diff --git a/Cargo.toml b/Cargo.toml
index 5dfdcf7..39b2d15 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -10,6 +10,7 @@ homepage = "https://github.com/ISibboI/evalexpr"
 documentation = "https://docs.rs/evalexpr"
 readme = "README.md"
 license = "MIT"
+edition = "2018"
 
 [lib]
 name = "evalexpr"
diff --git a/src/context/mod.rs b/src/context/mod.rs
index 3161027..092d064 100644
--- a/src/context/mod.rs
+++ b/src/context/mod.rs
@@ -1,9 +1,9 @@
 use std::collections::HashMap;
 
-use function::Function;
-use value::value_type::ValueType;
-use EvalexprError;
-use EvalexprResult;
+use crate::function::Function;
+use crate::value::value_type::ValueType;
+use crate::EvalexprError;
+use crate::EvalexprResult;
 
 use crate::value::Value;
 
diff --git a/src/error/display.rs b/src/error/display.rs
index 8b15a49..c583a36 100644
--- a/src/error/display.rs
+++ b/src/error/display.rs
@@ -1,10 +1,10 @@
 use std::fmt;
 
-use EvalexprError;
+use crate::EvalexprError;
 
 impl fmt::Display for EvalexprError {
     fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-        use EvalexprError::*;
+        use crate::EvalexprError::*;
         match self {
             WrongOperatorArgumentAmount { expected, actual } => write!(
                 f,
diff --git a/src/error/mod.rs b/src/error/mod.rs
index a053371..0066602 100644
--- a/src/error/mod.rs
+++ b/src/error/mod.rs
@@ -5,8 +5,8 @@
 //! The module also contains some helper functions starting with `expect_` that check for a condition and return `Err(_)` if the condition is not fulfilled.
 //! They are meant as shortcuts to not write the same error checking code everywhere.
 
-use token::PartialToken;
-use value::{value_type::ValueType, TupleType};
+use crate::token::PartialToken;
+use crate::value::{value_type::ValueType, TupleType};
 
 use crate::value::Value;
 
diff --git a/src/function/builtin.rs b/src/function/builtin.rs
index 0d46512..699d9ad 100644
--- a/src/function/builtin.rs
+++ b/src/function/builtin.rs
@@ -2,10 +2,10 @@
 use regex::Regex;
 
 use crate::error::*;
-use value::{FloatType, IntType};
-use EvalexprError;
-use Function;
-use Value;
+use crate::value::{FloatType, IntType};
+use crate::EvalexprError;
+use crate::Function;
+use crate::Value;
 
 pub fn builtin_function(identifier: &str) -> Option<Function> {
     match identifier {
diff --git a/src/function/mod.rs b/src/function/mod.rs
index 0f9ad9f..bd5b290 100644
--- a/src/function/mod.rs
+++ b/src/function/mod.rs
@@ -1,7 +1,7 @@
 use std::fmt;
 
-use error::EvalexprResult;
-use value::Value;
+use crate::error::EvalexprResult;
+use crate::value::Value;
 
 pub(crate) mod builtin;
 
@@ -20,14 +20,14 @@ pub(crate) mod builtin;
 /// assert_eq!(eval_with_context("id(4)", &context), Ok(Value::from(4)));
 /// ```
 pub struct Function {
-    function: Box<Fn(&Value) -> EvalexprResult<Value>>,
+    function: Box<dyn Fn(&Value) -> EvalexprResult<Value>>,
 }
 
 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: Box<Fn(&Value) -> EvalexprResult<Value>>) -> Self {
+    pub fn new(function: Box<dyn Fn(&Value) -> EvalexprResult<Value>>) -> Self {
         Self { function }
     }
 
diff --git a/src/interface/mod.rs b/src/interface/mod.rs
index d6fa5e8..2e59209 100644
--- a/src/interface/mod.rs
+++ b/src/interface/mod.rs
@@ -1,16 +1,16 @@
-use token;
-use tree;
-use value::TupleType;
-use Context;
-use EmptyContext;
-use EmptyType;
-use EvalexprError;
-use EvalexprResult;
-use FloatType;
-use IntType;
-use Node;
-use Value;
-use EMPTY_VALUE;
+use crate::token;
+use crate::tree;
+use crate::value::TupleType;
+use crate::Context;
+use crate::EmptyContext;
+use crate::EmptyType;
+use crate::EvalexprError;
+use crate::EvalexprResult;
+use crate::FloatType;
+use crate::IntType;
+use crate::Node;
+use crate::Value;
+use crate::EMPTY_VALUE;
 
 /// Evaluate the given expression string.
 ///
@@ -42,7 +42,7 @@ pub fn eval(string: &str) -> EvalexprResult<Value> {
 /// ```
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_with_context(string: &str, context: &Context) -> EvalexprResult<Value> {
+pub fn eval_with_context(string: &str, context: &dyn Context) -> EvalexprResult<Value> {
     tree::tokens_to_operator_tree(token::tokenize(string)?)?.eval_with_context(context)
 }
 
@@ -61,7 +61,7 @@ pub fn eval_with_context(string: &str, context: &Context) -> EvalexprResult<Valu
 /// ```
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_with_context_mut(string: &str, context: &mut Context) -> EvalexprResult<Value> {
+pub fn eval_with_context_mut(string: &str, context: &mut dyn Context) -> EvalexprResult<Value> {
     tree::tokens_to_operator_tree(token::tokenize(string)?)?.eval_with_context_mut(context)
 }
 
@@ -146,7 +146,7 @@ pub fn eval_empty(string: &str) -> EvalexprResult<EmptyType> {
 /// Evaluate the given expression string into a string with the given context.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_string_with_context(string: &str, context: &Context) -> EvalexprResult<String> {
+pub fn eval_string_with_context(string: &str, context: &dyn Context) -> EvalexprResult<String> {
     match eval_with_context(string, context) {
         Ok(Value::String(string)) => Ok(string),
         Ok(value) => Err(EvalexprError::expected_string(value)),
@@ -157,7 +157,7 @@ pub fn eval_string_with_context(string: &str, context: &Context) -> EvalexprResu
 /// Evaluate the given expression string into an integer with the given context.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_int_with_context(string: &str, context: &Context) -> EvalexprResult<IntType> {
+pub fn eval_int_with_context(string: &str, context: &dyn Context) -> EvalexprResult<IntType> {
     match eval_with_context(string, context) {
         Ok(Value::Int(int)) => Ok(int),
         Ok(value) => Err(EvalexprError::expected_int(value)),
@@ -168,7 +168,7 @@ pub fn eval_int_with_context(string: &str, context: &Context) -> EvalexprResult<
 /// Evaluate the given expression string into a float with the given context.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_float_with_context(string: &str, context: &Context) -> EvalexprResult<FloatType> {
+pub fn eval_float_with_context(string: &str, context: &dyn Context) -> EvalexprResult<FloatType> {
     match eval_with_context(string, context) {
         Ok(Value::Float(float)) => Ok(float),
         Ok(value) => Err(EvalexprError::expected_float(value)),
@@ -180,7 +180,7 @@ pub fn eval_float_with_context(string: &str, context: &Context) -> EvalexprResul
 /// If the result of the expression is an integer, it is silently converted into a float.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_number_with_context(string: &str, context: &Context) -> EvalexprResult<FloatType> {
+pub fn eval_number_with_context(string: &str, context: &dyn Context) -> EvalexprResult<FloatType> {
     match eval_with_context(string, context) {
         Ok(Value::Float(float)) => Ok(float),
         Ok(Value::Int(int)) => Ok(int as FloatType),
@@ -192,7 +192,7 @@ pub fn eval_number_with_context(string: &str, context: &Context) -> EvalexprResu
 /// Evaluate the given expression string into a boolean with the given context.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_boolean_with_context(string: &str, context: &Context) -> EvalexprResult<bool> {
+pub fn eval_boolean_with_context(string: &str, context: &dyn Context) -> EvalexprResult<bool> {
     match eval_with_context(string, context) {
         Ok(Value::Boolean(boolean)) => Ok(boolean),
         Ok(value) => Err(EvalexprError::expected_boolean(value)),
@@ -203,7 +203,7 @@ pub fn eval_boolean_with_context(string: &str, context: &Context) -> EvalexprRes
 /// Evaluate the given expression string into a tuple with the given context.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_tuple_with_context(string: &str, context: &Context) -> EvalexprResult<TupleType> {
+pub fn eval_tuple_with_context(string: &str, context: &dyn Context) -> EvalexprResult<TupleType> {
     match eval_with_context(string, context) {
         Ok(Value::Tuple(tuple)) => Ok(tuple),
         Ok(value) => Err(EvalexprError::expected_tuple(value)),
@@ -214,7 +214,7 @@ pub fn eval_tuple_with_context(string: &str, context: &Context) -> EvalexprResul
 /// Evaluate the given expression string into an empty value with the given context.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_empty_with_context(string: &str, context: &Context) -> EvalexprResult<EmptyType> {
+pub fn eval_empty_with_context(string: &str, context: &dyn Context) -> EvalexprResult<EmptyType> {
     match eval_with_context(string, context) {
         Ok(Value::Empty) => Ok(EMPTY_VALUE),
         Ok(value) => Err(EvalexprError::expected_empty(value)),
@@ -225,7 +225,7 @@ pub fn eval_empty_with_context(string: &str, context: &Context) -> EvalexprResul
 /// Evaluate the given expression string into a string with the given mutable context.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_string_with_context_mut(string: &str, context: &mut Context) -> EvalexprResult<String> {
+pub fn eval_string_with_context_mut(string: &str, context: &mut dyn Context) -> EvalexprResult<String> {
     match eval_with_context_mut(string, context) {
         Ok(Value::String(string)) => Ok(string),
         Ok(value) => Err(EvalexprError::expected_string(value)),
@@ -236,7 +236,7 @@ pub fn eval_string_with_context_mut(string: &str, context: &mut Context) -> Eval
 /// Evaluate the given expression string into an integer with the given mutable context.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_int_with_context_mut(string: &str, context: &mut Context) -> EvalexprResult<IntType> {
+pub fn eval_int_with_context_mut(string: &str, context: &mut dyn Context) -> EvalexprResult<IntType> {
     match eval_with_context_mut(string, context) {
         Ok(Value::Int(int)) => Ok(int),
         Ok(value) => Err(EvalexprError::expected_int(value)),
@@ -249,7 +249,7 @@ pub fn eval_int_with_context_mut(string: &str, context: &mut Context) -> Evalexp
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
 pub fn eval_float_with_context_mut(
     string: &str,
-    context: &mut Context,
+    context: &mut dyn Context,
 ) -> EvalexprResult<FloatType> {
     match eval_with_context_mut(string, context) {
         Ok(Value::Float(float)) => Ok(float),
@@ -264,7 +264,7 @@ pub fn eval_float_with_context_mut(
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
 pub fn eval_number_with_context_mut(
     string: &str,
-    context: &mut Context,
+    context: &mut dyn Context,
 ) -> EvalexprResult<FloatType> {
     match eval_with_context_mut(string, context) {
         Ok(Value::Float(float)) => Ok(float),
@@ -277,7 +277,7 @@ pub fn eval_number_with_context_mut(
 /// Evaluate the given expression string into a boolean with the given mutable context.
 ///
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
-pub fn eval_boolean_with_context_mut(string: &str, context: &mut Context) -> EvalexprResult<bool> {
+pub fn eval_boolean_with_context_mut(string: &str, context: &mut dyn Context) -> EvalexprResult<bool> {
     match eval_with_context_mut(string, context) {
         Ok(Value::Boolean(boolean)) => Ok(boolean),
         Ok(value) => Err(EvalexprError::expected_boolean(value)),
@@ -290,7 +290,7 @@ pub fn eval_boolean_with_context_mut(string: &str, context: &mut Context) -> Eva
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
 pub fn eval_tuple_with_context_mut(
     string: &str,
-    context: &mut Context,
+    context: &mut dyn Context,
 ) -> EvalexprResult<TupleType> {
     match eval_with_context_mut(string, context) {
         Ok(Value::Tuple(tuple)) => Ok(tuple),
@@ -304,7 +304,7 @@ pub fn eval_tuple_with_context_mut(
 /// *See the [crate doc](index.html) for more examples and explanations of the expression format.*
 pub fn eval_empty_with_context_mut(
     string: &str,
-    context: &mut Context,
+    context: &mut dyn Context,
 ) -> EvalexprResult<EmptyType> {
     match eval_with_context_mut(string, context) {
         Ok(Value::Empty) => Ok(EMPTY_VALUE),
diff --git a/src/lib.rs b/src/lib.rs
index 88a0155..2fd78b3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -372,12 +372,12 @@ extern crate serde;
 #[macro_use]
 extern crate serde_derive;
 
-pub use context::{Context, EmptyContext, HashMapContext};
-pub use error::{EvalexprError, EvalexprResult};
-pub use function::Function;
-pub use interface::*;
-pub use tree::Node;
-pub use value::{
+pub use crate::context::{Context, EmptyContext, HashMapContext};
+pub use crate::error::{EvalexprError, EvalexprResult};
+pub use crate::function::Function;
+pub use crate::interface::*;
+pub use crate::tree::Node;
+pub use crate::value::{
     value_type::ValueType, EmptyType, FloatType, IntType, TupleType, Value, EMPTY_VALUE,
 };
 
diff --git a/src/operator/display.rs b/src/operator/display.rs
index b713b20..96b0987 100644
--- a/src/operator/display.rs
+++ b/src/operator/display.rs
@@ -1,6 +1,6 @@
 use std::fmt::{Display, Error, Formatter};
 
-use operator::*;
+use crate::operator::*;
 
 impl Display for Operator {
     fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
diff --git a/src/operator/mod.rs b/src/operator/mod.rs
index a8ddda6..56862dc 100644
--- a/src/operator/mod.rs
+++ b/src/operator/mod.rs
@@ -1,4 +1,4 @@
-use function::builtin::builtin_function;
+use crate::function::builtin::builtin_function;
 
 use crate::{context::Context, error::*, value::Value};
 
diff --git a/src/token/display.rs b/src/token/display.rs
index f696e6c..ed8126e 100644
--- a/src/token/display.rs
+++ b/src/token/display.rs
@@ -1,6 +1,6 @@
 use std::fmt;
 
-use token::{PartialToken, Token};
+use crate::token::{PartialToken, Token};
 
 impl fmt::Display for Token {
     fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
diff --git a/src/token/mod.rs b/src/token/mod.rs
index d3a3fe4..66662dc 100644
--- a/src/token/mod.rs
+++ b/src/token/mod.rs
@@ -1,5 +1,5 @@
-use error::{EvalexprError, EvalexprResult};
-use value::{FloatType, IntType};
+use crate::error::{EvalexprError, EvalexprResult};
+use crate::value::{FloatType, IntType};
 
 mod display;
 
diff --git a/src/tree/display.rs b/src/tree/display.rs
index 1460187..6b7152a 100644
--- a/src/tree/display.rs
+++ b/src/tree/display.rs
@@ -1,5 +1,5 @@
 use std::fmt::{Display, Error, Formatter};
-use Node;
+use crate::Node;
 
 impl Display for Node {
     fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
diff --git a/src/tree/iter.rs b/src/tree/iter.rs
index f2481e1..6648005 100644
--- a/src/tree/iter.rs
+++ b/src/tree/iter.rs
@@ -1,5 +1,5 @@
 use std::slice::Iter;
-use Node;
+use crate::Node;
 
 /// An iterator that traverses an operator tree in pre-order.
 pub struct NodeIter<'a> {
diff --git a/src/tree/mod.rs b/src/tree/mod.rs
index 9ae775b..6724ad8 100644
--- a/src/tree/mod.rs
+++ b/src/tree/mod.rs
@@ -1,9 +1,9 @@
-use token::Token;
-use value::{TupleType, EMPTY_VALUE};
-use EmptyContext;
-use EmptyType;
-use FloatType;
-use IntType;
+use crate::token::Token;
+use crate::value::{TupleType, EMPTY_VALUE};
+use crate::EmptyContext;
+use crate::EmptyType;
+use crate::FloatType;
+use crate::IntType;
 
 use crate::{
     context::Context,
@@ -120,7 +120,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node with the given context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_with_context(&self, context: &Context) -> EvalexprResult<Value> {
+    pub fn eval_with_context(&self, context: &dyn Context) -> EvalexprResult<Value> {
         let mut arguments = Vec::new();
         for child in self.children() {
             arguments.push(child.eval_with_context(context)?);
@@ -131,7 +131,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node with the given mutable context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_with_context_mut(&self, context: &mut Context) -> EvalexprResult<Value> {
+    pub fn eval_with_context_mut(&self, context: &mut dyn Context) -> EvalexprResult<Value> {
         let mut arguments = Vec::new();
         for child in self.children() {
             arguments.push(child.eval_with_context_mut(context)?);
@@ -149,7 +149,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into a string with an the given context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_string_with_context(&self, context: &Context) -> EvalexprResult<String> {
+    pub fn eval_string_with_context(&self, context: &dyn Context) -> EvalexprResult<String> {
         match self.eval_with_context(context) {
             Ok(Value::String(string)) => Ok(string),
             Ok(value) => Err(EvalexprError::expected_string(value)),
@@ -160,7 +160,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into a float with an the given context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_float_with_context(&self, context: &Context) -> EvalexprResult<FloatType> {
+    pub fn eval_float_with_context(&self, context: &dyn Context) -> EvalexprResult<FloatType> {
         match self.eval_with_context(context) {
             Ok(Value::Float(float)) => Ok(float),
             Ok(value) => Err(EvalexprError::expected_float(value)),
@@ -171,7 +171,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into an integer with an the given context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_int_with_context(&self, context: &Context) -> EvalexprResult<IntType> {
+    pub fn eval_int_with_context(&self, context: &dyn Context) -> EvalexprResult<IntType> {
         match self.eval_with_context(context) {
             Ok(Value::Int(int)) => Ok(int),
             Ok(value) => Err(EvalexprError::expected_int(value)),
@@ -183,7 +183,7 @@ impl Node {
     /// If the result of the expression is an integer, it is silently converted into a float.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_number_with_context(&self, context: &Context) -> EvalexprResult<FloatType> {
+    pub fn eval_number_with_context(&self, context: &dyn Context) -> EvalexprResult<FloatType> {
         match self.eval_with_context(context) {
             Ok(Value::Int(int)) => Ok(int as FloatType),
             Ok(Value::Float(float)) => Ok(float),
@@ -195,7 +195,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into a boolean with an the given context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_boolean_with_context(&self, context: &Context) -> EvalexprResult<bool> {
+    pub fn eval_boolean_with_context(&self, context: &dyn Context) -> EvalexprResult<bool> {
         match self.eval_with_context(context) {
             Ok(Value::Boolean(boolean)) => Ok(boolean),
             Ok(value) => Err(EvalexprError::expected_boolean(value)),
@@ -206,7 +206,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into a tuple with an the given context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_tuple_with_context(&self, context: &Context) -> EvalexprResult<TupleType> {
+    pub fn eval_tuple_with_context(&self, context: &dyn Context) -> EvalexprResult<TupleType> {
         match self.eval_with_context(context) {
             Ok(Value::Tuple(tuple)) => Ok(tuple),
             Ok(value) => Err(EvalexprError::expected_tuple(value)),
@@ -217,7 +217,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into an empty value with an the given context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_empty_with_context(&self, context: &Context) -> EvalexprResult<EmptyType> {
+    pub fn eval_empty_with_context(&self, context: &dyn Context) -> EvalexprResult<EmptyType> {
         match self.eval_with_context(context) {
             Ok(Value::Empty) => Ok(EMPTY_VALUE),
             Ok(value) => Err(EvalexprError::expected_empty(value)),
@@ -228,7 +228,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into a string with an the given mutable context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_string_with_context_mut(&self, context: &mut Context) -> EvalexprResult<String> {
+    pub fn eval_string_with_context_mut(&self, context: &mut dyn Context) -> EvalexprResult<String> {
         match self.eval_with_context_mut(context) {
             Ok(Value::String(string)) => Ok(string),
             Ok(value) => Err(EvalexprError::expected_string(value)),
@@ -239,7 +239,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into a float with an the given mutable context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_float_with_context_mut(&self, context: &mut Context) -> EvalexprResult<FloatType> {
+    pub fn eval_float_with_context_mut(&self, context: &mut dyn Context) -> EvalexprResult<FloatType> {
         match self.eval_with_context_mut(context) {
             Ok(Value::Float(float)) => Ok(float),
             Ok(value) => Err(EvalexprError::expected_float(value)),
@@ -250,7 +250,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into an integer with an the given mutable context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_int_with_context_mut(&self, context: &mut Context) -> EvalexprResult<IntType> {
+    pub fn eval_int_with_context_mut(&self, context: &mut dyn Context) -> EvalexprResult<IntType> {
         match self.eval_with_context_mut(context) {
             Ok(Value::Int(int)) => Ok(int),
             Ok(value) => Err(EvalexprError::expected_int(value)),
@@ -262,7 +262,7 @@ impl Node {
     /// If the result of the expression is an integer, it is silently converted into a float.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_number_with_context_mut(&self, context: &mut Context) -> EvalexprResult<FloatType> {
+    pub fn eval_number_with_context_mut(&self, context: &mut dyn Context) -> EvalexprResult<FloatType> {
         match self.eval_with_context_mut(context) {
             Ok(Value::Int(int)) => Ok(int as FloatType),
             Ok(Value::Float(float)) => Ok(float),
@@ -274,7 +274,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into a boolean with an the given mutable context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_boolean_with_context_mut(&self, context: &mut Context) -> EvalexprResult<bool> {
+    pub fn eval_boolean_with_context_mut(&self, context: &mut dyn Context) -> EvalexprResult<bool> {
         match self.eval_with_context_mut(context) {
             Ok(Value::Boolean(boolean)) => Ok(boolean),
             Ok(value) => Err(EvalexprError::expected_boolean(value)),
@@ -285,7 +285,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into a tuple with an the given mutable context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_tuple_with_context_mut(&self, context: &mut Context) -> EvalexprResult<TupleType> {
+    pub fn eval_tuple_with_context_mut(&self, context: &mut dyn Context) -> EvalexprResult<TupleType> {
         match self.eval_with_context_mut(context) {
             Ok(Value::Tuple(tuple)) => Ok(tuple),
             Ok(value) => Err(EvalexprError::expected_tuple(value)),
@@ -296,7 +296,7 @@ impl Node {
     /// Evaluates the operator tree rooted at this node into an empty value with an the given mutable context.
     ///
     /// Fails, if one of the operators in the expression tree fails.
-    pub fn eval_empty_with_context_mut(&self, context: &mut Context) -> EvalexprResult<EmptyType> {
+    pub fn eval_empty_with_context_mut(&self, context: &mut dyn Context) -> EvalexprResult<EmptyType> {
         match self.eval_with_context_mut(context) {
             Ok(Value::Empty) => Ok(EMPTY_VALUE),
             Ok(value) => Err(EvalexprError::expected_empty(value)),
diff --git a/src/value/display.rs b/src/value/display.rs
index 4e98d39..599baa4 100644
--- a/src/value/display.rs
+++ b/src/value/display.rs
@@ -1,6 +1,6 @@
 use std::fmt::{Display, Error, Formatter};
 
-use Value;
+use crate::Value;
 
 impl Display for Value {
     fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
diff --git a/src/value/mod.rs b/src/value/mod.rs
index e12bef9..7dd0c96 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -1,4 +1,4 @@
-use error::{EvalexprError, EvalexprResult};
+use crate::error::{EvalexprError, EvalexprResult};
 
 mod display;
 pub mod value_type;
@@ -202,7 +202,7 @@ impl From<()> for Value {
 
 #[cfg(test)]
 mod tests {
-    use value::{TupleType, Value};
+    use crate::value::{TupleType, Value};
 
     #[test]
     fn test_value_conversions() {
diff --git a/src/value/value_type.rs b/src/value/value_type.rs
index 997b09e..b6837e7 100644
--- a/src/value/value_type.rs
+++ b/src/value/value_type.rs
@@ -1,4 +1,4 @@
-use Value;
+use crate::Value;
 
 /// The type of a `Value`.
 #[derive(Clone, Copy, Eq, PartialEq)]