+use EmptyContext;
 use EvalexprError;
 use FloatType;
 use IntType;
@@ -21,7 +21,7 @@ use value::TupleType;
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
 pub fn eval(string: &str) -> Result<Value, EvalexprError> {
-    eval_with_configuration(string, &EmptyConfiguration)
+    eval_with_configuration(string, &EmptyContext)
 }
 
 /// Evaluate the given expression string with the given configuration.
@@ -31,10 +31,10 @@ pub fn eval(string: &str) -> Result<Value, EvalexprError> {
 /// ```rust
 /// use evalexpr::*;
 ///
-/// let mut configuration = HashMapConfiguration::new();
-/// configuration.insert_variable("one", 1);
-/// configuration.insert_variable("two", 2);
-/// configuration.insert_variable("three", 3);
+/// let mut configuration = HashMapContext::new();
+/// configuration.set_value("one", 1).unwrap(); // Do proper error handling here
+/// configuration.set_value("two", 2).unwrap(); // Do proper error handling here
+/// configuration.set_value("three", 3).unwrap(); // Do proper error handling here
 /// assert_eq!(eval_with_configuration("one + two + three", &configuration), Ok(Value::from(6)));
 /// ```
 ///
@@ -58,14 +58,14 @@ pub fn eval_with_configuration(
 ///
 /// let precomputed = build_operator_tree("one + two + three").unwrap(); // Do proper error handling here
 ///
-/// let mut configuration = HashMapConfiguration::new();
-/// configuration.insert_variable("one", 1);
-/// configuration.insert_variable("two", 2);
-/// configuration.insert_variable("three", 3);
+/// let mut configuration = HashMapContext::new();
+/// configuration.set_value("one", 1).unwrap(); // Do proper error handling here
+/// configuration.set_value("two", 2).unwrap(); // Do proper error handling here
+/// configuration.set_value("three", 3).unwrap(); // Do proper error handling here
 ///
 /// assert_eq!(precomputed.eval_with_configuration(&configuration), Ok(Value::from(6)));
 ///
-/// configuration.insert_variable("three", 5);
+/// configuration.set_value("three", 5).unwrap(); // Do proper error handling here
 /// assert_eq!(precomputed.eval_with_configuration(&configuration), Ok(Value::from(8)));
 /// ```
 ///
diff --git a/src/lib.rs b/src/lib.rs
index 04d6a6b..f523f63 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -35,10 +35,10 @@
 //! use evalexpr::*;
 //! use evalexpr::error::expect_number;
 //!
-//! let mut configuration = HashMapConfiguration::new();
-//! configuration.insert_variable("five", 5);
-//! configuration.insert_variable("twelve", 12);
-//! configuration.insert_function("f", Function::new(Some(1) /* argument amount */, Box::new(|arguments| {
+//! let mut configuration = HashMapContext::new();
+//! configuration.set_value("five", 5).unwrap(); // Do proper error handling here
+//! configuration.set_value("twelve", 12).unwrap(); // Do proper error handling here
+//! configuration.set_function("f", Function::new(Some(1) /* argument amount */, Box::new(|arguments| {
 //!     if let Value::Int(int) = arguments[0] {
 //!         Ok(Value::Int(int / 2))
 //!     } else if let Value::Float(float) = arguments[0] {
@@ -46,8 +46,8 @@
 //!     } else {
 //!         Err(EvalexprError::expected_number(arguments[0].clone()))
 //!     }
-//! })));
-//! configuration.insert_function("avg", Function::new(Some(2) /* argument amount */, Box::new(|arguments| {
+//! }))).unwrap(); // Do proper error handling here
+//! configuration.set_function("avg", Function::new(Some(2) /* argument amount */, Box::new(|arguments| {
 //!     expect_number(&arguments[0])?;
 //!     expect_number(&arguments[1])?;
 //!
@@ -56,7 +56,7 @@
 //!     } else {
 //!         Ok(Value::Float((arguments[0].as_float()? + arguments[1].as_float()?) / 2.0))
 //!     }
-//! })));
+//! }))).unwrap(); // Do proper error handling here
 //!
 //! assert_eq!(eval_with_configuration("five + 8 > f(twelve)", &configuration), Ok(Value::from(true)));
 //! // `eval_with_configuration` returns a variant of the `Value` enum,
@@ -73,13 +73,13 @@
 //!
 //! let precompiled = build_operator_tree("a * b - c > 5").unwrap(); // Do proper error handling here
 //!
-//! let mut configuration = HashMapConfiguration::new();
-//! configuration.insert_variable("a", 6);
-//! configuration.insert_variable("b", 2);
-//! configuration.insert_variable("c", 3);
+//! let mut configuration = HashMapContext::new();
+//! configuration.set_value("a", 6).unwrap(); // Do proper error handling here
+//! configuration.set_value("b", 2).unwrap(); // Do proper error handling here
+//! configuration.set_value("c", 3).unwrap(); // Do proper error handling here
 //! assert_eq!(precompiled.eval_with_configuration(&configuration), Ok(Value::from(true)));
 //!
-//! configuration.insert_variable("c", 8);
+//! configuration.set_value("c", 8).unwrap(); // Do proper error handling here
 //! assert_eq!(precompiled.eval_with_configuration(&configuration), Ok(Value::from(false)));
 //! // `Node::eval_with_configuration` returns a variant of the `Value` enum,
 //! // while `Node::eval_[type]_with_configuration` returns the respective type directly.
@@ -241,8 +241,8 @@
 //! extern crate ron;
 //! use evalexpr::*;
 //!
-//! let mut configuration = HashMapConfiguration::new();
-//! configuration.insert_variable("five", 5);
+//! let mut configuration = HashMapContext::new();
+//! configuration.set_value("five", 5).unwrap(); // Do proper error handling here
 //!
 //! // In ron format, strings are surrounded by "
 //! let serialized_free = "\"five * five\"";
@@ -269,14 +269,14 @@ extern crate ron;
 #[cfg(feature = "serde")]
 extern crate serde;
 
-pub use configuration::{Configuration, EmptyConfiguration, HashMapConfiguration};
+pub use context::{Configuration, Context, EmptyContext, HashMapContext};
 pub use error::{EvalexprError, EvalexprResult};
 pub use function::Function;
 pub use interface::*;
 pub use tree::Node;
 pub use value::{FloatType, IntType, Value};
 
-mod configuration;
+mod context;
 pub mod error;
 #[cfg(feature = "serde")]
 mod feature_serde;
@@ -288,4 +288,3 @@ mod tree;
 mod value;
 
 // Exports
-
diff --git a/src/operator/mod.rs b/src/operator/mod.rs
index 56b9f3b..0b40a27 100644
--- a/src/operator/mod.rs
+++ b/src/operator/mod.rs
@@ -2,7 +2,7 @@ use std::fmt::{Debug, Display};
 
 use function::builtin::builtin_function;
 
-use crate::{configuration::Configuration, error::*, value::Value};
+use crate::{context::Configuration, error::*, value::Value};
 
 mod display;
 
@@ -29,7 +29,11 @@ pub trait Operator: Debug + Display {
     fn argument_amount(&self) -> usize;
 
     /// Evaluates the operator with the given arguments and configuration.
-    fn eval(&self, arguments: &[Value], configuration: &Configuration) -> Result<Value, EvalexprError>;
+    fn eval(
+        &self,
+        arguments: &[Value],
+        configuration: &Configuration,
+    ) -> Result<Value, EvalexprError>;
 }
 
 #[derive(Debug)]
@@ -118,7 +122,11 @@ impl Operator for RootNode {
         1
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 1)?;
         Ok(arguments[0].clone())
     }
@@ -137,7 +145,11 @@ impl Operator for Add {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -173,7 +185,11 @@ impl Operator for Sub {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -209,7 +225,11 @@ impl Operator for Neg {
         1
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 1)?;
         expect_number(&arguments[0])?;
 
@@ -239,7 +259,11 @@ impl Operator for Mul {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -275,7 +299,11 @@ impl Operator for Div {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -311,7 +339,11 @@ impl Operator for Mod {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -347,7 +379,11 @@ impl Operator for Exp {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -374,7 +410,11 @@ impl Operator for Eq {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
 
         if arguments[0] == arguments[1] {
@@ -398,7 +438,11 @@ impl Operator for Neq {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
 
         if arguments[0] != arguments[1] {
@@ -422,7 +466,11 @@ impl Operator for Gt {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -456,7 +504,11 @@ impl Operator for Lt {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -490,7 +542,11 @@ impl Operator for Geq {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -524,7 +580,11 @@ impl Operator for Leq {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         expect_number(&arguments[0])?;
         expect_number(&arguments[1])?;
@@ -558,7 +618,11 @@ impl Operator for And {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         let a = expect_boolean(&arguments[0])?;
         let b = expect_boolean(&arguments[1])?;
@@ -584,7 +648,11 @@ impl Operator for Or {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 2)?;
         let a = expect_boolean(&arguments[0])?;
         let b = expect_boolean(&arguments[1])?;
@@ -610,7 +678,11 @@ impl Operator for Not {
         1
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 1)?;
         let a = expect_boolean(&arguments[0])?;
 
@@ -635,7 +707,11 @@ impl Operator for Tuple {
         2
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         if let Value::Tuple(tuple) = &arguments[0] {
             let mut tuple = tuple.clone();
             if let Value::Tuple(tuple2) = &arguments[1] {
@@ -672,7 +748,11 @@ impl Operator for Const {
         0
     }
 
-    fn eval(&self, arguments: &[Value], _configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        _configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 0)?;
 
         Ok(self.value.clone())
@@ -692,11 +772,17 @@ impl Operator for VariableIdentifier {
         0
     }
 
-    fn eval(&self, _arguments: &[Value], configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        _arguments: &[Value],
+        configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         if let Some(value) = configuration.get_value(&self.identifier).cloned() {
             Ok(value)
         } else {
-            Err(EvalexprError::VariableIdentifierNotFound(self.identifier.clone()))
+            Err(EvalexprError::VariableIdentifierNotFound(
+                self.identifier.clone(),
+            ))
         }
     }
 }
@@ -714,7 +800,11 @@ impl Operator for FunctionIdentifier {
         1
     }
 
-    fn eval(&self, arguments: &[Value], configuration: &Configuration) -> Result<Value, EvalexprError> {
+    fn eval(
+        &self,
+        arguments: &[Value],
+        configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         expect_operator_argument_amount(arguments.len(), 1)?;
 
         let arguments = if let Value::Tuple(arguments) = &arguments[0] {
@@ -728,7 +818,9 @@ impl Operator for FunctionIdentifier {
         } else if let Some(builtin_function) = builtin_function(&self.identifier) {
             builtin_function.call(arguments)
         } else {
-            Err(EvalexprError::FunctionIdentifierNotFound(self.identifier.clone()))
+            Err(EvalexprError::FunctionIdentifierNotFound(
+                self.identifier.clone(),
+            ))
         }
     }
 }
diff --git a/src/tree/mod.rs b/src/tree/mod.rs
index c65137b..2578ccc 100644
--- a/src/tree/mod.rs
+++ b/src/tree/mod.rs
@@ -1,10 +1,10 @@
-use EmptyConfiguration;
+use EmptyContext;
 use FloatType;
 use IntType;
 use token::Token;
 use value::TupleType;
 
-use crate::{configuration::Configuration, error::EvalexprError, operator::*, value::Value};
+use crate::{context::Configuration, error::EvalexprError, operator::*, value::Value};
 
 mod display;
 
@@ -19,8 +19,8 @@ mod display;
 /// ```rust
 /// use evalexpr::*;
 ///
-/// let mut configuration = HashMapConfiguration::new();
-/// configuration.insert_variable("alpha", 2);
+/// let mut configuration = HashMapContext::new();
+/// configuration.set_value("alpha", 2).unwrap(); // Do proper error handling here
 /// let node = build_operator_tree("1 + alpha").unwrap(); // Do proper error handling here
 /// assert_eq!(node.eval_with_configuration(&configuration), Ok(Value::from(3)));
 /// ```
@@ -46,7 +46,10 @@ impl Node {
     /// Evaluates the operator tree rooted at this node with the given configuration.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_with_configuration(&self, configuration: &Configuration) -> Result<Value, EvalexprError> {
+    pub fn eval_with_configuration(
+        &self,
+        configuration: &Configuration,
+    ) -> Result<Value, EvalexprError> {
         let mut arguments = Vec::new();
         for child in self.children() {
             arguments.push(child.eval_with_configuration(configuration)?);
@@ -58,7 +61,7 @@ impl Node {
     ///
     /// Fails, if one of the operators in the expression tree fails.
     pub fn eval(&self) -> Result<Value, EvalexprError> {
-        self.eval_with_configuration(&EmptyConfiguration)
+        self.eval_with_configuration(&EmptyContext)
     }
 
     /// Evaluates the operator tree rooted at this node into a string with an the given configuration.
@@ -135,35 +138,35 @@ impl Node {
     ///
     /// Fails, if one of the operators in the expression tree fails.
     pub fn eval_string(&self) -> Result<String, EvalexprError> {
-        self.eval_string_with_configuration(&EmptyConfiguration)
+        self.eval_string_with_configuration(&EmptyContext)
     }
 
     /// Evaluates the operator tree rooted at this node into a float with an empty configuration.
     ///
     /// Fails, if one of the operators in the expression tree fails.
     pub fn eval_float(&self) -> Result<FloatType, EvalexprError> {
-        self.eval_float_with_configuration(&EmptyConfiguration)
+        self.eval_float_with_configuration(&EmptyContext)
     }
 
     /// Evaluates the operator tree rooted at this node into an integer with an empty configuration.
     ///
     /// Fails, if one of the operators in the expression tree fails.
     pub fn eval_int(&self) -> Result<IntType, EvalexprError> {
-        self.eval_int_with_configuration(&EmptyConfiguration)
+        self.eval_int_with_configuration(&EmptyContext)
     }
 
     /// Evaluates the operator tree rooted at this node into a boolean with an empty configuration.
     ///
     /// Fails, if one of the operators in the expression tree fails.
     pub fn eval_boolean(&self) -> Result<bool, EvalexprError> {
-        self.eval_boolean_with_configuration(&EmptyConfiguration)
+        self.eval_boolean_with_configuration(&EmptyContext)
     }
 
     /// Evaluates the operator tree rooted at this node into a tuple with an empty configuration.
     ///
     /// Fails, if one of the operators in the expression tree fails.
     pub fn eval_tuple(&self) -> Result<TupleType, EvalexprError> {
-        self.eval_tuple_with_configuration(&EmptyConfiguration)
+        self.eval_tuple_with_configuration(&EmptyContext)
     }
 
     fn children(&self) -> &[Node] {
@@ -178,7 +181,11 @@ impl Node {
         self.children().len() == self.operator().argument_amount()
     }
 
-    fn insert_back_prioritized(&mut self, node: Node, is_root_node: bool) -> Result<(), EvalexprError> {
+    fn insert_back_prioritized(
+        &mut self,
+        node: Node,
+        is_root_node: bool,
+    ) -> Result<(), EvalexprError> {
         if self.operator().precedence() < node.operator().precedence() || is_root_node
             // Right-to-left chaining
             || (self.operator().precedence() == node.operator().precedence() && !self.operator().is_left_to_right() && !node.operator().is_left_to_right())
diff --git a/src/value/mod.rs b/src/value/mod.rs
index 9073546..88792cb 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -1,6 +1,7 @@
 use error::EvalexprError;
 
 mod display;
+pub mod value_type;
 
 /// The type used to represent integers in `Value::Int`.
 pub type IntType = i64;
diff --git a/src/value/value_type.rs b/src/value/value_type.rs
new file mode 100644
index 0000000..ded6cc0
--- /dev/null
+++ b/src/value/value_type.rs
@@ -0,0 +1,35 @@
+use Value;
+
+/// The type of a `Value`.
+#[derive(Clone, Copy, Eq, PartialEq)]
+pub enum ValueType {
+    String,
+    Float,
+    Int,
+    Boolean,
+    Tuple,
+}
+
+impl From<&Value> for ValueType {
+    fn from(value: &Value) -> Self {
+        match value {
+            Value::String(_) => ValueType::String,
+            Value::Float(_) => ValueType::Float,
+            Value::Int(_) => ValueType::Int,
+            Value::Boolean(_) => ValueType::Boolean,
+            Value::Tuple(_) => ValueType::Tuple,
+        }
+    }
+}
+
+impl From<&mut Value> for ValueType {
+    fn from(value: &mut Value) -> Self {
+        From::<&Value>::from(value)
+    }
+}
+
+impl From<&&mut Value> for ValueType {
+    fn from(value: &&mut Value) -> Self {
+        From::<&Value>::from(*value)
+    }
+}
\ No newline at end of file
diff --git a/tests/integration.rs b/tests/integration.rs
index 0ad0ddf..40cf21f 100644
--- a/tests/integration.rs
+++ b/tests/integration.rs
@@ -10,7 +10,9 @@ fn test_unary_examples() {
     assert_eq!(eval("false"), Ok(Value::Boolean(false)));
     assert_eq!(
         eval("blub"),
-        Err(EvalexprError::VariableIdentifierNotFound("blub".to_string()))
+        Err(EvalexprError::VariableIdentifierNotFound(
+            "blub".to_string()
+        ))
     );
     assert_eq!(eval("-3"), Ok(Value::Int(-3)));
     assert_eq!(eval("-3.6"), Ok(Value::Float(-3.6)));
@@ -97,13 +99,13 @@ fn test_boolean_examples() {
 
 #[test]
 fn test_with_configuration() {
-    let mut configuration = HashMapConfiguration::new();
-    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));
+    let mut configuration = HashMapContext::new();
+    configuration.set_value("tr", Value::Boolean(true)).unwrap();
+    configuration.set_value("fa", Value::Boolean(false)).unwrap();
+    configuration.set_value("five", Value::Int(5)).unwrap();
+    configuration.set_value("six", Value::Int(6)).unwrap();
+    configuration.set_value("half", Value::Float(0.5)).unwrap();
+    configuration.set_value("zero", Value::Int(0)).unwrap();
 
     assert_eq!(
         eval_with_configuration("tr", &configuration),
@@ -133,8 +135,8 @@ fn test_with_configuration() {
 
 #[test]
 fn test_functions() {
-    let mut configuration = HashMapConfiguration::new();
-    configuration.insert_function(
+    let mut configuration = HashMapContext::new();
+    configuration.set_function(
         "sub2".to_string(),
         Function::new(
             Some(1),
@@ -148,8 +150,8 @@ fn test_functions() {
                 }
             }),
         ),
-    );
-    configuration.insert_variable("five".to_string(), Value::Int(5));
+    ).unwrap();
+    configuration.set_value("five".to_string(), Value::Int(5)).unwrap();
 
     assert_eq!(
         eval_with_configuration("sub2 5", &configuration),
@@ -175,8 +177,8 @@ fn test_functions() {
 
 #[test]
 fn test_n_ary_functions() {
-    let mut configuration = HashMapConfiguration::new();
-    configuration.insert_function(
+    let mut configuration = HashMapContext::new();
+    configuration.set_function(
         "sub2",
         Function::new(
             Some(1),
@@ -190,8 +192,8 @@ fn test_n_ary_functions() {
                 }
             }),
         ),
-    );
-    configuration.insert_function(
+    ).unwrap();
+    configuration.set_function(
         "avg",
         Function::new(
             Some(2),
@@ -208,8 +210,8 @@ fn test_n_ary_functions() {
                 }
             }),
         ),
-    );
-    configuration.insert_function(
+    ).unwrap();
+    configuration.set_function(
         "muladd",
         Function::new(
             Some(3),
@@ -230,15 +232,15 @@ fn test_n_ary_functions() {
                 }
             }),
         ),
-    );
-    configuration.insert_function(
+    ).unwrap();
+    configuration.set_function(
         "count",
         Function::new(
             None,
             Box::new(|arguments| Ok(Value::Int(arguments.len() as IntType))),
         ),
-    );
-    configuration.insert_variable("five".to_string(), Value::Int(5));
+    ).unwrap();
+    configuration.set_value("five".to_string(), Value::Int(5)).unwrap();
 
     assert_eq!(
         eval_with_configuration("avg(7, 5)", &configuration),
@@ -339,8 +341,8 @@ fn test_no_panic() {
 
 #[test]
 fn test_shortcut_functions() {
-    let mut configuration = HashMapConfiguration::new();
-    configuration.insert_variable("string", Value::from("a string"));
+    let mut configuration = HashMapContext::new();
+    configuration.set_value("string", Value::from("a string")).unwrap();
 
     // assert_eq!(eval_string("???"));
     assert_eq!(