Merge pull request #2 from typst/expressions

Add parsing capabilities for mathematical expressions

6 files changed, 301 insertions, 91 deletions

diff --git a/src/func.rs b/src/func.rs
index 2cdd026c..33cc6e83 100644
--- a/src/func.rs
+++ b/src/func.rs
@@ -132,7 +132,7 @@ macro_rules! function {
                 let func = $code;
 
                 for arg in header.args.into_iter() {
-                    feedback.errors.push(err!(arg.span(); "unexpected argument"));
+                    feedback.errors.push(err!(arg.span; "unexpected argument"));
                 }
 
                 $crate::Pass::new(func, feedback)
diff --git a/src/syntax/expr.rs b/src/syntax/expr.rs
index 9d690266..1dc47d03 100644
--- a/src/syntax/expr.rs
+++ b/src/syntax/expr.rs
@@ -1,6 +1,6 @@
 //! Expressions in function headers.
 
-use std::fmt::{self, Write, Debug, Formatter};
+use std::fmt::{self, Debug, Formatter};
 use std::iter::FromIterator;
 use std::ops::Deref;
 use std::str::FromStr;
@@ -26,7 +26,7 @@ pub enum Expr {
     Size(Size),
     /// A bool: `true, false`.
     Bool(bool),
-    /// A color value, including the alpha channel: `#f79143ff`
+    /// A color value, including the alpha channel: `#f79143ff`.
     Color(RgbaColor),
     /// A tuple: `(false, 12cm, "hi")`.
     Tuple(Tuple),
@@ -34,6 +34,16 @@ pub enum Expr {
     NamedTuple(NamedTuple),
     /// An object: `{ fit: false, size: 12pt }`.
     Object(Object),
+    /// An operator that negates the contained expression.
+    Neg(Box<Spanned<Expr>>),
+    /// An operator that adds the contained expressions.
+    Add(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
+    /// An operator that subtracts contained expressions.
+    Sub(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
+    /// An operator that multiplies the contained expressions.
+    Mul(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
+    /// An operator that divides the contained expressions.
+    Div(Box<Spanned<Expr>>, Box<Spanned<Expr>>),
 }
 
 impl Expr {
@@ -50,6 +60,11 @@ impl Expr {
             Tuple(_) => "tuple",
             NamedTuple(_) => "named tuple",
             Object(_) => "object",
+            Neg(_) => "negation",
+            Add(_, _) => "addition",
+            Sub(_, _) => "subtraction",
+            Mul(_, _) => "multiplication",
+            Div(_, _) => "division",
         }
     }
 }
@@ -67,6 +82,11 @@ impl Debug for Expr {
             Tuple(t) => t.fmt(f),
             NamedTuple(t) => t.fmt(f),
             Object(o) => o.fmt(f),
+            Neg(e) => write!(f, "-{:?}", e),
+            Add(a, b) => write!(f, "({:?} + {:?})", a, b),
+            Sub(a, b) => write!(f, "({:?} - {:?})", a, b),
+            Mul(a, b) => write!(f, "({:?} * {:?})", a, b),
+            Div(a, b) => write!(f, "({:?} / {:?})", a, b),
         }
     }
 }
@@ -102,9 +122,7 @@ impl Ident {
 
 impl Debug for Ident {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        f.write_char('`')?;
-        f.write_str(&self.0)?;
-        f.write_char('`')
+        write!(f, "`{}`", self.0)
     }
 }
 
@@ -346,7 +364,7 @@ impl Deref for NamedTuple {
 #[derive(Default, Clone, PartialEq)]
 pub struct Object {
     /// The key-value pairs of the object.
-    pub pairs: Vec<Pair>,
+    pub pairs: Vec<Spanned<Pair>>,
 }
 
 /// A key-value pair in an object.
@@ -373,7 +391,7 @@ impl Object {
     }
 
     /// Add a pair to object.
-    pub fn add(&mut self, pair: Pair) {
+    pub fn add(&mut self, pair: Spanned<Pair>) {
         self.pairs.push(pair);
     }
 
@@ -383,7 +401,7 @@ impl Object {
     /// Inserts an error if the value does not match. If the key is not
     /// contained, no error is inserted.
     pub fn get<V: Value>(&mut self, errors: &mut Errors, key: &str) -> Option<V> {
-        let index = self.pairs.iter().position(|pair| pair.key.v.as_str() == key)?;
+        let index = self.pairs.iter().position(|pair| pair.v.key.v.as_str() == key)?;
         self.get_index::<V>(errors, index)
     }
 
@@ -396,7 +414,7 @@ impl Object {
         errors: &mut Errors,
     ) -> Option<(K, V)> {
         for (index, pair) in self.pairs.iter().enumerate() {
-            let key = Spanned { v: pair.key.v.as_str(), span: pair.key.span };
+            let key = Spanned { v: pair.v.key.v.as_str(), span: pair.v.key.span };
             if let Some(key) = K::parse(key) {
                 return self.get_index::<V>(errors, index).map(|value| (key, value));
             }
@@ -414,7 +432,7 @@ impl Object {
         let mut index = 0;
         std::iter::from_fn(move || {
             if index < self.pairs.len() {
-                let key = &self.pairs[index].key;
+                let key = &self.pairs[index].v.key;
                 let key = Spanned { v: key.v.as_str(), span: key.span };
 
                 Some(if let Some(key) = K::parse(key) {
@@ -447,7 +465,7 @@ impl Object {
     /// Extract the argument at the given index and insert an error if the value
     /// does not match.
     fn get_index<V: Value>(&mut self, errors: &mut Errors, index: usize) -> Option<V> {
-        let expr = self.pairs.remove(index).value;
+        let expr = self.pairs.remove(index).v.value;
         let span = expr.span;
         match V::parse(expr) {
             Ok(output) => Some(output),
@@ -456,14 +474,14 @@ impl Object {
     }
 
     /// Iterate over the pairs of this object.
-    pub fn iter<'a>(&'a self) -> std::slice::Iter<'a, Pair> {
+    pub fn iter<'a>(&'a self) -> std::slice::Iter<'a, Spanned<Pair>> {
         self.pairs.iter()
     }
 }
 
 impl IntoIterator for Object {
-    type Item = Pair;
-    type IntoIter = std::vec::IntoIter<Pair>;
+    type Item = Spanned<Pair>;
+    type IntoIter = std::vec::IntoIter<Spanned<Pair>>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.pairs.into_iter()
@@ -471,16 +489,16 @@ impl IntoIterator for Object {
 }
 
 impl<'a> IntoIterator for &'a Object {
-    type Item = &'a Pair;
-    type IntoIter = std::slice::Iter<'a, Pair>;
+    type Item = &'a Spanned<Pair>;
+    type IntoIter = std::slice::Iter<'a, Spanned<Pair>>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.iter()
     }
 }
 
-impl FromIterator<Pair> for Object {
-    fn from_iter<I: IntoIterator<Item=Pair>>(iter: I) -> Self {
+impl FromIterator<Spanned<Pair>> for Object {
+    fn from_iter<I: IntoIterator<Item=Spanned<Pair>>>(iter: I) -> Self {
         Object { pairs: iter.into_iter().collect() }
     }
 }
@@ -488,7 +506,7 @@ impl FromIterator<Pair> for Object {
 impl Debug for Object {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
         f.debug_map()
-            .entries(self.pairs.iter().map(|p| (&p.key.v, &p.value.v)))
+            .entries(self.pairs.iter().map(|p| (&p.v.key.v, &p.v.value.v)))
             .finish()
     }
 }
diff --git a/src/syntax/func/mod.rs b/src/syntax/func/mod.rs
index 53bbc14e..fd516208 100644
--- a/src/syntax/func/mod.rs
+++ b/src/syntax/func/mod.rs
@@ -42,22 +42,27 @@ impl FuncArgs {
     }
 
     /// Add an argument.
-    pub fn add(&mut self, arg: FuncArg) {
-        match arg {
-            FuncArg::Pos(item) => self.pos.add(item),
-            FuncArg::Key(pair) => self.key.add(pair),
+    pub fn add(&mut self, arg: Spanned<FuncArg>) {
+        match arg.v {
+            FuncArg::Pos(item) => self.pos.add(Spanned::new(item, arg.span)),
+            FuncArg::Key(pair) => self.key.add(Spanned::new(pair, arg.span)),
         }
     }
 
     /// Iterate over all arguments.
-    pub fn into_iter(self) -> impl Iterator<Item=FuncArg> {
-        self.pos.items.into_iter().map(|item| FuncArg::Pos(item))
-            .chain(self.key.pairs.into_iter().map(|pair| FuncArg::Key(pair)))
+    pub fn into_iter(self) -> impl Iterator<Item=Spanned<FuncArg>> {
+        let pos = self.pos.items.into_iter()
+            .map(|spanned| spanned.map(|item| FuncArg::Pos(item)));
+
+        let key = self.key.pairs.into_iter()
+        .map(|spanned| spanned.map(|pair| FuncArg::Key(pair)));
+
+        pos.chain(key)
     }
 }
 
-impl FromIterator<FuncArg> for FuncArgs {
-    fn from_iter<I: IntoIterator<Item=FuncArg>>(iter: I) -> Self {
+impl FromIterator<Spanned<FuncArg>> for FuncArgs {
+    fn from_iter<I: IntoIterator<Item=Spanned<FuncArg>>>(iter: I) -> Self {
         let mut args = FuncArgs::new();
         for item in iter.into_iter() {
             args.add(item);
@@ -70,24 +75,11 @@ impl FromIterator<FuncArg> for FuncArgs {
 #[derive(Debug, Clone, PartialEq)]
 pub enum FuncArg {
     /// A positional argument.
-    Pos(Spanned<Expr>),
+    Pos(Expr),
     /// A keyword argument.
     Key(Pair),
 }
 
-impl FuncArg {
-    /// The full span of this argument.
-    ///
-    /// In case of a positional argument this is just the span of the expression
-    /// and in case of a keyword argument the combined span of key and value.
-    pub fn span(&self) -> Span {
-        match self {
-            FuncArg::Pos(item) => item.span,
-            FuncArg::Key(Pair { key, value }) => Span::merge(key.span, value.span),
-        }
-    }
-}
-
 /// Extra methods on [`Options`](Option) used for argument parsing.
 pub trait OptionExt: Sized {
     /// Add an error about a missing argument `arg` with the given span if the
diff --git a/src/syntax/parsing.rs b/src/syntax/parsing.rs
index cfbf2bf3..9636df21 100644
--- a/src/syntax/parsing.rs
+++ b/src/syntax/parsing.rs
@@ -190,10 +190,8 @@ impl<'s> FuncParser<'s> {
             if let Some(ident) = p.parse_ident() {
                 // This could still be a named tuple
                 if let Some(Token::LeftParen) = p.peekv() {
-                    return Ok(FuncArg::Pos(
-                        p.parse_named_tuple(ident)
-                            .map(|t| Expr::NamedTuple(t))
-                    ));
+                    let tuple = p.parse_named_tuple(ident);
+                    return Ok(tuple.map(|t| FuncArg::Pos(Expr::NamedTuple(t))));
                 }
 
                 p.skip_whitespace();
@@ -209,23 +207,100 @@ impl<'s> FuncParser<'s> {
                     let value = p.parse_expr().ok_or(("value", None))?;
 
                     // Add a keyword argument.
-                    Ok(FuncArg::Key(Pair { key: ident, value }))
+                    let span = Span::merge(ident.span, value.span);
+                    let pair = Pair { key: ident, value };
+                    Ok(Spanned::new(FuncArg::Key(pair), span))
                 } else {
                     // Add a positional argument because there was no equals
                     // sign after the identifier that could have been a key.
-                    Ok(FuncArg::Pos(ident.map(|id| Expr::Ident(id))))
+                    Ok(ident.map(|id| FuncArg::Pos(Expr::Ident(id))))
                 }
             } else {
                 // Add a positional argument because we haven't got an
                 // identifier that could be an argument key.
-                p.parse_expr().map(|expr| FuncArg::Pos(expr))
-                    .ok_or(("argument", None))
+                let value = p.parse_expr().ok_or(("argument", None))?;
+                Ok(value.map(|expr| FuncArg::Pos(expr)))
             }
         }).v
     }
 
-    /// Parse an atomic or compound (tuple / object) expression.
+    /// Parse an expression which may contain math operands. For this, this
+    /// method looks for operators in descending order of associativity, i.e. we
+    /// first drill down to find all negations, brackets and tuples, the next
+    /// level, we look for multiplication and division and here finally, for
+    /// addition and subtraction.
     fn parse_expr(&mut self) -> Option<Spanned<Expr>> {
+        let o1 = self.parse_term()?;
+        self.parse_binop(o1, "summand", Self::parse_expr, |token| match token {
+            Token::Plus => Some(Expr::Add),
+            Token::Hyphen => Some(Expr::Sub),
+            _ => None,
+        })
+    }
+
+    fn parse_term(&mut self) -> Option<Spanned<Expr>> {
+        let o1 = self.parse_factor()?;
+        self.parse_binop(o1, "factor", Self::parse_term, |token| match token {
+            Token::Star => Some(Expr::Mul),
+            Token::Slash => Some(Expr::Div),
+            _ => None,
+        })
+    }
+
+    fn parse_binop<F, G>(
+        &mut self,
+        o1: Spanned<Expr>,
+        operand_name: &str,
+        parse_operand: F,
+        parse_op: G,
+    ) -> Option<Spanned<Expr>>
+    where
+        F: FnOnce(&mut Self) -> Option<Spanned<Expr>>,
+        G: FnOnce(Token) -> Option<fn(Box<Spanned<Expr>>, Box<Spanned<Expr>>) -> Expr>,
+    {
+        self.skip_whitespace();
+
+        if let Some(next) = self.peek() {
+            if let Some(binop) = parse_op(next.v) {
+                self.eat();
+                self.skip_whitespace();
+
+                if let Some(o2) = parse_operand(self) {
+                    let span = Span::merge(o1.span, o2.span);
+                    let expr = binop(Box::new(o1), Box::new(o2));
+                    return Some(Spanned::new(expr, span));
+                } else {
+                    self.feedback.errors.push(err!(
+                        Span::merge(next.span, o1.span);
+                        "missing right {}", operand_name,
+                    ));
+                }
+            }
+        }
+
+        Some(o1)
+    }
+
+    /// Parse expressions that are of the form value or -value.
+    fn parse_factor(&mut self) -> Option<Spanned<Expr>> {
+        let first = self.peek()?;
+        if first.v == Token::Hyphen {
+            self.eat();
+            self.skip_whitespace();
+
+            if let Some(factor) = self.parse_value() {
+                let span = Span::merge(first.span, factor.span);
+                Some(Spanned::new(Expr::Neg(Box::new(factor)), span))
+            } else {
+                self.feedback.errors.push(err!(first.span; "dangling minus"));
+                None
+            }
+        } else {
+            self.parse_value()
+        }
+    }
+
+    fn parse_value(&mut self) -> Option<Spanned<Expr>> {
         let first = self.peek()?;
         macro_rules! take {
             ($v:expr) => ({ self.eat(); Spanned { v: $v, span: first.span } });
@@ -263,27 +338,36 @@ impl<'s> FuncParser<'s> {
                 }
             },
 
-            Token::LeftParen => self.parse_tuple().map(|t| Expr::Tuple(t)),
+            Token::LeftParen => {
+                let (mut tuple, can_be_coerced) = self.parse_tuple();
+                // Coerce 1-tuple into value
+                if can_be_coerced && tuple.v.items.len() > 0 {
+                    tuple.v.items.pop().expect("length is at least one")
+                } else {
+                    tuple.map(|t| Expr::Tuple(t))
+                }
+            },
             Token::LeftBrace => self.parse_object().map(|o| Expr::Object(o)),
 
             _ => return None,
         })
     }
 
-    /// Parse a tuple expression: `(<expr>, ...)`.
-    fn parse_tuple(&mut self) -> Spanned<Tuple> {
+    /// Parse a tuple expression: `(<expr>, ...)`. The boolean in the return
+    /// values showes whether the tuple can be coerced into a single value.
+    fn parse_tuple(&mut self) -> (Spanned<Tuple>, bool) {
         let token = self.eat();
         debug_assert_eq!(token.map(Spanned::value), Some(Token::LeftParen));
 
         // Parse a collection until a right paren appears and complain about
         // missing a `value` when an invalid token is encoutered.
-        self.parse_collection(Some(Token::RightParen),
+        self.parse_collection_comma_aware(Some(Token::RightParen),
             |p| p.parse_expr().ok_or(("value", None)))
     }
 
     /// Parse a tuple expression: `name(<expr>, ...)` with a given identifier.
     fn parse_named_tuple(&mut self, name: Spanned<Ident>) -> Spanned<NamedTuple> {
-        let tuple = self.parse_tuple();
+        let tuple = self.parse_tuple().0;
         let span = Span::merge(name.span, tuple.span);
         Spanned::new(NamedTuple::new(name, tuple), span)
     }
@@ -314,7 +398,8 @@ impl<'s> FuncParser<'s> {
 
             let value = p.parse_expr().ok_or(("value", None))?;
 
-            Ok(Pair { key, value })
+            let span = Span::merge(key.span, value.span);
+            Ok(Spanned::new(Pair { key, value }, span))
         })
     }
 
@@ -323,13 +408,30 @@ impl<'s> FuncParser<'s> {
     fn parse_collection<C, I, F>(
         &mut self,
         end: Option<Token>,
-        mut parse_item: F
+        parse_item: F
     ) -> Spanned<C>
     where
-        C: FromIterator<I>,
-        F: FnMut(&mut Self) -> Result<I, (&'static str, Option<Position>)>,
+        C: FromIterator<Spanned<I>>,
+        F: FnMut(&mut Self) -> Result<Spanned<I>, (&'static str, Option<Position>)>,
+    {
+        self.parse_collection_comma_aware(end, parse_item).0
+    }
+
+    /// Parse a comma-separated collection where each item is parsed through
+    /// `parse_item` until the `end` token is met. The first item in the return
+    /// tuple is the collection, the second item indicates whether the
+    /// collection can be coerced into a single item (i.e. no comma appeared).
+    fn parse_collection_comma_aware<C, I, F>(
+        &mut self,
+        end: Option<Token>,
+        mut parse_item: F
+    ) -> (Spanned<C>, bool)
+    where
+        C: FromIterator<Spanned<I>>,
+        F: FnMut(&mut Self) -> Result<Spanned<I>, (&'static str, Option<Position>)>,
     {
         let start = self.pos();
+        let mut can_be_coerced = true;
 
         // Parse the comma separated items.
         let collection = std::iter::from_fn(|| {
@@ -356,11 +458,16 @@ impl<'s> FuncParser<'s> {
                 Ok(item) => {
                     // Expect a comma behind the item (only separated by
                     // whitespace).
-                    let behind_item = self.pos();
                     self.skip_whitespace();
                     match self.peekv() {
-                        Some(Token::Comma) => { self.eat(); }
-                        t @ Some(_) if t != end => self.expected_at("comma", behind_item),
+                        Some(Token::Comma) => {
+                            can_be_coerced = false;
+                            self.eat();
+                        }
+                        t @ Some(_) if t != end => {
+                            can_be_coerced = false;
+                            self.expected_at("comma", item.span.end);
+                        },
                         _ => {}
                     }
 
@@ -383,7 +490,7 @@ impl<'s> FuncParser<'s> {
         }).filter_map(|x| x).collect();
 
         let end = self.pos();
-        Spanned::new(collection, Span { start, end })
+        (Spanned::new(collection, Span { start, end }), can_be_coerced)
     }
 
     /// Try to parse an identifier and do nothing if the peekable token is no
@@ -546,6 +653,19 @@ mod tests {
     fn Id(text: &str) -> Expr { Expr::Ident(Ident(text.to_string())) }
     fn Str(text: &str) -> Expr { Expr::Str(text.to_string()) }
     fn Pt(points: f32) -> Expr { Expr::Size(Size::pt(points)) }
+    fn Neg(e1: Expr) -> Expr { Expr::Neg(Box::new(zspan(e1))) }
+    fn Add(e1: Expr, e2: Expr) -> Expr {
+        Expr::Add(Box::new(zspan(e1)), Box::new(zspan(e2)))
+    }
+    fn Sub(e1: Expr, e2: Expr) -> Expr {
+        Expr::Sub(Box::new(zspan(e1)), Box::new(zspan(e2)))
+    }
+    fn Mul(e1: Expr, e2: Expr) -> Expr {
+        Expr::Mul(Box::new(zspan(e1)), Box::new(zspan(e2)))
+    }
+    fn Div(e1: Expr, e2: Expr) -> Expr {
+        Expr::Div(Box::new(zspan(e1)), Box::new(zspan(e2)))
+    }
 
     fn Clr(r: u8, g: u8, b: u8, a: u8) -> Expr {
         Expr::Color(RgbaColor::new(r, g, b, a))
@@ -589,10 +709,10 @@ mod tests {
     macro_rules! object {
         ($($key:expr => $value:expr),* $(,)?) => {
             Expr::Object(Object {
-                pairs: vec![$(Pair {
+                pairs: vec![$(zspan(Pair {
                     key: zspan(Ident($key.to_string())),
                     value: zspan($value),
-                }),*]
+                })),*]
             })
         };
     }
@@ -814,6 +934,12 @@ mod tests {
         p!("[val: 12e1pt]" => [func!("val": (Pt(12e1)), {})]);
         p!("[val: #f7a20500]" => [func!("val": (ClrStr("f7a20500")), {})]);
 
+        // Math
+        p!("[val: 3.2in + 6pt]" => [func!("val": (Add(Sz(Size::inches(3.2)), Sz(Size::pt(6.0)))), {})]);
+        p!("[val: 5 - 0.01]"    => [func!("val": (Sub(Num(5.0), Num(0.01))), {})]);
+        p!("[val: (3mm * 2)]"   => [func!("val": (Mul(Sz(Size::mm(3.0)), Num(2.0))), {})]);
+        p!("[val: 12e-3cm/1pt]" => [func!("val": (Div(Sz(Size::cm(12e-3)), Sz(Size::pt(1.0)))), {})]);
+
         // Unclosed string.
         p!("[val: \"hello]" => [func!("val": (Str("hello]")), {})], [
             (0:13, 0:13, "expected quote"),
@@ -836,6 +962,30 @@ mod tests {
     }
 
     #[test]
+    fn parse_complex_mathematical_expressions() {
+        p!("[val: (3.2in + 6pt)*(5/2-1)]" => [func!("val": (
+            Mul(
+                Add(Sz(Size::inches(3.2)), Sz(Size::pt(6.0))),
+                Sub(Div(Num(5.0), Num(2.0)), Num(1.0))
+            )
+        ), {})]);
+        p!("[val: (6.3E+2+4* - 3.2pt)/2]"  => [func!("val": (
+            Div(Add(Num(6.3e2),Mul(Num(4.0), Neg(Pt(3.2)))), Num(2.0))
+        ), {})]);
+        p!("[val: 4pt--]" =>
+            [func!("val": (Pt(4.0)), {})],
+            [
+                (0:10, 0:11, "dangling minus"),
+                (0:6, 0:10, "missing right summand")
+            ],
+        );
+        p!("[val: 3mm+4pt*]" =>
+            [func!("val": (Add(Sz(Size::mm(3.0)), Pt(4.0))), {})],
+            [(0:10, 0:14, "missing right factor")],
+        );
+    }
+
+    #[test]
     fn parse_tuples() {
         // Empty tuple
         p!("[val: ()]" => [func!("val": (tuple!()), {})]);
@@ -858,9 +1008,9 @@ mod tests {
         );
 
         // Unclosed tuple
-        p!("[val: (hello]" =>
-            [func!("val": (tuple!(Id("hello"))), {})],
-            [(0:12, 0:12, "expected closing paren")],
+        p!("[val: (hello,]" =>
+            [func!("val": (tuple!(Id("hello"),)), {})],
+            [(0:13, 0:13, "expected closing paren")],
         );
         p!("[val: lang(中文]" =>
             [func!("val": (named_tuple!("lang", Id("中文"))), {})],
@@ -882,8 +1032,8 @@ mod tests {
         );
 
         // Nested tuples
-        p!("[val: (1, (2))]" =>
-            [func!("val": (tuple!(Num(1.0), tuple!(Num(2.0)))), {})]
+        p!("[val: (1, (2, 3))]" =>
+            [func!("val": (tuple!(Num(1.0), tuple!(Num(2.0), Num(3.0)))), {})]
         );
         p!("[val: css(1pt, rgb(90, 102, 254), \"solid\")]" =>
             [func!("val": (named_tuple!(
@@ -1085,7 +1235,7 @@ mod tests {
         // Body nodes in bodies.
         p!("[val:*][*Hi*]" =>
             [func!("val"; [Bold, T("Hi"), Bold])],
-            [(0:5, 0:6, "expected argument, found invalid token")],
+            [(0:5, 0:6, "expected argument, found star")],
         );
 
         // Errors in bodies.
diff --git a/src/syntax/test.rs b/src/syntax/test.rs
index ca47c0c5..47680487 100644
--- a/src/syntax/test.rs
+++ b/src/syntax/test.rs
@@ -88,13 +88,6 @@ pub trait SpanlessEq<Rhs=Self> {
     fn spanless_eq(&self, other: &Rhs) -> bool;
 }
 
-impl<T: SpanlessEq> SpanlessEq for Vec<Spanned<T>> {
-    fn spanless_eq(&self, other: &Vec<Spanned<T>>) -> bool {
-        self.len() == other.len()
-        && self.iter().zip(other).all(|(x, y)| x.v.spanless_eq(&y.v))
-    }
-}
-
 impl SpanlessEq for SyntaxModel {
     fn spanless_eq(&self, other: &SyntaxModel) -> bool {
         self.nodes.spanless_eq(&other.nodes)
@@ -130,6 +123,11 @@ impl SpanlessEq for Expr {
             (Expr::NamedTuple(a), Expr::NamedTuple(b)) => a.spanless_eq(b),
             (Expr::Tuple(a), Expr::Tuple(b)) => a.spanless_eq(b),
             (Expr::Object(a), Expr::Object(b)) => a.spanless_eq(b),
+            (Expr::Neg(a), Expr::Neg(b)) => a.spanless_eq(&b),
+            (Expr::Add(a1, a2), Expr::Add(b1, b2)) => a1.spanless_eq(&b1) && a2.spanless_eq(&b2),
+            (Expr::Sub(a1, a2), Expr::Sub(b1, b2)) => a1.spanless_eq(&b1) && a2.spanless_eq(&b2),
+            (Expr::Mul(a1, a2), Expr::Mul(b1, b2)) => a1.spanless_eq(&b1) && a2.spanless_eq(&b2),
+            (Expr::Div(a1, a2), Expr::Div(b1, b2)) => a1.spanless_eq(&b1) && a2.spanless_eq(&b2),
             (a, b) => a == b,
         }
     }
@@ -154,7 +152,26 @@ impl SpanlessEq for Object {
     fn spanless_eq(&self, other: &Object) -> bool {
         self.pairs.len() == other.pairs.len()
         && self.pairs.iter().zip(&other.pairs)
-            .all(|(x, y)| x.key.v == y.key.v && x.value.v.spanless_eq(&y.value.v))
+            .all(|(x, y)| x.v.key.v == y.v.key.v && x.v.value.v.spanless_eq(&y.v.value.v))
+    }
+}
+
+impl<T: SpanlessEq> SpanlessEq for Vec<T> {
+    fn spanless_eq(&self, other: &Vec<T>) -> bool {
+        self.len() == other.len()
+        && self.iter().zip(other).all(|(x, y)| x.spanless_eq(&y))
+    }
+}
+
+impl<T: SpanlessEq> SpanlessEq for Spanned<T> {
+    fn spanless_eq(&self, other: &Spanned<T>) -> bool {
+        self.v.spanless_eq(&other.v)
+    }
+}
+
+impl<T: SpanlessEq> SpanlessEq for Box<T> {
+    fn spanless_eq(&self, other: &Box<T>) -> bool {
+        (&**self).spanless_eq(&**other)
     }
 }
 
diff --git a/src/syntax/tokens.rs b/src/syntax/tokens.rs
index adf29de2..3b34019d 100644
--- a/src/syntax/tokens.rs
+++ b/src/syntax/tokens.rs
@@ -78,10 +78,18 @@ pub enum Token<'s> {
     ExprSize(Size),
     /// A boolean in a function header: `true | false`.
     ExprBool(bool),
-    /// A hex value in a function header: `#20d82a`
+    /// A hex value in a function header: `#20d82a`.
     ExprHex(&'s str),
-
-    /// A star in body-text.
+    /// A plus in a function header, signifying the addition of expressions.
+    Plus,
+    /// A hyphen in a function header,
+    /// signifying the subtraction of expressions.
+    Hyphen,
+    /// A slash in a function header, signifying the division of expressions.
+    Slash,
+
+    /// A star. It can appear in a function header where it signifies the
+    /// multiplication of expressions or the body where it modifies the styling.
     Star,
     /// An underscore in body-text.
     Underscore,
@@ -125,6 +133,9 @@ impl<'s> Token<'s> {
             ExprSize(_)     => "size",
             ExprBool(_)     => "bool",
             ExprHex(_)      => "hex value",
+            Plus            => "plus",
+            Hyphen          => "minus",
+            Slash           => "slash",
             Star            => "star",
             Underscore      => "underscore",
             Backslash       => "backslash",
@@ -213,11 +224,19 @@ impl<'s> Iterator for Tokens<'s> {
             ',' if self.mode == Header => Comma,
             '=' if self.mode == Header => Equals,
 
+            // Expression operators.
+            '+' if self.mode == Header => Plus,
+            '-' if self.mode == Header => Hyphen,
+            '/' if self.mode == Header => Slash,
+
             // String values.
             '"' if self.mode == Header => self.parse_string(),
 
+            // Star serves a double purpose as a style modifier
+            // and a expression operator in the header.
+            '*' => Star,
+
             // Style toggles.
-            '*' if self.mode == Body => Star,
             '_' if self.mode == Body => Underscore,
             '`' if self.mode == Body => self.parse_raw(),
 
@@ -231,15 +250,20 @@ impl<'s> Iterator for Tokens<'s> {
             c => {
                 let body = self.mode == Body;
 
+                let mut last_was_e = false;
                 let text = self.read_string_until(|n| {
-                    match n {
+                    let val = match n {
                         c if c.is_whitespace() => true,
-                        '['  | ']' | '/' => true,
-                        '\\' | '*' | '_' | '`' if body => true,
+                        '['  | ']' | '/' | '*' => true,
+                        '\\' | '_' | '`' if body => true,
                         ':'  | '=' | ',' | '"' |
                         '('  | ')' | '{' | '}' if !body => true,
+                        '+'  | '-' if !body && !last_was_e => true,
                         _ => false,
-                    }
+                    };
+
+                    last_was_e = n == 'e' || n == 'E';
+                    val
                 }, false, -(c.len_utf8() as isize), 0).0;
 
                 if self.mode == Header {
@@ -411,6 +435,8 @@ impl<'s> Tokens<'s> {
         }
     }
 
+    /// Will read the input stream until the argument F evaluates to `true`
+    /// for the current character.
     fn read_string_until<F>(
         &mut self,
         mut f: F,
@@ -517,6 +543,10 @@ mod tests {
         ExprBool as Bool,
         ExprHex as Hex,
         Text as T,
+        Plus,
+        Hyphen as Min,
+        Star,
+        Slash,
     };
 
     #[allow(non_snake_case)]
@@ -595,7 +625,7 @@ mod tests {
         t!(Body, "`]"            => [Raw("]", false)]);
         t!(Body, "`\\``"         => [Raw("\\`", true)]);
         t!(Body, "\\ "           => [Backslash, S(0)]);
-        t!(Header, "_*`"         => [Invalid("_*`")]);
+        t!(Header, "_`"          => [Invalid("_`")]);
     }
 
     #[test]
@@ -613,10 +643,13 @@ mod tests {
         t!(Header, "12e4%"             => [Num(1200.0)]);
         t!(Header, "__main__"          => [Id("__main__")]);
         t!(Header, ".func.box"         => [Id(".func.box")]);
-        t!(Header, "--arg, _b, _1"     => [Id("--arg"), Comma, S(0), Id("_b"), Comma, S(0), Id("_1")]);
+        t!(Header, "arg, _b, _1"       => [Id("arg"), Comma, S(0), Id("_b"), Comma, S(0), Id("_1")]);
         t!(Header, "12_pt, 12pt"       => [Invalid("12_pt"), Comma, S(0), Sz(Size::pt(12.0))]);
         t!(Header, "1e5in"             => [Sz(Size::inches(100000.0))]);
         t!(Header, "2.3cm"             => [Sz(Size::cm(2.3))]);
+        t!(Header, "12e-3in"           => [Sz(Size::inches(12e-3))]);
+        t!(Header, "6.1cm + 4pt,a=1*2" => [Sz(Size::cm(6.1)), S(0), Plus, S(0), Sz(Size::pt(4.0)), Comma, Id("a"), Equals, Num(1.0), Star, Num(2.0)]);
+        t!(Header, "(5 - 1) / 2.1"     => [LP, Num(5.0), S(0), Min, S(0), Num(1.0), RP, S(0), Slash, S(0), Num(2.1)]);
         t!(Header, "02.4mm"            => [Sz(Size::mm(2.4))]);
         t!(Header, "2.4.cm"            => [Invalid("2.4.cm")]);
         t!(Header, "(1,2)"             => [LP, Num(1.0), Comma, Num(2.0), RP]);