diff --git a/dust-lang/src/analyzer.rs b/dust-lang/src/analyzer.rs
index c17e2cb..c41124d 100644
--- a/dust-lang/src/analyzer.rs
+++ b/dust-lang/src/analyzer.rs
@@ -17,8 +17,8 @@ use crate::{
         PrimitiveValueExpression, RangeExpression, Span, Statement, StructDefinition,
         StructExpression, TupleAccessExpression,
     },
-    core_library, parse, Context, ContextError, DustError, Expression, Identifier, StructType,
-    Type,
+    core_library, parse, Context, ContextError, DustError, Expression, Identifier, Rangeable,
+    RangeableType, StructType, Type,
 };
 
 /// Analyzes the abstract syntax tree for errors.
@@ -255,6 +255,18 @@ impl<'recovered, 'a: 'recovered> Analyzer<'a> {
                             list: list.clone(),
                         });
                     }
+                } else if let Type::Range { r#type } = &index_type {
+                    if let RangeableType::Integer = r#type {
+                        // Ok
+                    } else {
+                        return Err(AnalysisError::ExpectedType {
+                            expected: Type::Range {
+                                r#type: RangeableType::Integer,
+                            },
+                            actual: index_type,
+                            actual_expression: index.clone(),
+                        });
+                    }
                 } else {
                     return Err(AnalysisError::ExpectedType {
                         expected: Type::Integer,
diff --git a/dust-lang/src/core_library.rs b/dust-lang/src/core_library.rs
index 8b14ae4..6c771a2 100644
--- a/dust-lang/src/core_library.rs
+++ b/dust-lang/src/core_library.rs
@@ -16,24 +16,6 @@ pub fn core_library<'a>() -> &'a Context {
                     (0, 0),
                 ),
             ),
-            (
-                Identifier::new("is_even"),
-                (
-                    ContextData::VariableValue(Value::Function(Function::BuiltIn(
-                        BuiltInFunction::IsEven,
-                    ))),
-                    (0, 0),
-                ),
-            ),
-            (
-                Identifier::new("is_odd"),
-                (
-                    ContextData::VariableValue(Value::Function(Function::BuiltIn(
-                        BuiltInFunction::IsOdd,
-                    ))),
-                    (0, 0),
-                ),
-            ),
             (
                 Identifier::new("read_line"),
                 (
diff --git a/dust-lang/src/value.rs b/dust-lang/src/value.rs
index 11ac735..bf364a4 100644
--- a/dust-lang/src/value.rs
+++ b/dust-lang/src/value.rs
@@ -234,27 +234,36 @@ impl Value {
     }
 
     pub fn get_field(&self, field: &Identifier) -> Option<Value> {
-        let built_in_function = match field.as_str() {
-            "to_string" => BuiltInFunction::ToString,
-            "length" => {
-                return match self {
-                    Value::List(values) => Some(Value::Integer(values.len() as i64)),
-                    Value::String(string) => Some(Value::Integer(string.len() as i64)),
-                    Value::Map(map) => Some(Value::Integer(map.len() as i64)),
-                    _ => None,
-                }
-            }
-            _ => {
-                return match self {
-                    Value::Mutable(inner) => inner.read().unwrap().get_field(field),
-                    Value::Struct(Struct::Fields { fields, .. }) => fields.get(field).cloned(),
-                    Value::Map(pairs) => pairs.get(field).cloned(),
-                    _ => None,
-                };
-            }
-        };
+        if let Value::Mutable(locked) = self {
+            return locked.read().unwrap().get_field(field);
+        }
 
-        Some(Value::Function(Function::BuiltIn(built_in_function)))
+        match field.as_str() {
+            "is_even" => match self {
+                Value::Integer(integer) => Some(Value::Boolean(integer % 2 == 0)),
+                Value::Float(float) => Some(Value::Boolean(float % 2.0 == 0.0)),
+                _ => None,
+            },
+            "is_odd" => match self {
+                Value::Integer(integer) => Some(Value::Boolean(integer % 2 != 0)),
+                Value::Float(float) => Some(Value::Boolean(float % 2.0 != 0.0)),
+                _ => None,
+            },
+            "to_string" => Some(Value::Function(Function::BuiltIn(
+                BuiltInFunction::ToString,
+            ))),
+            "length" => match self {
+                Value::List(values) => Some(Value::Integer(values.len() as i64)),
+                Value::String(string) => Some(Value::Integer(string.len() as i64)),
+                Value::Map(map) => Some(Value::Integer(map.len() as i64)),
+                _ => None,
+            },
+            _ => match self {
+                Value::Struct(Struct::Fields { fields, .. }) => fields.get(field).cloned(),
+                Value::Map(pairs) => pairs.get(field).cloned(),
+                _ => None,
+            },
+        }
     }
 
     pub fn get_index(&self, index: Value) -> Result<Option<Value>, ValueError> {
diff --git a/dust-lang/src/vm.rs b/dust-lang/src/vm.rs
index 6be144d..ca8d206 100644
--- a/dust-lang/src/vm.rs
+++ b/dust-lang/src/vm.rs
@@ -737,7 +737,7 @@ impl Vm {
                     });
                 }
             } else {
-                return Err(RuntimeError::UndefinedProperty {
+                return Err(RuntimeError::UndefinedField {
                     value: container_value,
                     value_position: container_position,
                     property: field.inner,
@@ -1130,7 +1130,7 @@ pub enum RuntimeError {
         identifier: Identifier,
         position: Span,
     },
-    UndefinedProperty {
+    UndefinedField {
         value: Value,
         value_position: Span,
         property: Identifier,
@@ -1175,7 +1175,7 @@ impl RuntimeError {
             } => (start_position.0, end_position.1),
 
             Self::UndefinedType { position, .. } => *position,
-            Self::UndefinedProperty {
+            Self::UndefinedField {
                 property_position, ..
             } => *property_position,
         }
@@ -1326,7 +1326,7 @@ impl Display for RuntimeError {
                     "Identifier \"{identifier}\" is not associated with a value or constructor"
                 )
             }
-            Self::UndefinedProperty {
+            Self::UndefinedField {
                 value, property, ..
             } => {
                 write!(f, "Value {} does not have the property {}", value, property)
@@ -1672,14 +1672,14 @@ mod tests {
 
     #[test]
     fn is_even() {
-        let input = "is_even(42)";
+        let input = "42.is_even";
 
         assert_eq!(run(input), Ok(Some(Value::Boolean(true))));
     }
 
     #[test]
     fn is_odd() {
-        let input = "is_odd(42)";
+        let input = "42.is_odd";
 
         assert_eq!(run(input), Ok(Some(Value::Boolean(false))));
     }