Implement Display and std::error::Error for Error

Relates to #19

src/error/display.rs

@@ -0,0 +1,111 @@
+use std::fmt;
+use Error;
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        use Error::*;
+        match self {
+            WrongOperatorArgumentAmount { expected, actual } => write!(
+                f,
+                "An operator expected {} arguments, but got {}.",
+                expected, actual
+            ),
+
+            WrongFunctionArgumentAmount { expected, actual } => write!(
+                f,
+                "A function expected {} arguments, but got {}.",
+                expected, actual
+            ),
+
+            ExpectedString { actual } => {
+                write!(f, "Expected a Value::String, but got {:?}.", actual)
+            },
+
+            ExpectedInt { actual } => write!(f, "Expected a Value::Int, but got {:?}.", actual),
+
+            ExpectedFloat { actual } => write!(f, "Expected a Value::Float, but got {:?}.", actual),
+
+            ExpectedNumber { actual } => {
+                write!(f, "Expected a Value::Number, but got {:?}.", actual)
+            },
+
+            ExpectedBoolean { actual } => {
+                write!(f, "Expected a Value::Boolean, but got {:?}.", actual)
+            },
+
+            ExpectedTuple { actual } => write!(f, "Expected a Value::Tuple, but got {:?}.", actual),
+
+            EmptyExpression => write!(
+                f,
+                "Got an empty expression that cannot be parsed into a node tree, because it \
+                 returns nothing."
+            ),
+
+            AppendedToLeafNode => write!(f, "Tried to append a node to a leaf node."),
+
+            PrecedenceViolation => write!(
+                f,
+                "Tried to append a node to another node with higher precedence."
+            ),
+
+            VariableIdentifierNotFound(identifier) => write!(
+                f,
+                "Variable identifier is not bound to anything by configuration: {:?}.",
+                identifier
+            ),
+
+            FunctionIdentifierNotFound(identifier) => write!(
+                f,
+                "Function identifier is not bound to anything by configuration: {:?}.",
+                identifier
+            ),
+
+            TypeError { expected, actual } => {
+                write!(f, "Expected one of {:?}, but got {:?}.", expected, actual)
+            },
+
+            UnmatchedLBrace => write!(f, "Found an unmatched opening parenthesis '('."),
+
+            UnmatchedRBrace => write!(f, "Found an unmatched closing parenthesis ')'."),
+
+            UnmatchedPartialToken { first, second } => {
+                if let Some(second) = second {
+                    write!(
+                        f,
+                        "Found a partial token '{}' that should not be followed by '{}'.",
+                        first, second
+                    )
+                } else {
+                    write!(
+                        f,
+                        "Found a partial token '{}' that should be followed by another partial \
+                         token.",
+                        first
+                    )
+                }
+            },
+
+            AdditionError { augend, addend } => write!(f, "Error adding {} + {}", augend, addend),
+
+            SubtractionError {
+                minuend,
+                subtrahend,
+            } => write!(f, "Error subtracting {} - {}", minuend, subtrahend),
+
+            NegationError { argument } => write!(f, "Error negating -{}", argument),
+
+            MultiplicationError {
+                multiplicand,
+                multiplier,
+            } => write!(f, "Error multiplying {} * {}", multiplicand, multiplier),
+
+            DivisionError { dividend, divisor } => {
+                write!(f, "Error dividing {} / {}", dividend, divisor)
+            },
+
+            ModulationError { dividend, divisor } => {
+                write!(f, "Error modulating {} % {}", dividend, divisor)
+            },
+        }
+    }
+}

src/error/mod.rs

@@ -9,6 +9,8 @@ use crate::value::Value;
 use token::PartialToken;
 use value::TupleType;
 
+mod display;
+
 /// Errors used in this crate.
 #[derive(Debug, PartialEq)]
 pub enum Error {
@@ -68,10 +70,6 @@ pub enum Error {
     /// The given expression is empty
     EmptyExpression,
 
-    /// Tried to evaluate the root node.
-    /// The root node should only be used as dummy node.
-    EvaluatedRootNode,
-
     /// Tried to append a child to a leaf node.
     /// Leaf nodes cannot have children.
     AppendedToLeafNode,
@@ -274,3 +272,5 @@ pub fn expect_boolean(actual: &Value) -> Result<bool, Error> {
         _ => Err(Error::expected_boolean(actual.clone())),
     }
 }
+
+impl std::error::Error for Error {}