diff --git a/src/func/helpers.rs b/src/func/helpers.rs
index a7c270c26..6e6896c9f 100644
--- a/src/func/helpers.rs
+++ b/src/func/helpers.rs
@@ -1,8 +1,9 @@
-use std::iter::Peekable;
-use std::slice::Iter;
-use super::prelude::*;
+//! Helper types and macros for creating custom functions.
 
-/// Implement the function trait more concisely.
+use super::prelude::*;
+use Expression::*;
+
+/// Lets you implement the function trait more concisely.
 #[macro_export]
 macro_rules! function {
     (data: $ident:ident, $($tts:tt)*) => (
@@ -15,15 +16,15 @@ macro_rules! function {
     );
 
     (@parse $ident:ident, parse: plain, $($tts:tt)*) => (
-        fn parse(header: &FuncHeader, body: Option<&str>, _: ParseContext)
-            -> ParseResult<Self> where Self: Sized
-        {
-            Arguments::new(header).done()?;
+        fn parse(header: &FuncHeader, body: Option<&str>, _: ParseContext) -> ParseResult<Self>
+        where Self: Sized {
+            ArgParser::new(&header.args).done()?;
             if body.is_some() {
                 err!("expected no body");
             }
             Ok($ident)
         }
+
         function!(@layout $($tts)*);
     );
 
@@ -33,14 +34,15 @@ macro_rules! function {
         $block:block
         $($tts:tt)*
     ) => (
-        fn parse(header: &FuncHeader, body: Option<&str>, ctx: ParseContext)
-            -> ParseResult<Self> where Self: Sized
-        {
-            #[allow(unused_mut)] let mut $args = Arguments::new(header);
+        fn parse(header: &FuncHeader, body: Option<&str>, ctx: ParseContext) -> ParseResult<Self>
+        where Self: Sized {
+            #[allow(unused_mut)]
+            let mut $args = ArgParser::new(&header.args);
             let $body = body;
             let $ctx = ctx;
             $block
         }
+
         function!(@layout $($tts)*);
     );
 
@@ -82,51 +84,110 @@ macro_rules! parse {
     )
 }
 
-/// Return a formatted parsing error.
+/// Early-return with a formatted parsing error or yield
+/// an error expression without returning when prefixed with `@`.
 #[macro_export]
 macro_rules! err {
     (@$($tts:tt)*) => ($crate::syntax::ParseError::new(format!($($tts)*)));
     ($($tts:tt)*) => (return Err(err!(@$($tts)*)););
 }
 
-/// Convenient interface for parsing function arguments.
-pub struct Arguments<'a> {
-    args: Peekable<Iter<'a, Spanned<Expression>>>,
+/// Easy parsing of function arguments.
+pub struct ArgParser<'a> {
+    args: &'a FuncArgs,
+    positional_index: usize,
 }
 
-impl<'a> Arguments<'a> {
-    pub fn new(header: &'a FuncHeader) -> Arguments<'a> {
-        Arguments {
-            args: header.args.positional.iter().peekable()
+impl<'a> ArgParser<'a> {
+    pub fn new(args: &'a FuncArgs) -> ArgParser<'a> {
+        ArgParser {
+            args,
+            positional_index: 0,
         }
     }
 
-    pub fn get_expr(&mut self) -> ParseResult<&'a Spanned<Expression>> {
-        self.args.next()
-            .ok_or_else(|| ParseError::new("expected expression"))
+    /// Get the next positional argument of the given type.
+    ///
+    /// If there are no more arguments or the type is wrong,
+    /// this will return an error.
+    pub fn get_pos<T>(&mut self) -> ParseResult<Spanned<T::Output>> where T: Argument<'a> {
+        self.get_pos_opt::<T>()?
+            .ok_or_else(|| err!(@"expected {}", T::ERROR_MESSAGE))
     }
 
-    pub fn get_ident(&mut self) -> ParseResult<Spanned<&'a str>> {
-        let expr = self.get_expr()?;
-        match &expr.val {
-            Expression::Ident(s) => Ok(Spanned::new(s.as_str(), expr.span)),
-            _ => err!("expected identifier"),
+    /// Get the next positional argument if there is any.
+    ///
+    /// If the argument is of the wrong type, this will return an error.
+    pub fn get_pos_opt<T>(&mut self) -> ParseResult<Option<Spanned<T::Output>>>
+    where T: Argument<'a> {
+        let arg = self.args.positional
+            .get(self.positional_index)
+            .map(T::from_expr)
+            .transpose();
+
+        if let Ok(Some(_)) = arg {
+            self.positional_index += 1;
         }
+
+        arg
     }
 
-    pub fn get_ident_if_present(&mut self) -> ParseResult<Option<Spanned<&'a str>>> {
-        if self.args.peek().is_some() {
-            self.get_ident().map(|s| Some(s))
-        } else {
-            Ok(None)
-        }
+    /// Get a keyword argument with the given key and type.
+    pub fn get_key<T>(&mut self, key: &str) -> ParseResult<Spanned<T::Output>>
+    where T: Argument<'a> {
+        self.get_key_opt::<T>(key)?
+            .ok_or_else(|| err!(@"expected {}", T::ERROR_MESSAGE))
     }
 
-    pub fn done(&mut self) -> ParseResult<()> {
-        if self.args.peek().is_none() {
+    /// Get a keyword argument with the given key and type if it is present.
+    pub fn get_key_opt<T>(&mut self, key: &str) -> ParseResult<Option<Spanned<T::Output>>>
+    where T: Argument<'a> {
+        self.args.keyword.iter()
+            .find(|entry| entry.val.0.val == key)
+            .map(|entry| T::from_expr(&entry.val.1))
+            .transpose()
+    }
+
+    /// Assert that there are no positional arguments left. Returns an error, otherwise.
+    pub fn done(&self) -> ParseResult<()> {
+        if self.positional_index == self.args.positional.len() {
             Ok(())
         } else {
-            Err(ParseError::new("unexpected argument"))
+            err!("unexpected argument");
         }
     }
 }
+
+/// A kind of argument.
+pub trait Argument<'a> {
+    type Output;
+    const ERROR_MESSAGE: &'static str;
+
+    fn from_expr(expr: &'a Spanned<Expression>) -> ParseResult<Spanned<Self::Output>>;
+}
+
+macro_rules! arg {
+    ($type:ident, $err:expr, $doc:expr, $output:ty, $wanted:pat => $converted:expr) => (
+        #[doc = $doc]
+        #[doc = " argument for use with the [`ArgParser`]."]
+        pub struct $type;
+        impl<'a> Argument<'a> for $type {
+            type Output = $output;
+            const ERROR_MESSAGE: &'static str = $err;
+
+            fn from_expr(expr: &'a Spanned<Expression>) -> ParseResult<Spanned<Self::Output>> {
+                match &expr.val {
+                    $wanted => Ok(Spanned::new($converted, expr.span)),
+                    #[allow(unreachable_patterns)] _ => err!("expected {}", $err),
+                }
+            }
+        }
+    );
+}
+
+arg!(ArgExpr,  "expression", "A generic expression", &'a Expression, expr => &expr);
+arg!(ArgIdent, "identifier", "An identifier (e.g. `horizontal`)", &'a str, Ident(s) => s.as_str());
+arg!(ArgStr,   "string", "A string (e.g. `\"Hello\"`)", &'a str, Str(s) => s.as_str());
+arg!(ArgNum,   "number", "A number (e.g. `5.4`)", f64, Num(n) => *n);
+arg!(ArgSize,  "size", "A size (e.g. `12pt`)", crate::size::Size, Size(s) => *s);
+arg!(ArgBool,  "bool", "A boolean (`true` or `false`)", bool, Bool(b) => *b);
diff --git a/src/func/mod.rs b/src/func/mod.rs
index 402f0111f..bd61204ec 100644
--- a/src/func/mod.rs
+++ b/src/func/mod.rs
@@ -7,8 +7,7 @@ use std::fmt::{self, Debug, Formatter};
 use self::prelude::*;
 
 #[macro_use]
-mod helpers;
-pub use helpers::Arguments;
+pub mod helpers;
 
 /// Useful imports for creating your own functions.
 pub mod prelude {
@@ -24,11 +23,12 @@ pub mod prelude {
 
 /// Typesetting function types.
 ///
-/// These types have to be able to parse tokens into themselves and store the
-/// relevant information from the parsing to do their role in typesetting later.
+/// These types have to be able to parse themselves from a string and build
+/// a list of layouting commands corresponding to the parsed source.
 ///
-/// The trait `FunctionBounds` is automatically implemented for types which can
-/// be used as functions, that is they fulfill the bounds `Debug + PartialEq +
+/// This trait is a supertrait of `FunctionBounds` for technical reasons.  The
+/// trait `FunctionBounds` is automatically implemented for types which can
+/// be used as functions, that is, all types which fulfill the bounds `Debug + PartialEq +
 /// 'static`.
 pub trait Function: FunctionBounds {
     /// Parse the header and body into this function given a context.
@@ -84,6 +84,19 @@ where T: Debug + PartialEq + 'static
     }
 }
 
+/// Commands requested for execution by functions.
+#[derive(Debug)]
+pub enum Command<'a> {
+    LayoutTree(&'a SyntaxTree),
+    Add(Layout),
+    AddMany(MultiLayout),
+    AddFlex(Layout),
+    SetAlignment(Alignment),
+    SetStyle(TextStyle),
+    FinishLayout,
+    FinishFlexRun,
+}
+
 /// A sequence of commands requested for execution by a function.
 #[derive(Debug)]
 pub struct CommandList<'a> {
@@ -130,19 +143,8 @@ impl<'a> IntoIterator for &'a CommandList<'a> {
     }
 }
 
-/// Commands requested for execution by functions.
-#[derive(Debug)]
-pub enum Command<'a> {
-    Layout(&'a SyntaxTree),
-    Add(Layout),
-    AddMany(MultiLayout),
-    AddFlex(Layout),
-    SetAlignment(Alignment),
-    SetStyle(TextStyle),
-    FinishLayout,
-    FinishFlexRun,
-}
-
+/// Create a list of commands.
+#[macro_export]
 macro_rules! commands {
     ($($x:expr),*$(,)*) => (
         $crate::func::CommandList::from_vec(vec![$($x,)*])
diff --git a/src/layout/actions.rs b/src/layout/actions.rs
index 67ad48b41..707d6113e 100644
--- a/src/layout/actions.rs
+++ b/src/layout/actions.rs
@@ -62,7 +62,7 @@ debug_display!(LayoutAction);
 /// content is written.
 ///
 /// Furthermore, the action list can translate absolute position into a coordinate system
-/// with a different. This is realized in the `add_box` method, which allows a layout to
+/// with a different origin. This is realized in the `add_box` method, which allows a layout to
 /// be added at a position, effectively translating all movement actions inside the layout
 /// by the position.
 #[derive(Debug, Clone)]
diff --git a/src/layout/stacked.rs b/src/layout/stacked.rs
index 7521967b3..379579f50 100644
--- a/src/layout/stacked.rs
+++ b/src/layout/stacked.rs
@@ -2,7 +2,7 @@ use super::*;
 
 /// Layouts boxes stack-like.
 ///
-/// The boxes are arranged vertically, each layout gettings it's own "line".
+/// The boxes are arranged along an axis, each layout gettings it's own "line".
 #[derive(Debug, Clone)]
 pub struct StackLayouter {
     ctx: StackContext,
diff --git a/src/layout/tree.rs b/src/layout/tree.rs
index 131570df5..1d88751fb 100644
--- a/src/layout/tree.rs
+++ b/src/layout/tree.rs
@@ -108,7 +108,7 @@ impl<'a, 'p> TreeLayouter<'a, 'p> {
 
         for command in commands {
             match command {
-                Command::Layout(tree) => self.layout(tree)?,
+                Command::LayoutTree(tree) => self.layout(tree)?,
 
                 Command::Add(layout) => {
                     self.finish_flex()?;
diff --git a/src/library/structure.rs b/src/library/structure.rs
index 977d7499b..e6d242a00 100644
--- a/src/library/structure.rs
+++ b/src/library/structure.rs
@@ -1,6 +1,7 @@
 use crate::func::prelude::*;
+use Command::*;
 
-/// Ends the current page.
+/// 📜 `page.break`: Ends the current page.
 #[derive(Debug, PartialEq)]
 pub struct Pagebreak;
 
@@ -9,11 +10,11 @@ function! {
     parse: plain,
 
     layout(_, _) {
-        Ok(commands![Command::FinishLayout])
+        Ok(commands![FinishLayout])
     }
 }
 
-/// Ends the current line.
+/// 🔙 `line.break`, `n`: Ends the current line.
 #[derive(Debug, PartialEq)]
 pub struct Linebreak;
 
@@ -22,11 +23,14 @@ function! {
     parse: plain,
 
     layout(_, _) {
-        Ok(commands![Command::FinishFlexRun])
+        Ok(commands![FinishFlexRun])
     }
 }
 
-/// Aligns content in different ways.
+/// 📐 `align`: Aligns content in different ways.
+///
+/// **Positional arguments:**
+/// - `left`, `right` or `center` _(required)_.
 #[derive(Debug, PartialEq)]
 pub struct Align {
     body: Option<SyntaxTree>,
@@ -38,7 +42,7 @@ function! {
 
     parse(args, body, ctx) {
         let body = parse!(optional: body, ctx);
-        let arg = args.get_ident()?;
+        let arg = args.get_pos::<ArgIdent>()?;
         let alignment = match arg.val {
             "left" => Alignment::Left,
             "right" => Alignment::Right,
@@ -56,17 +60,22 @@ function! {
     layout(this, ctx) {
         Ok(commands![match &this.body {
             Some(body) => {
-                Command::AddMany(layout_tree(body, LayoutContext {
+                AddMany(layout_tree(body, LayoutContext {
                     alignment: this.alignment,
                     .. ctx
                 })?)
             }
-            None => Command::SetAlignment(this.alignment)
+            None => SetAlignment(this.alignment)
         }])
     }
 }
 
-/// Layouts content into a box.
+/// 📦 `box`: Layouts content into a box.
+///
+/// **Positional arguments:** None.
+///
+/// **Keyword arguments:**
+/// - flow: either `horizontal` or `vertical` _(optional)_.
 #[derive(Debug, PartialEq)]
 pub struct Boxed {
     body: SyntaxTree,
@@ -80,7 +89,7 @@ function! {
         let body = parse!(required: body, ctx);
 
         let mut flow = Flow::Vertical;
-        if let Some(ident) = args.get_ident_if_present()? {
+        if let Some(ident) = args.get_key_opt::<ArgIdent>("flow")? {
             flow = match ident.val {
                 "vertical" => Flow::Vertical,
                 "horizontal" => Flow::Horizontal,
@@ -97,7 +106,7 @@ function! {
 
     layout(this, ctx) {
         Ok(commands![
-            Command::AddMany(layout_tree(&this.body, LayoutContext {
+            AddMany(layout_tree(&this.body, LayoutContext {
                 flow: this.flow,
                 .. ctx
             })?)
@@ -106,8 +115,12 @@ function! {
 }
 
 macro_rules! spacefunc {
-    ($ident:ident, $name:expr, $var:ident => $command:expr) => (
-        /// Adds whitespace.
+    ($ident:ident, $doc:expr, $var:ident => $command:expr) => (
+        #[doc = $doc]
+        ///
+        /// **Positional arguments:**
+        /// - Spacing as a size or number, which is interpreted as a multiple
+        ///   of the font size _(required)_.
         #[derive(Debug, PartialEq)]
         pub struct $ident(Spacing);
 
@@ -117,10 +130,10 @@ macro_rules! spacefunc {
             parse(args, body, _ctx) {
                 parse!(forbidden: body);
 
-                let arg = args.get_expr()?;
+                let arg = args.get_pos::<ArgExpr>()?;
                 let spacing = match arg.val {
-                    Expression::Size(s) => Spacing::Absolute(s),
-                    Expression::Number(f) => Spacing::Relative(f as f32),
+                    Expression::Size(s) => Spacing::Absolute(*s),
+                    Expression::Num(f) => Spacing::Relative(*f as f32),
                     _ => err!("invalid spacing, expected size or number"),
                 };
 
@@ -146,5 +159,8 @@ enum Spacing {
     Relative(f32),
 }
 
-spacefunc!(HorizontalSpace, "h", space => Command::AddFlex(Layout::empty(space, Size::zero())));
-spacefunc!(VerticalSpace, "v", space => Command::Add(Layout::empty(Size::zero(), space)));
+spacefunc!(HorizontalSpace, "📖 `h`: Adds horizontal whitespace.",
+    space => AddFlex(Layout::empty(space, Size::zero())));
+
+spacefunc!(VerticalSpace, "📑 `v`: Adds vertical whitespace.",
+    space => Add(Layout::empty(Size::zero(), space)));
diff --git a/src/library/style.rs b/src/library/style.rs
index 48f4d4db2..9bcdcccd7 100644
--- a/src/library/style.rs
+++ b/src/library/style.rs
@@ -2,8 +2,8 @@ use crate::func::prelude::*;
 use toddle::query::FontClass;
 
 macro_rules! stylefunc {
-    ($ident:ident) => (
-        /// Styles text.
+    ($ident:ident, $doc:expr) => (
+        #[doc = $doc]
         #[derive(Debug, PartialEq)]
         pub struct $ident {
             body: Option<SyntaxTree>
@@ -24,7 +24,7 @@ macro_rules! stylefunc {
                 Ok(match &this.body {
                     Some(body) => commands![
                         Command::SetStyle(new_style),
-                        Command::Layout(body),
+                        Command::LayoutTree(body),
                         Command::SetStyle(ctx.style.clone()),
                     ],
                     None => commands![Command::SetStyle(new_style)]
@@ -34,6 +34,6 @@ macro_rules! stylefunc {
     );
 }
 
-stylefunc!(Italic);
-stylefunc!(Bold);
-stylefunc!(Monospace);
+stylefunc!(Italic, "💡 `italic`: Sets text in _italics_.");
+stylefunc!(Bold, "🧱 `bold`: Sets text in **bold**.");
+stylefunc!(Monospace, "👩‍💻 `mono`: Sets text in `monospace`.");
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index 286bb2c77..09a5bdc39 100644
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -128,7 +128,7 @@ pub enum FuncArg {
 pub enum Expression {
     Ident(String),
     Str(String),
-    Number(f64),
+    Num(f64),
     Size(Size),
     Bool(bool),
 }
@@ -145,7 +145,7 @@ impl Display for Expression {
         match self {
             Ident(s) => write!(f, "{}", s),
             Str(s) => write!(f, "{:?}", s),
-            Number(n) => write!(f, "{}", n),
+            Num(n) => write!(f, "{}", n),
             Size(s) => write!(f, "{}", s),
             Bool(b) => write!(f, "{}", b),
         }
diff --git a/src/syntax/parsing.rs b/src/syntax/parsing.rs
index f952f5e6a..e4cda6a59 100644
--- a/src/syntax/parsing.rs
+++ b/src/syntax/parsing.rs
@@ -211,7 +211,7 @@ impl<'s> Parser<'s> {
                 if let Ok(b) = text.parse::<bool>() {
                     Expression::Bool(b)
                 } else if let Ok(num) = text.parse::<f64>() {
-                    Expression::Number(num)
+                    Expression::Num(num)
                 } else if let Ok(size) = text.parse::<Size>() {
                     Expression::Size(size)
                 } else {
@@ -499,7 +499,7 @@ mod tests {
 
     mod args {
         use super::Expression;
-        pub use Expression::{Number as N, Size as Z, Bool as B};
+        pub use Expression::{Num as N, Size as Z, Bool as B};
 
         pub fn S(string: &str) -> Expression { Expression::Str(string.to_owned()) }
         pub fn I(string: &str) -> Expression { Expression::Ident(string.to_owned()) }
diff --git a/tests/layouts/coma.typ b/tests/layouts/coma.typ
index b56299eca..83900e03c 100644
--- a/tests/layouts/coma.typ
+++ b/tests/layouts/coma.typ
@@ -1,6 +1,6 @@
 {size:420pt*300pt}
 
-[box: horizontal][
+[box: flow=horizontal][
     *Technical University Berlin* [n]
     *Faculty II, Institute for Mathematics* [n]
     Secretary Example [n]
@@ -17,7 +17,7 @@
     *Alle Antworten sind zu beweisen.*
 ]
 
-[box: horizontal][
+[box: flow=horizontal][
     *1. Aufgabe* [align: right][(1 + 1 + 2 Punkte)]
 ]