diff --git a/src/parsing.rs b/src/parsing.rs
index 5333cbd3c..9d921fe18 100644
--- a/src/parsing.rs
+++ b/src/parsing.rs
@@ -1,5 +1,6 @@
 //! Tokenization and parsing of source code into syntax trees.
 
+use std::collections::HashMap;
 use std::fmt;
 use std::iter::Peekable;
 use std::mem::swap;
@@ -208,7 +209,7 @@ impl<'s> Tokens<'s> {
 pub struct Parser<'s, T> where T: Iterator<Item=Token<'s>> {
     tokens: Peekable<T>,
     state: ParserState,
-    stack: Vec<Function<'s>>,
+    stack: Vec<FuncInvocation>,
     tree: SyntaxTree<'s>,
 }
 
@@ -296,9 +297,13 @@ impl<'s, T> Parser<'s, T> where T: Iterator<Item=Token<'s>> {
                 },
 
                 PS::Function => {
-                    let name = match token {
-                        Token::Word(word) if word.is_identifier() => word,
-                        _ => return self.err("expected identifier"),
+                    let name = if let Token::Word(word) = token {
+                        match Ident::new(word) {
+                            Some(ident) => ident,
+                            None => return self.err("invalid identifier"),
+                        }
+                    } else {
+                        return self.err("expected identifier");
                     };
 
                     self.advance();
@@ -306,9 +311,15 @@ impl<'s, T> Parser<'s, T> where T: Iterator<Item=Token<'s>> {
                         return self.err("expected closing bracket");
                     }
 
-                    let mut func = Function {
+                    let header = FuncHeader {
                         name,
-                        body: None,
+                        args: vec![],
+                        kwargs: HashMap::new(),
+                    };
+
+                    let func = FuncInvocation {
+                        header,
+                        body: unimplemented!(),
                     };
 
                     // This function has a body.
diff --git a/src/syntax.rs b/src/syntax.rs
index 46b848323..fdb71d145 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -1,5 +1,10 @@
 //! Tokenized and syntax tree representations of source code.
 
+use std::fmt::Debug;
+use std::collections::HashMap;
+use std::ops::Deref;
+use crate::utility::StrExt;
+
 
 /// A logical unit of the incoming text stream.
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
@@ -32,7 +37,7 @@ pub enum Token<'s> {
 }
 
 /// A tree representation of the source.
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Debug, PartialEq)]
 pub struct SyntaxTree<'s> {
     /// The children.
     pub nodes: Vec<Node<'s>>,
@@ -47,7 +52,7 @@ impl<'s> SyntaxTree<'s> {
 }
 
 /// A node in the abstract syntax tree.
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Debug, PartialEq)]
 pub enum Node<'s> {
     /// Whitespace between other nodes.
     Space,
@@ -62,15 +67,98 @@ pub enum Node<'s> {
     /// A literal word.
     Word(&'s str),
     /// A function invocation.
-    Func(Function<'s>),
+    Func(FuncInvocation),
 }
 
-/// A node representing a function invocation.
-#[derive(Debug, Clone, PartialEq)]
-pub struct Function<'s> {
-    /// The name of the function.
-    pub name: &'s str,
-    /// Some syntax tree if the function had a body (second set of brackets),
-    /// otherwise nothing.
-    pub body: Option<SyntaxTree<'s>>,
+/// A complete function invocation consisting of header and body.
+#[derive(Debug, PartialEq)]
+pub struct FuncInvocation {
+    pub header: FuncHeader,
+    pub body: Box<dyn Function>,
+}
+
+/// Contains header information of a function invocation.
+#[derive(Debug, Clone, PartialEq)]
+pub struct FuncHeader {
+    pub name: Ident,
+    pub args: Vec<Expression>,
+    pub kwargs: HashMap<Ident, Expression>
+}
+
+use std::any::Any;
+
+/// Types that act as functions.
+pub trait Function: Debug + FunctionHelper {
+    /// Parse the function.
+    fn parse() -> Self where Self: Sized;
+
+    /// Execute the function and optionally yield a return value.
+    fn typeset(&self, header: &FuncHeader) -> Option<Expression>;
+}
+
+trait FunctionHelper {
+    fn help_as_any(&self) -> &dyn Any;
+    fn help_eq(&self, other: &dyn Function) -> bool;
+}
+
+impl<T> FunctionHelper for T where T: Clone + PartialEq + 'static {
+    fn help_as_any(&self) -> &dyn Any {
+        self
+    }
+
+    fn help_eq(&self, other: &dyn Function) -> bool {
+        if let Some(other) = other.help_as_any().downcast_ref::<Self>() {
+            self == other
+        } else {
+            false
+        }
+    }
+}
+
+impl PartialEq<dyn Function> for &dyn Function {
+    fn eq(&self, other: &dyn Function) -> bool {
+        self.help_eq(other)
+    }
+}
+
+impl PartialEq<Box<dyn Function>> for Box<dyn Function> {
+    fn eq(&self, other: &Box<dyn Function>) -> bool {
+        &*self == &*other
+    }
+}
+
+/// A potentially unevaluated expression.
+#[derive(Debug, Clone, PartialEq)]
+pub enum Expression {}
+
+/// A valid identifier.
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+pub struct Ident(String);
+
+impl Ident {
+    /// Create a new identifier if the string is a valid one.
+    #[inline]
+    pub fn new<S: Into<String>>(ident: S) -> Option<Ident> {
+        let ident = ident.into();
+        if ident.is_identifier() {
+            Some(Ident(ident))
+        } else {
+            None
+        }
+    }
+
+    /// Consume self and return the underlying string.
+    #[inline]
+    pub fn into_inner(self) -> String {
+        self.0
+    }
+}
+
+impl Deref for Ident {
+    type Target = str;
+
+    #[inline]
+    fn deref(&self) -> &str {
+        &*self.0
+    }
 }