diff --git a/CHANGELOG.md b/CHANGELOG.md
index bf3241f..79648f6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -22,12 +22,13 @@
 - Formatter
 - Simple language server
 - Decide how to handle comments in the syntax (?)(should comments mean something like in rust?)
+- Not ignore comments & whitespace, for code formatting
 
 ## v0.0.12
 
 - [x] Infer datatype of an identifier
 - [x] Infer datatype of a binary operatior
-- [ ] Infer datatype of unary operator
+- [x] Infer datatype of unary operator
 - [ ] Infer datatype of a function call expression
 - [ ] Infer datatype of binary operators
 - [x] Infer Int & Float as different types
diff --git a/src/main.rs b/src/main.rs
index 3852a10..d524ba7 100755
--- a/src/main.rs
+++ b/src/main.rs
@@ -11,6 +11,8 @@ mod syntax;
 mod lexic;
 // Module to handle semantic analysis
 mod semantic;
+// Defines the PHP AST
+mod php_ast;
 // Transforms an AST to JS
 mod codegen;
 
diff --git a/src/php_ast/mod.rs b/src/php_ast/mod.rs
new file mode 100644
index 0000000..d1f1486
--- /dev/null
+++ b/src/php_ast/mod.rs
@@ -0,0 +1,7 @@
+
+// Follows https://phplang.org/spec/09-lexical-structure.html
+
+struct PhpAst {
+
+}
+
diff --git a/src/semantic/types/expression.rs b/src/semantic/types/expression.rs
index 1572a80..167d6c6 100644
--- a/src/semantic/types/expression.rs
+++ b/src/semantic/types/expression.rs
@@ -30,7 +30,46 @@ impl Typed for Expression<'_> {
                 Ok(datatype)
             }
             Expression::FunctionCall(_) => todo!(),
-            Expression::UnaryOperator(_, _) => todo!(),
+            Expression::UnaryOperator(op, exp) => {
+                let expr_type = match exp.get_type(scope) {
+                    Ok(t) => t,
+                    Err(_reason) => {
+                        return Err(MistiError::Semantic(SemanticError {
+                            error_start: 0,
+                            error_end: 1,
+                            reason: format!("Error getting type of expression"),
+                        }))
+                    }
+                };
+
+                // Only supported unary operator: - & !
+                if *op == "-" {
+                    if expr_type != "Int" && expr_type != "Float" {
+                        return Err(MistiError::Semantic(SemanticError {
+                            error_start: 0,
+                            error_end: 1,
+                            reason: format!(
+                                "Expected a Int or Float after unary `-`, got {}",
+                                expr_type
+                            ),
+                        }));
+                    } else {
+                        return Ok("Int".into());
+                    }
+                } else if *op == "!" {
+                    if expr_type != "Bool" {
+                        return Err(MistiError::Semantic(SemanticError {
+                            error_start: 0,
+                            error_end: 1,
+                            reason: format!("Expected a Bool after unary `!`, got {}", expr_type),
+                        }));
+                    } else {
+                        return Ok("Bool".into());
+                    }
+                }
+
+                panic!("Illegal state: Found an unexpected unary operator during semantic analysis: {}", *op);
+            }
             Expression::BinaryOperator(exp1, exp2, operator) => {
                 let t1 = exp1.get_type(scope)?;
                 let t2 = exp2.get_type(scope)?;