diff --git a/src/ast/mod.rs b/src/ast/mod.rs
index 257df08..39c6626 100644
--- a/src/ast/mod.rs
+++ b/src/ast/mod.rs
@@ -1,5 +1,5 @@
-use pest::{iterators::{Pair, Pairs}, Parser};
 use crate::parser::{DeimosParser, Rule};
+use pest::{iterators::Pair, Parser};
 
 #[derive(Debug)]
 pub struct CompilationUnit {
@@ -9,7 +9,7 @@ pub struct CompilationUnit {
 
 #[derive(Debug)]
 pub struct Fqn {
-    identifiers: Vec<Identifier>
+    identifiers: Vec<Identifier>,
 }
 
 #[derive(Debug)]
@@ -25,7 +25,7 @@ pub enum Declaration {
         is_extern: bool,
         is_public: bool,
         type_declaration: TypeDeclaration,
-        impl_ctor: Option<ImplCtor>
+        impl_ctor: Option<ImplCtor>,
     },
     Module {
         identifier: Identifier,
@@ -37,92 +37,98 @@ pub enum Declaration {
         identifier: Identifier,
         is_extern: bool,
         is_public: bool,
-        statement: Statement
+        parameters: Vec<Parameter>,
+        declared_type: Option<TypeUse>,
+        statement: Statement,
     },
     Prop(Prop),
-    Field(Field)
+    Field(Field),
 }
 
 #[derive(Debug)]
 pub struct TypeDeclaration {
     generics: Vec<GenericParameter>,
-    extends: Vec<Type>,
+    extends: Vec<TypeUse>,
     declarations: Vec<Declaration>,
 }
 
 impl TypeDeclaration {
-    pub fn new(generics: Vec<GenericParameter>, extends: Vec<Type>, declarations: Vec<Declaration>) -> TypeDeclaration {
+    pub fn new(
+        generics: Vec<GenericParameter>,
+        extends: Vec<TypeUse>,
+        declarations: Vec<Declaration>,
+    ) -> TypeDeclaration {
         TypeDeclaration {
             generics,
             extends,
-            declarations
+            declarations,
         }
     }
 }
 
 #[derive(Debug)]
 pub struct Identifier {
-    name: String
+    name: String,
 }
 
 #[derive(Debug)]
 pub struct GenericParameter {
     identifier: Identifier,
-    bound: Option<GenericBound>
+    bound: Option<GenericBound>,
 }
 
 #[derive(Debug)]
 pub enum GenericBound {
-    Extends {
-        fqn: Fqn
-    },
-    Super {
-        fqn: Fqn
-    }
+    Extends { fqn: Fqn },
+    Super { fqn: Fqn },
 }
 
 #[derive(Debug)]
 pub struct GenericArgument {
-    fqn: Fqn
+    fqn: Fqn,
 }
 
 #[derive(Debug)]
-pub struct Type {
+pub struct TypeUse {
     fqn: Fqn,
-    generics: Vec<GenericArgument>
+    generics: Vec<GenericArgument>,
 }
 
 #[derive(Debug)]
 pub struct ImplCtor {
-    args: Vec<ImplCtorArg>
+    args: Vec<ImplCtorArg>,
 }
 
 #[derive(Debug)]
 pub struct ImplCtorArg {
     is_field: bool,
     identifier: Identifier,
-    r#type: Option<Type>
+    r#type: Option<TypeUse>,
 }
 
 #[derive(Debug)]
 pub struct Prop {
     identifier: Identifier,
-    r#type: Type
+    r#type: TypeUse,
 }
 
 #[derive(Debug)]
 pub struct Field {
     identifier: Identifier,
-    r#type: Type
+    r#type: TypeUse,
 }
 
 #[derive(Debug)]
 pub enum Statement {
-    BlockStatement {
-        statements: Vec<Statement>
-    },
+    BlockStatement { statements: Vec<Statement> },
     CallStatement,
-    AssignmentStatement
+    AssignmentStatement,
+}
+
+#[derive(Debug)]
+pub struct Parameter {
+    identifier: Identifier,
+    declared_type: Option<TypeUse>,
 }
 
 fn build_field(field_pair: Pair<Rule>) -> Declaration {
@@ -136,27 +142,27 @@ fn build_prop(prop_pair: Pair<Rule>) -> Declaration {
 fn build_impl_ctor_arg(impl_ctor_arg_pair: Pair<Rule>) -> ImplCtorArg {
     let mut is_field = false;
     let mut identifier: Option<Identifier> = None;
-    let mut r#type: Option<Type> = None;
+    let mut r#type: Option<TypeUse> = None;
 
     for pair in impl_ctor_arg_pair.into_inner() {
         match pair.as_rule() {
             Rule::fld => {
                 is_field = true;
-            },
+            }
             Rule::identifier => {
                 identifier = Some(build_identifier(pair));
-            },
-            Rule::r#type => {
-                r#type = Some(build_type(pair));
-            },
-            _ => panic!("Unexpected rule: {}", pair)
+            }
+            Rule::type_use => {
+                r#type = Some(build_type_use(pair));
+            }
+            _ => panic!("Unexpected rule: {}", pair),
         }
     }
 
     ImplCtorArg {
         is_field,
         identifier: identifier.unwrap(),
-        r#type
+        r#type,
     }
 }
 
@@ -169,30 +175,30 @@ fn build_impl_ctor(impl_ctor_pair: Pair<Rule>) -> ImplCtor {
             Rule::impl_ctor_arg => {
                 args.push(build_impl_ctor_arg(pair));
             }
-            _ => panic!("Unexpected rule: {}", pair)
+            _ => panic!("Unexpected rule: {}", pair),
         }
     }
 
-    ImplCtor {
-        args
-    }
+    ImplCtor { args }
 }
 
 fn build_generic_argument(generic_argument_pair: Pair<Rule>) -> GenericArgument {
     let fqn_pair = generic_argument_pair.into_inner().next().unwrap();
     GenericArgument {
-        fqn: build_fqn(fqn_pair)
+        fqn: build_fqn(fqn_pair),
     }
 }
 
-fn build_generic_arguments_declaration(generic_arguments_declaration_pair: Pair<Rule>) -> Vec<GenericArgument> {
+fn build_generic_arguments_declaration(
+    generic_arguments_declaration_pair: Pair<Rule>,
+) -> Vec<GenericArgument> {
     let mut generic_arguments: Vec<GenericArgument> = vec![];
     for pair in generic_arguments_declaration_pair.into_inner() {
         match pair.as_rule() {
             Rule::generic_argument => {
                 generic_arguments.push(build_generic_argument(pair));
             }
-            _ => panic!("Expected only generic_argument rules. Found: {}", pair)
+            _ => panic!("Expected only generic_argument rules. Found: {}", pair),
         }
     }
     generic_arguments
@@ -206,7 +212,7 @@ fn build_generic_parameter(generic_parameter_pair: Pair<Rule>) -> GenericParamet
     }
 }
 
-fn build_type(type_pair: Pair<Rule>) -> Type {
+fn build_type_use(type_pair: Pair<Rule>) -> TypeUse {
     let mut fqn: Option<Fqn> = None;
     let mut generic_arguments_declaration: Option<Vec<GenericArgument>> = None;
 
@@ -218,38 +224,43 @@ fn build_type(type_pair: Pair<Rule>) -> Type {
             Rule::generic_arguments_declaration => {
                 generic_arguments_declaration = Some(build_generic_arguments_declaration(pair));
             }
-            _ => panic!("Expected only fqn or generic_arguments_declaration rules. Found: {}", pair)
+            _ => panic!(
+                "Expected only fqn or generic_arguments_declaration rules. Found: {}",
+                pair
+            ),
         }
     }
 
-    Type {
+    TypeUse {
         fqn: fqn.unwrap(),
-        generics: generic_arguments_declaration.unwrap_or(vec![])
+        generics: generic_arguments_declaration.unwrap_or(vec![]),
     }
 }
 
-fn build_generic_parameters_declaration(generic_parameters_declaration_pair: Pair<Rule>) -> Vec<GenericParameter> {
+fn build_generic_parameters_declaration(
+    generic_parameters_declaration_pair: Pair<Rule>,
+) -> Vec<GenericParameter> {
     let mut parameters: Vec<GenericParameter> = vec![];
     for pair in generic_parameters_declaration_pair.into_inner() {
         match pair.as_rule() {
             Rule::generic_parameter => {
                 parameters.push(build_generic_parameter(pair));
             }
-            _ => panic!("Expected only generic_parameter rule. Found: {}", pair)
+            _ => panic!("Expected only generic_parameter rule. Found: {}", pair),
         }
     }
     parameters
 }
 
-fn build_extends_list(extends_list_pair: Pair<Rule>) -> Vec<Type> {
-    let mut extensions: Vec<Type> = vec![];
+fn build_extends_list(extends_list_pair: Pair<Rule>) -> Vec<TypeUse> {
+    let mut extensions: Vec<TypeUse> = vec![];
 
     for pair in extends_list_pair.into_inner() {
         match pair.as_rule() {
-            Rule::r#type => {
-                extensions.push(build_type(pair));
+            Rule::type_use => {
+                extensions.push(build_type_use(pair));
             }
-            _ => panic!("Expected only type rule. Found: {}", pair)
+            _ => panic!("Expected only type rule. Found: {}", pair),
         }
     }
 
@@ -259,7 +270,7 @@ fn build_extends_list(extends_list_pair: Pair<Rule>) -> Vec<Type> {
 fn build_interface(is_extern: bool, is_public: bool, interface_pair: Pair<Rule>) -> Declaration {
     let mut identifier: Option<Identifier> = None;
     let mut generic_parameters: Option<Vec<GenericParameter>> = None;
-    let mut extends: Option<Vec<Type>> = None;
+    let mut extends: Option<Vec<TypeUse>> = None;
     let mut declarations: Vec<Declaration> = vec![];
 
     for pair in interface_pair.into_inner() {
@@ -290,36 +301,40 @@ fn build_interface(is_extern: bool, is_public: bool, interface_pair: Pair<Rule>)
         type_declaration: TypeDeclaration::new(
             generic_parameters.unwrap_or(vec![]),
             extends.unwrap_or(vec![]),
-            declarations
-        )
+            declarations,
+        ),
     }
 }
 
-fn build_implementation(is_extern: bool, is_public: bool, implementation_pair: Pair<Rule>) -> Declaration {
+fn build_implementation(
+    is_extern: bool,
+    is_public: bool,
+    implementation_pair: Pair<Rule>,
+) -> Declaration {
     let mut identifier: Option<Identifier> = None;
     let mut generic_parameters: Option<Vec<GenericParameter>> = None;
     let mut impl_ctor: Option<ImplCtor> = None;
-    let mut extends: Option<Vec<Type>> = None;
+    let mut extends: Option<Vec<TypeUse>> = None;
     let mut declarations: Vec<Declaration> = vec![];
 
     for pair in implementation_pair.into_inner() {
         match pair.as_rule() {
             Rule::identifier => {
                 identifier = Some(build_identifier(pair));
-            },
+            }
             Rule::generic_parameters_declaration => {
                 generic_parameters = Some(build_generic_parameters_declaration(pair));
-            },
+            }
             Rule::impl_ctor => {
                 impl_ctor = Some(build_impl_ctor(pair));
-            },
+            }
             Rule::extends_list => {
                 extends = Some(build_extends_list(pair));
-            },
+            }
             Rule::declaration => {
                 declarations.push(build_declaration(pair));
             }
-            _ => panic!("Unexpected rule: {}", pair)
+            _ => panic!("Unexpected rule: {}", pair),
         }
     }
 
@@ -330,9 +345,9 @@ fn build_implementation(is_extern: bool, is_public: bool, implementation_pair: P
         type_declaration: TypeDeclaration::new(
             generic_parameters.unwrap_or(vec![]),
             extends.unwrap_or(vec![]),
-            declarations
+            declarations,
         ),
-        impl_ctor
+        impl_ctor,
     }
 }
 
@@ -340,10 +355,88 @@ fn build_module(is_extern: bool, is_public: bool, pair: Pair<Rule>) -> Declarati
     todo!()
 }
 
-fn build_function(is_extern: bool, is_public: bool, pair: Pair<Rule>) -> Declaration {
+fn build_parameter(pair: Pair<Rule>) -> Parameter {
+    let mut identifier: Option<Identifier> = None;
+    let mut declared_type: Option<TypeUse> = None;
+    for pair in pair.into_inner() {
+        match pair.as_rule() {
+            Rule::identifier => {
+                identifier = Some(build_identifier(pair));
+            }
+            Rule::type_use => {
+                declared_type = Some(build_type_use(pair));
+            }
+            _ => unreachable!(),
+        }
+    }
+    Parameter {
+        identifier: identifier.unwrap(),
+        declared_type,
+    }
+}
+
+fn build_parameters_list(pair: Pair<Rule>) -> Vec<Parameter> {
+    let mut parameters: Vec<Parameter> = vec![];
+    for pair in pair.into_inner() {
+        match pair.as_rule() {
+            Rule::parameter => {
+                parameters.push(build_parameter(pair));
+            }
+            _ => unreachable!(),
+        }
+    }
+    parameters
+}
+
+fn build_function_body(pair: Pair<Rule>) -> Statement {
     todo!()
 }
 
+fn build_function_equals_body(pair: Pair<Rule>) -> Statement {
+    todo!()
+}
+
+fn build_function(is_extern: bool, is_public: bool, pair: Pair<Rule>) -> Declaration {
+    let mut generic_parameters: Option<Vec<GenericParameter>> = None;
+    let mut identifier: Option<Identifier> = None;
+    let mut parameters: Option<Vec<Parameter>> = None;
+    let mut return_type: Option<TypeUse> = None;
+    let mut statement: Option<Statement> = None;
+
+    for pair in pair.into_inner() {
+        match pair.as_rule() {
+            Rule::generic_parameters_declaration => {
+                generic_parameters = Some(build_generic_parameters_declaration(pair));
+            }
+            Rule::identifier => {
+                identifier = Some(build_identifier(pair));
+            }
+            Rule::parameters_list => {
+                parameters = Some(build_parameters_list(pair));
+            }
+            Rule::type_use => {
+                return_type = Some(build_type_use(pair));
+            }
+            Rule::function_body => {
+                statement = Some(build_function_body(pair));
+            }
+            Rule::function_equals_body => {
+                statement = Some(build_function_equals_body(pair));
+            }
+            _ => unreachable!(),
+        }
+    }
+
+    Declaration::Function {
+        is_extern,
+        is_public,
+        identifier: identifier.unwrap(),
+        parameters: parameters.unwrap(),
+        declared_type: return_type,
+        statement: statement.unwrap(),
+    }
+}
+
 fn build_declaration(declaration_pair: Pair<Rule>) -> Declaration {
     let mut is_extern = false;
     let mut is_public = false;
@@ -353,29 +446,32 @@ fn build_declaration(declaration_pair: Pair<Rule>) -> Declaration {
         match pair.as_rule() {
             Rule::r#extern => {
                 is_extern = true;
-            },
+            }
             Rule::r#pub => {
                 is_public = true;
             }
             Rule::interface => {
                 declaration = Some(build_interface(is_extern, is_public, pair));
-            },
+            }
             Rule::implementation => {
                 declaration = Some(build_implementation(is_extern, is_public, pair));
-            },
+            }
             Rule::module => {
                 declaration = Some(build_module(is_extern, is_public, pair));
-            },
+            }
             Rule::function => {
                 declaration = Some(build_function(is_extern, is_public, pair));
-            },
+            }
             Rule::prop => {
                 declaration = Some(build_prop(pair));
             }
             Rule::field => {
                 declaration = Some(build_field(pair));
             }
-            _ => panic!("Expected only interface, implementation, module, or function rules; found {}", pair)
+            _ => panic!(
+                "Expected only interface, implementation, module, or function rules; found {}",
+                pair
+            ),
         }
     }
     declaration.expect("Expected declaration.")
@@ -383,12 +479,10 @@ fn build_declaration(declaration_pair: Pair<Rule>) -> Declaration {
 
 fn build_identifier(pair: Pair<Rule>) -> Identifier {
     match pair.as_rule() {
-        Rule::identifier => {
-            Identifier {
-                name: String::from(pair.as_str())
-            }
-        }
-        _ => panic!("Expected an identifier.")
+        Rule::identifier => Identifier {
+            name: String::from(pair.as_str()),
+        },
+        _ => panic!("Expected an identifier."),
     }
 }
 
@@ -399,15 +493,13 @@ fn build_fqn(fqn_pair: Pair<Rule>) -> Fqn {
             Rule::identifier => {
                 identifiers.push(build_identifier(pair));
             }
-            _ => panic!("Expected only identifiers.")
+            _ => panic!("Expected only identifiers."),
         }
     }
-    Fqn {
-        identifiers
-    }
+    Fqn { identifiers }
 }
 
-fn build_compilation_unit(pair: Pair<Rule>)-> CompilationUnit {
+fn build_compilation_unit(pair: Pair<Rule>) -> CompilationUnit {
     let mut namespace: Option<Fqn> = None;
     let mut declarations: Vec<Declaration> = vec![];
 
@@ -421,13 +513,16 @@ fn build_compilation_unit(pair: Pair<Rule>)-> CompilationUnit {
                 declarations.push(build_declaration(pair));
             }
             Rule::EOI => {} // ignore
-            _ => panic!("Expected only namespace, declaration, or EOI rules, found: {}", pair)
+            _ => panic!(
+                "Expected only namespace, declaration, or EOI rules, found: {}",
+                pair
+            ),
         }
     }
 
     CompilationUnit {
         namespace,
-        declarations
+        declarations,
     }
 }
 
@@ -437,10 +532,7 @@ pub fn build_ast(src: &str) -> CompilationUnit {
         .next()
         .expect("Expcted compilation_unit.");
     match pair.as_rule() {
-        Rule::compilation_unit => {
-            build_compilation_unit(pair)
-        },
-        _ => panic!("Expected compilation_unit rule.")
+        Rule::compilation_unit => build_compilation_unit(pair),
+        _ => panic!("Expected compilation_unit rule."),
     }
 }
-
diff --git a/src/parser/deimos.pest b/src/parser/deimos.pest
index 342a139..14c00f6 100644
--- a/src/parser/deimos.pest
+++ b/src/parser/deimos.pest
@@ -11,34 +11,34 @@ extern = { "extern" }
 pub = { "pub" }
 
 interface = { "int" ~ identifier ~ generic_parameters_declaration? ~ extends_list? ~ ( "{" ~ declaration* ~ "}" )? }
-extends_list = { ":" ~ type ~ ( "+" ~ type )* }
+extends_list = { ":" ~ type_use ~ ( "+" ~ type_use )* }
 
 implementation = { "impl" ~ identifier ~ generic_parameters_declaration? ~ impl_ctor? ~ extends_list? ~ ( "{" ~ declaration* ~ "}" )? }
 impl_ctor = { "(" ~ impl_ctor_args? ~ ")" }
 impl_ctor_args = { impl_ctor_arg ~ ( "," ~ impl_ctor_arg )* }
-impl_ctor_arg = { fld? ~ identifier ~ ( ":" ~ type )? }
+impl_ctor_arg = { fld? ~ identifier ~ ( ":" ~ type_use )? }
 fld = { "fld" }
 
 module = { "mod" ~ identifier ~ "{" ~ declaration* ~ "}" }
 
-property = { identifier ~ ( ":" ~ type )? }
+property = { identifier ~ ( ":" ~ type_use )? }
 
-function = { "fn" ~ generic_parameters_declaration? ~ identifier ~ "(" ~ args_list? ~ ")" ~ ( ":" ~ type )? ~ ( function_body | function_equals_body ) }
+function = { "fn" ~ generic_parameters_declaration? ~ identifier ~ "(" ~ parameters_list? ~ ")" ~ ( ":" ~ type_use )? ~ ( function_body | function_equals_body ) }
 function_equals_body = { "=" ~ expression }
 function_body = { "{" ~ statement* ~ "}" }
 
-type = { fqn ~ generic_arguments_declaration? }
+type_use = { fqn ~ generic_arguments_declaration? }
 generic_parameters_declaration = { "<" ~ generic_parameter ~ ( "," ~ generic_parameter )* ~ ">" }
 generic_parameter = { identifier }
 
 generic_arguments_declaration = { "<" ~ generic_argument ~ ( "," ~ generic_argument )* ~ ">" }
 generic_argument = { fqn }
 
-args_list = { arg ~ ( "," ~ arg )* }
-arg = { identifier ~ ( ":" ~ "..."? ~ type )? }
+parameters_list = { parameter ~ ( "," ~ parameter )* }
+parameter = { identifier ~ ( ":" ~ "..."? ~ type_use )? }
 
-prop = { identifier ~ ":" ~ type }
-field = { fld ~ identifier ~ ":" ~ type }
+prop = { identifier ~ ":" ~ type_use }
+field = { fld ~ identifier ~ ":" ~ type_use }
 
 statement = { call_stmt }