diff --git a/src/lexer.rs b/src/lexer.rs
index d7a8fca..00a19ae 100644
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -131,6 +131,7 @@ pub fn lexer<'src>() -> impl Parser<
         .or_not()
         .then(text::int(10))
         .then(just('.').then(text::digits(10)))
+        .then(just('e').then(text::digits(10)).or_not())
         .to_slice()
         .map(|text: &str| Token::Float(text.parse().unwrap()));
 
diff --git a/src/value.rs b/src/value.rs
index bb2f978..cc3a5f3 100644
--- a/src/value.rs
+++ b/src/value.rs
@@ -143,22 +143,54 @@ impl Value {
     }
 
     pub fn add(&self, other: &Self) -> Result<Value, ValidationError> {
-        if let (ValueInner::Integer(left), ValueInner::Integer(right)) =
-            (self.inner().as_ref(), other.inner().as_ref())
-        {
-            Ok(Value::integer(left + right))
-        } else {
-            Err(ValidationError::ExpectedIntegerOrFloat)
+        match (self.inner().as_ref(), other.inner().as_ref()) {
+            (ValueInner::Integer(left), ValueInner::Integer(right)) => {
+                let sum = left.saturating_add(*right);
+
+                Ok(Value::integer(sum))
+            }
+            (ValueInner::Float(left), ValueInner::Float(right)) => {
+                let sum = left + right;
+
+                Ok(Value::float(sum))
+            }
+            (ValueInner::Float(left), ValueInner::Integer(right)) => {
+                let sum = left + *right as f64;
+
+                Ok(Value::float(sum))
+            }
+            (ValueInner::Integer(left), ValueInner::Float(right)) => {
+                let sum = *left as f64 + right;
+
+                Ok(Value::float(sum))
+            }
+            _ => Err(ValidationError::ExpectedIntegerOrFloat),
         }
     }
 
     pub fn subtract(&self, other: &Self) -> Result<Value, ValidationError> {
-        if let (ValueInner::Integer(left), ValueInner::Integer(right)) =
-            (self.inner().as_ref(), other.inner().as_ref())
-        {
-            Ok(Value::integer(left - right))
-        } else {
-            Err(ValidationError::ExpectedIntegerOrFloat)
+        match (self.inner().as_ref(), other.inner().as_ref()) {
+            (ValueInner::Integer(left), ValueInner::Integer(right)) => {
+                let sum = left.saturating_sub(*right);
+
+                Ok(Value::integer(sum))
+            }
+            (ValueInner::Float(left), ValueInner::Float(right)) => {
+                let sum = left - right;
+
+                Ok(Value::float(sum))
+            }
+            (ValueInner::Float(left), ValueInner::Integer(right)) => {
+                let sum = left - *right as f64;
+
+                Ok(Value::float(sum))
+            }
+            (ValueInner::Integer(left), ValueInner::Float(right)) => {
+                let sum = *left as f64 - right;
+
+                Ok(Value::float(sum))
+            }
+            _ => Err(ValidationError::ExpectedIntegerOrFloat),
         }
     }
 }
diff --git a/tests/values.rs b/tests/values.rs
new file mode 100644
index 0000000..a0b8698
--- /dev/null
+++ b/tests/values.rs
@@ -0,0 +1,151 @@
+use std::collections::BTreeMap;
+
+use dust_lang::{
+    abstract_tree::{Identifier, Type},
+    error::{Error, TypeCheckError, ValidationError},
+    *,
+};
+
+#[test]
+fn none() {
+    assert_eq!(interpret("x = 9"), Ok(None));
+    assert_eq!(interpret("x = 1 + 1"), Ok(None));
+}
+
+#[test]
+fn integer() {
+    assert_eq!(interpret("1"), Ok(Some(Value::integer(1))));
+    assert_eq!(interpret("123"), Ok(Some(Value::integer(123))));
+    assert_eq!(interpret("-666"), Ok(Some(Value::integer(-666))));
+}
+
+#[test]
+fn integer_saturation() {
+    assert_eq!(
+        interpret("9223372036854775807 + 1"),
+        Ok(Some(Value::integer(i64::MAX)))
+    );
+    assert_eq!(
+        interpret("-9223372036854775808 - 1"),
+        Ok(Some(Value::integer(i64::MIN)))
+    );
+}
+
+#[test]
+fn float() {
+    assert_eq!(
+        interpret("1.7976931348623157e308"),
+        Ok(Some(Value::float(f64::MAX)))
+    );
+    assert_eq!(
+        interpret("-1.7976931348623157e308"),
+        Ok(Some(Value::float(f64::MIN)))
+    );
+}
+
+#[test]
+fn float_saturation() {
+    assert_eq!(
+        interpret("1.7976931348623157e308 + 1"),
+        Ok(Some(Value::float(f64::MAX)))
+    );
+    assert_eq!(
+        interpret("-1.7976931348623157e308 - 1"),
+        Ok(Some(Value::float(f64::MIN)))
+    );
+}
+
+#[test]
+fn string() {
+    assert_eq!(
+        interpret("\"one\""),
+        Ok(Some(Value::string("one".to_string())))
+    );
+    assert_eq!(
+        interpret("'one'"),
+        Ok(Some(Value::string("one".to_string())))
+    );
+    assert_eq!(
+        interpret("`one`"),
+        Ok(Some(Value::string("one".to_string())))
+    );
+    assert_eq!(
+        interpret("`'one'`"),
+        Ok(Some(Value::string("'one'".to_string())))
+    );
+    assert_eq!(
+        interpret("'`one`'"),
+        Ok(Some(Value::string("`one`".to_string())))
+    );
+    assert_eq!(
+        interpret("\"'one'\""),
+        Ok(Some(Value::string("'one'".to_string())))
+    );
+}
+
+#[test]
+fn list() {
+    assert_eq!(
+        interpret("[1, 2, 'foobar']"),
+        Ok(Some(Value::list(vec![
+            Value::integer(1),
+            Value::integer(2),
+            Value::string("foobar".to_string()),
+        ])))
+    );
+}
+
+#[test]
+fn empty_list() {
+    assert_eq!(interpret("[]"), Ok(Some(Value::list(Vec::new()))));
+}
+
+#[test]
+fn map() {
+    let mut map = BTreeMap::new();
+
+    map.insert(Identifier::new("x"), Value::integer(1));
+    map.insert(Identifier::new("foo"), Value::string("bar".to_string()));
+
+    assert_eq!(
+        interpret("{ x = 1, foo = 'bar' }"),
+        Ok(Some(Value::map(map)))
+    );
+}
+
+#[test]
+fn empty_map() {
+    assert_eq!(interpret("{}"), Ok(Some(Value::map(BTreeMap::new()))));
+}
+
+#[test]
+fn map_types() {
+    let mut map = BTreeMap::new();
+
+    map.insert(Identifier::new("x"), Value::integer(1));
+    map.insert(Identifier::new("foo"), Value::string("bar".to_string()));
+
+    assert_eq!(
+        interpret("{ x <int> = 1, foo <str> = 'bar' }"),
+        Ok(Some(Value::map(map)))
+    );
+}
+
+#[test]
+fn map_type_errors() {
+    assert_eq!(
+        interpret("{ foo <bool> = 'bar' }"),
+        Err(vec![Error::Validation {
+            error: ValidationError::TypeCheck(TypeCheckError {
+                actual: Type::String,
+                expected: Type::Boolean
+            }),
+            span: (0..0).into()
+        }])
+    );
+}
+
+#[test]
+fn range() {
+    assert_eq!(interpret("0..100"), Ok(Some(Value::range(0..100))));
+}
diff --git a/tests/variables.rs b/tests/variables.rs
index 12a1d20..32a33ce 100644
--- a/tests/variables.rs
+++ b/tests/variables.rs
@@ -1,5 +1,5 @@
 use dust_lang::{
-    abstract_tree::Type,
+    abstract_tree::{Expression, Identifier, Statement, Type},
     error::{Error, TypeCheckError, ValidationError},
     *,
 };
@@ -33,3 +33,15 @@ fn set_variable_with_type_error() {
         }])
     );
 }
+
+#[test]
+fn function_variable() {
+    assert_eq!(
+        interpret("foobar = (x: int): int x; foobar"),
+        Ok(Some(Value::function(
+            vec![(Identifier::new("x"), Type::Integer)],
+            Type::Integer,
+            Statement::Expression(Expression::Identifier(Identifier::new("x")))
+        )))
+    );
+}