Pretty printing 🦋

- Syntax tree and value pretty printing
- Better value evaluation (top-level strings and content are evaluated plainly, everything else is pretty printed)

9 files changed, 533 insertions, 46 deletions

diff --git a/src/color.rs b/src/color.rs
index d74584ae..11cc5b3b 100644
--- a/src/color.rs
+++ b/src/color.rs
@@ -126,6 +126,7 @@ mod tests {
 
     #[test]
     fn test_parse_color_strings() {
+        #[track_caller]
         fn test(hex: &str, r: u8, g: u8, b: u8, a: u8) {
             assert_eq!(RgbaColor::from_str(hex), Ok(RgbaColor::new(r, g, b, a)));
         }
@@ -139,6 +140,7 @@ mod tests {
 
     #[test]
     fn test_parse_invalid_colors() {
+        #[track_caller]
         fn test(hex: &str) {
             assert_eq!(RgbaColor::from_str(hex), Err(ParseRgbaError));
         }
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index 42a5555d..1c6e3d51 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -169,7 +169,7 @@ impl Eval for Spanned<&Lit> {
 
     fn eval(self, ctx: &mut EvalContext) -> Self::Output {
         match *self.v {
-            Lit::Ident(ref v) => match ctx.state.scope.get(v.as_str()) {
+            Lit::Ident(ref v) => match ctx.state.scope.get(&v) {
                 Some(value) => value.clone(),
                 None => {
                     ctx.diag(error!(self.span, "unknown variable"));
@@ -286,6 +286,7 @@ fn add(ctx: &mut EvalContext, span: Span, lhs: Value, rhs: Value) -> Value {
 
         // Complex data types to themselves.
         (Str(a), Str(b)) => Str(a + &b),
+        (Array(a), Array(b)) => Array(concat(a, b)),
         (Dict(a), Dict(b)) => Dict(concat(a, b)),
         (Content(a), Content(b)) => Content(concat(a, b)),
 
diff --git a/src/eval/value.rs b/src/eval/value.rs
index 80c6b820..50741987 100644
--- a/src/eval/value.rs
+++ b/src/eval/value.rs
@@ -1,5 +1,5 @@
 use std::any::Any;
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 use std::fmt::{self, Debug, Display, Formatter};
 use std::ops::Deref;
 use std::rc::Rc;
@@ -7,10 +7,11 @@ use std::rc::Rc;
 use super::{Args, Eval, EvalContext};
 use crate::color::Color;
 use crate::geom::{Length, Linear, Relative};
-use crate::syntax::{Spanned, Tree, WithSpan};
+use crate::pretty::{pretty, Pretty, Printer};
+use crate::syntax::{pretty_content_expr, Spanned, Tree, WithSpan};
 
 /// A computational value.
-#[derive(Clone, PartialEq)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum Value {
     /// The value that indicates the absence of a meaningful value.
     None,
@@ -82,21 +83,9 @@ impl Eval for &Value {
     fn eval(self, ctx: &mut EvalContext) -> Self::Output {
         ctx.push(ctx.make_text_node(match self {
             Value::None => return,
-            Value::Bool(v) => v.to_string(),
-            Value::Int(v) => v.to_string(),
-            Value::Float(v) => v.to_string(),
-            Value::Length(v) => v.to_string(),
-            Value::Relative(v) => v.to_string(),
-            Value::Linear(v) => v.to_string(),
-            Value::Color(v) => v.to_string(),
-            Value::Str(v) => v.clone(),
-            // TODO: Find good representation for composite types.
-            Value::Array(_v) => "(array)".into(),
-            Value::Dict(_v) => "(dictionary)".into(),
+            Value::Str(s) => s.clone(),
             Value::Content(tree) => return tree.eval(ctx),
-            Value::Func(v) => v.to_string(),
-            Value::Any(v) => v.to_string(),
-            Value::Error => "(error)".into(),
+            other => pretty(other),
         }));
     }
 }
@@ -107,24 +96,24 @@ impl Default for Value {
     }
 }
 
-impl Debug for Value {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+impl Pretty for Value {
+    fn pretty(&self, p: &mut Printer) {
         match self {
-            Self::None => f.pad("None"),
-            Self::Bool(v) => Debug::fmt(v, f),
-            Self::Int(v) => Debug::fmt(v, f),
-            Self::Float(v) => Debug::fmt(v, f),
-            Self::Length(v) => Debug::fmt(v, f),
-            Self::Relative(v) => Debug::fmt(v, f),
-            Self::Linear(v) => Debug::fmt(v, f),
-            Self::Color(v) => Debug::fmt(v, f),
-            Self::Str(v) => Debug::fmt(v, f),
-            Self::Array(v) => Debug::fmt(v, f),
-            Self::Dict(v) => Debug::fmt(v, f),
-            Self::Content(v) => Debug::fmt(v, f),
-            Self::Func(v) => Debug::fmt(v, f),
-            Self::Any(v) => Debug::fmt(v, f),
-            Self::Error => f.pad("Error"),
+            Value::None => p.push_str("none"),
+            Value::Bool(v) => write!(p, "{}", v).unwrap(),
+            Value::Int(v) => write!(p, "{}", v).unwrap(),
+            Value::Float(v) => write!(p, "{}", v).unwrap(),
+            Value::Length(v) => write!(p, "{}", v).unwrap(),
+            Value::Relative(v) => write!(p, "{}", v).unwrap(),
+            Value::Linear(v) => write!(p, "{}", v).unwrap(),
+            Value::Color(v) => write!(p, "{}", v).unwrap(),
+            Value::Str(v) => write!(p, "{:?}", v).unwrap(),
+            Value::Array(array) => array.pretty(p),
+            Value::Dict(dict) => dict.pretty(p),
+            Value::Content(content) => pretty_content_expr(content, p),
+            Value::Func(v) => v.pretty(p),
+            Value::Any(v) => v.pretty(p),
+            Value::Error => p.push_str("(error)"),
         }
     }
 }
@@ -132,8 +121,35 @@ impl Debug for Value {
 /// An array value: `(1, "hi", 12cm)`.
 pub type ValueArray = Vec<Value>;
 
+impl Pretty for ValueArray {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str("(");
+        p.join(self, ", ", |item, p| item.pretty(p));
+        if self.len() == 1 {
+            p.push_str(",");
+        }
+        p.push_str(")");
+    }
+}
+
 /// A dictionary value: `(color: #f79143, pattern: dashed)`.
-pub type ValueDict = HashMap<String, Value>;
+pub type ValueDict = BTreeMap<String, Value>;
+
+impl Pretty for ValueDict {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str("(");
+        if self.is_empty() {
+            p.push_str(":");
+        } else {
+            p.join(self, ", ", |(key, value), p| {
+                p.push_str(key);
+                p.push_str(": ");
+                value.pretty(p);
+            });
+        }
+        p.push_str(")");
+    }
+}
 
 /// A content value: `{*Hi* there}`.
 pub type ValueContent = Tree;
@@ -169,14 +185,15 @@ impl Deref for ValueFunc {
     }
 }
 
-impl Display for ValueFunc {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        write!(f, "<function {}>", self.name)
+impl Pretty for ValueFunc {
+    fn pretty(&self, p: &mut Printer) {
+        write!(p, "(function {})", self.name).unwrap();
     }
 }
+
 impl Debug for ValueFunc {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        Display::fmt(self, f)
+        f.debug_struct("ValueFunc").field("name", &self.name).finish()
     }
 }
 
@@ -229,15 +246,15 @@ impl PartialEq for ValueAny {
     }
 }
 
-impl Display for ValueAny {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        Display::fmt(&self.0, f)
+impl Pretty for ValueAny {
+    fn pretty(&self, p: &mut Printer) {
+        write!(p, "{}", self.0).unwrap();
     }
 }
 
 impl Debug for ValueAny {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        Debug::fmt(&self.0, f)
+        f.debug_tuple("ValueAny").field(&self.0).finish()
     }
 }
 
@@ -465,3 +482,47 @@ macro_rules! impl_type {
         }
     };
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::color::RgbaColor;
+    use crate::pretty::pretty;
+    use crate::syntax::Node;
+
+    #[track_caller]
+    fn test_pretty(value: impl Into<Value>, exp: &str) {
+        assert_eq!(pretty(&value.into()), exp);
+    }
+
+    #[test]
+    fn test_pretty_print_values() {
+        test_pretty(Value::None, "none");
+        test_pretty(false, "false");
+        test_pretty(12.4, "12.4");
+        test_pretty(Length::ZERO, "0pt");
+        test_pretty(Relative::ONE, "100%");
+        test_pretty(Relative::new(0.3) + Length::cm(2.0), "30% + 2cm");
+        test_pretty(Color::Rgba(RgbaColor::new(1, 1, 1, 0xff)), "#010101");
+        test_pretty("hello", r#""hello""#);
+        test_pretty(vec![Spanned::zero(Node::Strong)], "{*}");
+        test_pretty(ValueFunc::new("nil", |_, _| Value::None), "(function nil)");
+        test_pretty(ValueAny::new(1), "1");
+        test_pretty(Value::Error, "(error)");
+    }
+
+    #[test]
+    fn test_pretty_print_collections() {
+        // Array.
+        test_pretty(Value::Array(vec![]), "()");
+        test_pretty(vec![Value::None], "(none,)");
+        test_pretty(vec![Value::Int(1), Value::Int(2)], "(1, 2)");
+
+        // Dictionary.
+        let mut dict = BTreeMap::new();
+        dict.insert("one".into(), Value::Int(1));
+        dict.insert("two".into(), Value::Int(2));
+        test_pretty(BTreeMap::new(), "(:)");
+        test_pretty(dict, "(one: 1, two: 2)");
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 15d68803..05cc0569 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -38,6 +38,7 @@ pub mod library;
 pub mod paper;
 pub mod parse;
 pub mod prelude;
+pub mod pretty;
 pub mod shaping;
 pub mod syntax;
 
diff --git a/src/parse/resolve.rs b/src/parse/resolve.rs
index d6c6d8a4..c4afc430 100644
--- a/src/parse/resolve.rs
+++ b/src/parse/resolve.rs
@@ -124,6 +124,7 @@ mod tests {
 
     #[test]
     fn test_resolve_strings() {
+        #[track_caller]
         fn test(string: &str, expected: &str) {
             assert_eq!(resolve_string(string), expected.to_string());
         }
@@ -144,6 +145,7 @@ mod tests {
 
     #[test]
     fn test_split_at_lang_tag() {
+        #[track_caller]
         fn test(text: &str, lang: &str, inner: &str) {
             assert_eq!(split_at_lang_tag(text), (lang, inner));
         }
@@ -158,6 +160,7 @@ mod tests {
 
     #[test]
     fn test_resolve_raw() {
+        #[track_caller]
         fn test(
             raw: &str,
             backticks: usize,
@@ -190,6 +193,7 @@ mod tests {
 
     #[test]
     fn test_trim_raw() {
+        #[track_caller]
         fn test(text: &str, expected: Vec<&str>) {
             assert_eq!(trim_and_split_raw(text).0, expected);
         }
@@ -207,6 +211,7 @@ mod tests {
 
     #[test]
     fn test_split_lines() {
+        #[track_caller]
         fn test(text: &str, expected: Vec<&str>) {
             assert_eq!(split_lines(text), expected);
         }
diff --git a/src/pretty.rs b/src/pretty.rs
new file mode 100644
index 00000000..a7482869
--- /dev/null
+++ b/src/pretty.rs
@@ -0,0 +1,68 @@
+//! Pretty printing.
+
+use std::fmt::{Arguments, Result, Write};
+
+/// Pretty print an item and return the resulting string.
+pub fn pretty<T>(item: &T) -> String
+where
+    T: Pretty,
+{
+    let mut p = Printer::new();
+    item.pretty(&mut p);
+    p.finish()
+}
+
+/// Pretty printing.
+pub trait Pretty {
+    /// Pretty print this item into the given printer.
+    fn pretty(&self, p: &mut Printer);
+}
+
+/// A buffer into which items are printed.
+pub struct Printer {
+    buf: String,
+}
+
+impl Printer {
+    /// Create a new pretty printer.
+    pub fn new() -> Self {
+        Self { buf: String::new() }
+    }
+
+    /// Push a string into the buffer.
+    pub fn push_str(&mut self, string: &str) {
+        self.buf.push_str(string);
+    }
+
+    /// Write formatted items into the buffer.
+    pub fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result {
+        Write::write_fmt(self, fmt)
+    }
+
+    /// Write a comma-separated list of items.
+    pub fn join<T, I, F>(&mut self, items: I, joiner: &str, mut write_item: F)
+    where
+        I: IntoIterator<Item = T>,
+        F: FnMut(T, &mut Self),
+    {
+        let mut iter = items.into_iter();
+        if let Some(first) = iter.next() {
+            write_item(first, self);
+        }
+        for item in iter {
+            self.push_str(joiner);
+            write_item(item, self);
+        }
+    }
+
+    /// Finish pretty printing and return the underlying buffer.
+    pub fn finish(self) -> String {
+        self.buf
+    }
+}
+
+impl Write for Printer {
+    fn write_str(&mut self, s: &str) -> Result {
+        Ok(self.push_str(s))
+    }
+}
diff --git a/src/syntax/expr.rs b/src/syntax/expr.rs
index cf6611f9..4916f34f 100644
--- a/src/syntax/expr.rs
+++ b/src/syntax/expr.rs
@@ -21,6 +21,35 @@ pub enum Expr {
     Content(ExprContent),
 }
 
+impl Pretty for Expr {
+    fn pretty(&self, p: &mut Printer) {
+        match self {
+            Self::Lit(lit) => lit.pretty(p),
+            Self::Call(call) => call.pretty(p),
+            Self::Unary(unary) => unary.pretty(p),
+            Self::Binary(binary) => binary.pretty(p),
+            Self::Array(array) => array.pretty(p),
+            Self::Dict(dict) => dict.pretty(p),
+            Self::Content(content) => pretty_content_expr(content, p),
+        }
+    }
+}
+
+/// Pretty print content in an expression context.
+pub fn pretty_content_expr(tree: &Tree, p: &mut Printer) {
+    if let [Spanned { v: Node::Expr(Expr::Call(call)), .. }] = tree.as_slice() {
+        // Remove unncessary braces from content expression containing just a
+        // single function call.
+        //
+        // Example: Transforms "{(call: {[f]})}" => "{(call: [f])}"
+        pretty_bracket_call(call, p, false);
+    } else {
+        p.push_str("{");
+        tree.pretty(p);
+        p.push_str("}");
+    }
+}
+
 /// An invocation of a function: `[foo ...]`, `foo(...)`.
 #[derive(Debug, Clone, PartialEq)]
 pub struct ExprCall {
@@ -30,12 +59,68 @@ pub struct ExprCall {
     pub args: Spanned<ExprArgs>,
 }
 
+impl Pretty for ExprCall {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str(&self.name.v);
+        p.push_str("(");
+        self.args.v.pretty(p);
+        p.push_str(")");
+    }
+}
+
+/// Pretty print a bracketed function call, with body or chaining when possible.
+pub fn pretty_bracket_call(call: &ExprCall, p: &mut Printer, chained: bool) {
+    if chained {
+        p.push_str(" | ");
+    } else {
+        p.push_str("[");
+    }
+
+    // Function name.
+    p.push_str(&call.name.v);
+
+    // Find out whether this can be written as body or chain.
+    //
+    // Example: Transforms "[v {Hi}]" => "[v][Hi]".
+    if let [head @ .., Argument::Pos(Spanned { v: Expr::Content(content), .. })] =
+        call.args.v.as_slice()
+    {
+        // Previous arguments.
+        if !head.is_empty() {
+            p.push_str(" ");
+            p.join(head, ", ", |item, p| item.pretty(p));
+        }
+
+        // Find out whether this can written as a chain.
+        //
+        // Example: Transforms "[v][[f]]" => "[v | f]".
+        if let [Spanned { v: Node::Expr(Expr::Call(call)), .. }] = content.as_slice() {
+            return pretty_bracket_call(call, p, true);
+        } else {
+            p.push_str("][");
+            content.pretty(p);
+        }
+    } else if !call.args.v.is_empty() {
+        p.push_str(" ");
+        call.args.v.pretty(p);
+    }
+
+    // Either end of header or end of body.
+    p.push_str("]");
+}
+
 /// The arguments to a function: `12, draw: false`.
 ///
 /// In case of a bracketed invocation with a body, the body is _not_
 /// included in the span for the sake of clearer error messages.
 pub type ExprArgs = Vec<Argument>;
 
+impl Pretty for Vec<Argument> {
+    fn pretty(&self, p: &mut Printer) {
+        p.join(self, ", ", |item, p| item.pretty(p));
+    }
+}
+
 /// An argument to a function call: `12` or `draw: false`.
 #[derive(Debug, Clone, PartialEq)]
 pub enum Argument {
@@ -45,6 +130,15 @@ pub enum Argument {
     Named(Named),
 }
 
+impl Pretty for Argument {
+    fn pretty(&self, p: &mut Printer) {
+        match self {
+            Self::Pos(expr) => expr.v.pretty(p),
+            Self::Named(named) => named.pretty(p),
+        }
+    }
+}
+
 /// A pair of a name and an expression: `pattern: dashed`.
 #[derive(Debug, Clone, PartialEq)]
 pub struct Named {
@@ -54,6 +148,14 @@ pub struct Named {
     pub expr: Spanned<Expr>,
 }
 
+impl Pretty for Named {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str(&self.name.v);
+        p.push_str(": ");
+        self.expr.v.pretty(p);
+    }
+}
+
 /// A unary operation: `-x`.
 #[derive(Debug, Clone, PartialEq)]
 pub struct ExprUnary {
@@ -63,6 +165,13 @@ pub struct ExprUnary {
     pub expr: Box<Spanned<Expr>>,
 }
 
+impl Pretty for ExprUnary {
+    fn pretty(&self, p: &mut Printer) {
+        self.op.v.pretty(p);
+        self.expr.v.pretty(p);
+    }
+}
+
 /// A unary operator.
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub enum UnOp {
@@ -70,6 +179,14 @@ pub enum UnOp {
     Neg,
 }
 
+impl Pretty for UnOp {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str(match self {
+            Self::Neg => "-",
+        });
+    }
+}
+
 /// A binary operation: `a + b`, `a / b`.
 #[derive(Debug, Clone, PartialEq)]
 pub struct ExprBinary {
@@ -81,6 +198,16 @@ pub struct ExprBinary {
     pub rhs: Box<Spanned<Expr>>,
 }
 
+impl Pretty for ExprBinary {
+    fn pretty(&self, p: &mut Printer) {
+        self.lhs.v.pretty(p);
+        p.push_str(" ");
+        self.op.v.pretty(p);
+        p.push_str(" ");
+        self.rhs.v.pretty(p);
+    }
+}
+
 /// A binary operator.
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub enum BinOp {
@@ -94,12 +221,46 @@ pub enum BinOp {
     Div,
 }
 
+impl Pretty for BinOp {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str(match self {
+            Self::Add => "+",
+            Self::Sub => "-",
+            Self::Mul => "*",
+            Self::Div => "/",
+        });
+    }
+}
+
 /// An array expression: `(1, "hi", 12cm)`.
 pub type ExprArray = SpanVec<Expr>;
 
+impl Pretty for ExprArray {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str("(");
+        p.join(self, ", ", |item, p| item.v.pretty(p));
+        if self.len() == 1 {
+            p.push_str(",");
+        }
+        p.push_str(")");
+    }
+}
+
 /// A dictionary expression: `(color: #f79143, pattern: dashed)`.
 pub type ExprDict = Vec<Named>;
 
+impl Pretty for ExprDict {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str("(");
+        if self.is_empty() {
+            p.push_str(":");
+        } else {
+            p.join(self, ", ", |named, p| named.pretty(p));
+        }
+        p.push_str(")");
+    }
+}
+
 /// A content expression: `{*Hello* there!}`.
 pub type ExprContent = Tree;
 
@@ -128,3 +289,65 @@ pub enum Lit {
     /// A string literal: `"hello!"`.
     Str(String),
 }
+
+impl Pretty for Lit {
+    fn pretty(&self, p: &mut Printer) {
+        match self {
+            Self::Ident(v) => p.push_str(&v),
+            Self::None => p.push_str("none"),
+            Self::Bool(v) => write!(p, "{}", v).unwrap(),
+            Self::Int(v) => write!(p, "{}", v).unwrap(),
+            Self::Float(v) => write!(p, "{}", v).unwrap(),
+            Self::Length(v, u) => write!(p, "{}{}", v, u).unwrap(),
+            Self::Percent(v) => write!(p, "{}%", v).unwrap(),
+            Self::Color(v) => write!(p, "{}", v).unwrap(),
+            Self::Str(s) => write!(p, "{:?}", &s).unwrap(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::super::tests::test_pretty;
+
+    #[test]
+    fn test_pretty_print_chaining() {
+        // All equivalent.
+        test_pretty("[v [f]]", "[v | f]");
+        test_pretty("[v {[f]}]", "[v | f]");
+        test_pretty("[v][[f]]", "[v | f]");
+        test_pretty("[v | f]", "[v | f]");
+    }
+
+    #[test]
+    fn test_pretty_print_expressions() {
+        // Unary and binary operations.
+        test_pretty("{1 +}", "{1}");
+        test_pretty("{1 + func(-2)}", "{1 + func(-2)}");
+
+        // Array.
+        test_pretty("(-5,)", "(-5,)");
+        test_pretty("(1, 2, 3)", "(1, 2, 3)");
+
+        // Dictionary.
+        test_pretty("{(:)}", "{(:)}");
+        test_pretty("{(percent: 5%)}", "{(percent: 5%)}");
+
+        // Content expression without unncessary braces.
+        test_pretty("[v [f], 1]", "[v [f], 1]");
+        test_pretty("(func: {[f]})", "(func: [f])");
+    }
+
+    #[test]
+    fn test_pretty_print_literals() {
+        test_pretty("{none}", "{none}");
+        test_pretty("{true}", "{true}");
+        test_pretty("{25}", "{25}");
+        test_pretty("{2.50}", "{2.5}");
+        test_pretty("{1e2}", "{100}");
+        test_pretty("{12pt}", "{12pt}");
+        test_pretty("{50%}", "{50%}");
+        test_pretty("{#fff}", "{#ffffff}");
+        test_pretty(r#"{"hi\n"}"#, r#"{"hi\n"}"#);
+    }
+}
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index 9c78fbc3..22f51d82 100644
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -12,5 +12,33 @@ pub use node::*;
 pub use span::*;
 pub use token::*;
 
-/// A collection of nodes which form a tree together with their children.
+use crate::pretty::{Pretty, Printer};
+
+/// The abstract syntax tree.
 pub type Tree = SpanVec<Node>;
+
+impl Pretty for Tree {
+    fn pretty(&self, p: &mut Printer) {
+        for node in self {
+            node.v.pretty(p);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::parse::parse;
+    use crate::pretty::pretty;
+
+    #[track_caller]
+    pub fn test_pretty(src: &str, exp: &str) {
+        let tree = parse(src).output;
+        let found = pretty(&tree);
+        if exp != found {
+            println!("tree:     {:#?}", tree);
+            println!("expected: {}", exp);
+            println!("found:    {}", found);
+            panic!("test failed");
+        }
+    }
+}
diff --git a/src/syntax/node.rs b/src/syntax/node.rs
index d64d4fed..91fa72d7 100644
--- a/src/syntax/node.rs
+++ b/src/syntax/node.rs
@@ -27,15 +27,62 @@ pub enum Node {
     Expr(Expr),
 }
 
+impl Pretty for Node {
+    fn pretty(&self, p: &mut Printer) {
+        match self {
+            Self::Text(text) => p.push_str(&text),
+            Self::Space => p.push_str(" "),
+            Self::Linebreak => p.push_str(r"\"),
+            Self::Parbreak => p.push_str("\n\n"),
+            Self::Strong => p.push_str("*"),
+            Self::Emph => p.push_str("_"),
+            Self::Heading(heading) => heading.pretty(p),
+            Self::Raw(raw) => raw.pretty(p),
+            Self::Expr(expr) => pretty_expr_node(expr, p),
+        }
+    }
+}
+
+/// Pretty print an expression in a node context.
+pub fn pretty_expr_node(expr: &Expr, p: &mut Printer) {
+    match expr {
+        // Prefer bracket calls over expression blocks with just a single paren
+        // call.
+        //
+        // Example: Transforms "{v()}" => "[v]".
+        Expr::Call(call) => pretty_bracket_call(call, p, false),
+
+        // Remove unncessary nesting of content and expression blocks.
+        //
+        // Example: Transforms "{{Hi}}" => "Hi".
+        Expr::Content(content) => content.pretty(p),
+
+        _ => {
+            p.push_str("{");
+            expr.pretty(p);
+            p.push_str("}");
+        }
+    }
+}
+
 /// A section heading: `# Introduction`.
 #[derive(Debug, Clone, PartialEq)]
 pub struct NodeHeading {
-    /// The section depth (numer of hashtags minus 1).
+    /// The section depth (numer of hashtags minus 1, capped at 5).
     pub level: Spanned<u8>,
     /// The contents of the heading.
     pub contents: Tree,
 }
 
+impl Pretty for NodeHeading {
+    fn pretty(&self, p: &mut Printer) {
+        for _ in 0 ..= self.level.v {
+            p.push_str("#");
+        }
+        self.contents.pretty(p);
+    }
+}
+
 /// A raw block with optional syntax highlighting: `` `raw` ``.
 ///
 /// Raw blocks start with an arbitrary number of backticks and end with the same
@@ -114,3 +161,54 @@ pub struct NodeRaw {
     /// are inline-level when they contain no newlines.
     pub inline: bool,
 }
+
+impl Pretty for NodeRaw {
+    fn pretty(&self, p: &mut Printer) {
+        p.push_str("`");
+        if let Some(lang) = &self.lang {
+            p.push_str(&lang);
+            p.push_str(" ");
+        }
+        // TODO: Technically, we should handle backticks in the lines
+        // by wrapping with more backticks and possibly adding space
+        // before the first or after the last line.
+        p.join(&self.lines, "\n", |line, p| p.push_str(line));
+        p.push_str("`");
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::super::tests::test_pretty;
+
+    #[test]
+    fn test_pretty_print_removes_nesting() {
+        // Even levels of nesting do not matter.
+        test_pretty("{{Hi}}", "Hi");
+        test_pretty("{{{{Hi}}}}", "Hi");
+    }
+
+    #[test]
+    fn test_pretty_print_prefers_bracket_calls() {
+        // All reduces to a simple bracket call.
+        test_pretty("{v()}", "[v]");
+        test_pretty("[v]", "[v]");
+        test_pretty("{[v]}", "[v]");
+        test_pretty("{{[v]}}", "[v]");
+    }
+
+    #[test]
+    fn test_pretty_print_nodes() {
+        // Basic text and markup.
+        test_pretty(r"*Hi_\", r"*Hi_\");
+
+        // Whitespace.
+        test_pretty("  ", " ");
+        test_pretty("\n\n\n", "\n\n");
+
+        // Heading and raw.
+        test_pretty("# Ok", "# Ok");
+        test_pretty("``\none\ntwo\n``", "`one\ntwo`");
+        test_pretty("`lang one\ntwo`", "`lang one\ntwo`");
+    }
+}