Crate pretty

source ·
Expand description

This crate defines a Wadler-style pretty-printing API.

Start with with the static functions of Doc.

Quick start

Let’s pretty-print simple sexps! We want to pretty print sexps like

(1 2 3)

or, if the line would be too long, like

 (2 3)
 (4 5 6))

A simple symbolic expression consists of a numeric atom or a nested ordered list of symbolic expression children.

enum SExp {
use SExp::*;

We define a simple conversion to a Doc. Atoms are rendered as strings; lists are recursively rendered, with spaces between children where appropriate. Children are nested and grouped, allowing them to be laid out in a single line as appropriate.

impl SExp {
    /// Return a pretty printed format of self.
    pub fn to_doc(&self) -> Doc<BoxDoc<()>> {
        match *self {
            Atom(ref x) => Doc::as_string(x),
            List(ref xs) =>
                    .append(Doc::intersperse(xs.into_iter().map(|x| x.to_doc()), Doc::space()).nest(1).group())

Next, we convert the Doc to a plain old string.

impl SExp {
    pub fn to_pretty(&self, width: usize) -> String {
        let mut w = Vec::new();
        self.to_doc().render(width, &mut w).unwrap();

And finally we can test that the nesting and grouping behaves as we expected.

let atom = SExp::Atom(5);
assert_eq!("5", atom.to_pretty(10));
let list = SExp::List(vec![SExp::Atom(1), SExp::Atom(2), SExp::Atom(3)]);
assert_eq!("(1 2 3)", list.to_pretty(10));
 3)", list.to_pretty(5));

Advanced usage

There’s a more efficient pattern that uses the DocAllocator trait, as implemented by BoxAllocator, to allocate DocBuilder instances. See examples/ for this approach.


The DocBuilder type allows for convenient appending of documents even for arena allocated documents by storing the arena inline.
Writes to something implementing std::fmt::Write
Writes to something implementing std::io::Write
Newtype wrapper for &Doc


The concrete document type. This type is not meant to be used directly. Instead use the static functions on Doc or the methods on an DocAllocator.


The DocAllocator trait abstracts over a type which can allocate (pointers to) Doc.
Trait representing the operations necessary to render a document
Trait representing the operations necessary to write an annotated document.

Type Definitions

An arena which can be used to allocate Doc values.