diff --git a/src/codegen/function_declaration.rs b/src/codegen/function_declaration.rs
index d0c4359..ca5824a 100644
--- a/src/codegen/function_declaration.rs
+++ b/src/codegen/function_declaration.rs
@@ -2,11 +2,11 @@ use crate::syntax::ast::FunctionDeclaration;
 
 use super::Transpilable;
 
-impl Transpilable for FunctionDeclaration {
+impl Transpilable for FunctionDeclaration<'_> {
     fn transpile(&self) -> String {
         format!(
             "function {}() {{\n{}\n}}",
-            self.identifier,
+            self.identifier.value,
             self.block.transpile()
         )
     }
diff --git a/src/codegen/module_ast.rs b/src/codegen/module_ast.rs
index d4c4067..0c56af6 100644
--- a/src/codegen/module_ast.rs
+++ b/src/codegen/module_ast.rs
@@ -1,7 +1,7 @@
 use super::Transpilable;
 use crate::syntax::ast::ModuleAST;
 
-impl Transpilable for ModuleAST {
+impl Transpilable for ModuleAST<'_> {
     /// Transpiles the whole AST into PHP, using this same trait on the
     /// nodes and leaves of the AST
     fn transpile(&self) -> String {
diff --git a/src/codegen/top_level_construct.rs b/src/codegen/top_level_construct.rs
index 9c3f412..2e9fc9e 100644
--- a/src/codegen/top_level_construct.rs
+++ b/src/codegen/top_level_construct.rs
@@ -2,7 +2,7 @@ use crate::syntax::ast::TopLevelDeclaration;
 
 use super::Transpilable;
 
-impl Transpilable for TopLevelDeclaration {
+impl Transpilable for TopLevelDeclaration<'_> {
     fn transpile(&self) -> String {
         match self {
             TopLevelDeclaration::Binding(binding) => binding.transpile(),
diff --git a/src/semantic/impls.rs b/src/semantic/impls.rs
index 17735f7..d8600f9 100644
--- a/src/semantic/impls.rs
+++ b/src/semantic/impls.rs
@@ -10,7 +10,7 @@ pub trait SemanticCheck {
     fn check_semantics(&self, scope: &SymbolTable) -> Result<(), MistiError>;
 }
 
-impl SemanticCheck for ModuleAST {
+impl SemanticCheck for ModuleAST<'_> {
     /// Checks that this AST is semantically correct, given a symbol table
     fn check_semantics(&self, scope: &SymbolTable) -> Result<(), MistiError> {
         for declaration in &self.declarations {
@@ -21,7 +21,7 @@ impl SemanticCheck for ModuleAST {
     }
 }
 
-impl SemanticCheck for TopLevelDeclaration {
+impl SemanticCheck for TopLevelDeclaration<'_> {
     fn check_semantics(&self, scope: &SymbolTable) -> Result<(), MistiError> {
         match self {
             TopLevelDeclaration::Binding(_) => {
@@ -34,15 +34,13 @@ impl SemanticCheck for TopLevelDeclaration {
                 Err(MistiError::Semantic(error))
             }
             TopLevelDeclaration::FunctionDeclaration(function) => {
-                let function_name = function.identifier.as_ref().clone();
+                let function_name = function.identifier.value.clone();
 
                 if scope.test(&function_name) {
                     let error = SemanticError {
-                        // TODO: Get the position of the function name. For this, these structs
-                        // should store the token instead of just the string
-                        error_start: 0,
-                        error_end: 0,
-                        reason: format!("Function {} already defined", function_name),
+                        error_start: function.identifier.position,
+                        error_end: function.identifier.get_end_position(),
+                        reason: format!("Duplicated function: A function with name {} was already defined", function_name),
                     };
 
                     return Err(MistiError::Semantic(error));
diff --git a/src/syntax/ast/mod.rs b/src/syntax/ast/mod.rs
index cbd6da1..5b2cdd8 100644
--- a/src/syntax/ast/mod.rs
+++ b/src/syntax/ast/mod.rs
@@ -1,22 +1,24 @@
+use crate::lexic::token::Token;
+
 use self::functions::FunctionCall;
 
 pub mod functions;
 pub mod statement;
 pub mod var_binding;
 
-pub struct ModuleAST {
-    pub declarations: Vec<TopLevelDeclaration>,
+pub struct ModuleAST<'a> {
+    pub declarations: Vec<TopLevelDeclaration<'a>>,
 }
 
 #[derive(Debug)]
-pub enum TopLevelDeclaration {
+pub enum TopLevelDeclaration<'a> {
     Binding(var_binding::Binding),
-    FunctionDeclaration(FunctionDeclaration),
+    FunctionDeclaration(FunctionDeclaration<'a>),
 }
 
 #[derive(Debug)]
-pub struct FunctionDeclaration {
-    pub identifier: Box<String>,
+pub struct FunctionDeclaration<'a> {
+    pub identifier: &'a Token,
     pub params_list: Box<ParamsList>,
     pub block: Box<Block>,
 }
diff --git a/src/syntax/functions/function_declaration.rs b/src/syntax/functions/function_declaration.rs
index 128167a..5a60481 100644
--- a/src/syntax/functions/function_declaration.rs
+++ b/src/syntax/functions/function_declaration.rs
@@ -91,7 +91,7 @@ pub fn try_parse<'a>(tokens: &'a Vec<Token>, pos: usize) -> ParseResult<Function
     // Construct and return the function declaration
     ParseResult::Ok(
         FunctionDeclaration {
-            identifier: Box::new(identifier.value.clone()),
+            identifier: &identifier,
             params_list: Box::new(params_list),
             block: Box::new(block),
         },
@@ -311,10 +311,7 @@ mod tests {
             panic!("Expected a function declaration.")
         };
 
-        assert_eq!(
-            function_declaration.identifier,
-            Box::new(String::from("id"))
-        );
+        assert_eq!(function_declaration.identifier.value, String::from("id"));
     }
 }
 
@@ -331,7 +328,7 @@ mod whitespace_test {
             panic!("Expected a function declaration.")
         };
 
-        assert_eq!(declaration.identifier, Box::new(String::from("id")));
+        assert_eq!(declaration.identifier.value, (String::from("id")));
     }
 
     #[test]
@@ -341,7 +338,7 @@ mod whitespace_test {
             panic!("Expected a function declaration.")
         };
 
-        assert_eq!(declaration.identifier, Box::new(String::from("id")));
+        assert_eq!(declaration.identifier.value, (String::from("id")));
     }
 
     #[test]
@@ -351,7 +348,7 @@ mod whitespace_test {
             panic!("Expected a function declaration.")
         };
 
-        assert_eq!(declaration.identifier, Box::new(String::from("id")));
+        assert_eq!(declaration.identifier.value, (String::from("id")));
     }
 
     #[test]
@@ -360,7 +357,7 @@ mod whitespace_test {
         let ParseResult::Ok(declaration, _) = try_parse(&tokens, 0) else {
             panic!("Expected a function declaration.")
         };
-        assert_eq!(declaration.identifier, Box::new(String::from("id")));
+        assert_eq!(declaration.identifier.value, (String::from("id")));
     }
 
     #[test]
@@ -369,6 +366,6 @@ mod whitespace_test {
         let ParseResult::Ok(declaration, _) = try_parse(&tokens, 0) else {
             panic!("Expected a function declaration.")
         };
-        assert_eq!(declaration.identifier, Box::new(String::from("id")));
+        assert_eq!(declaration.identifier.value, (String::from("id")));
     }
 }