Gutenberg

A library for laying out text, based on Phil Wadler's pretty printing algorithm.

Installing

Gutenberg is available on Nuget. API docs are hosted on my website.

Tutorial

Gutenberg's core object is called Document<T>. It represents a textual document which can be laid out in a variety of ways.

You can create a document from a string,

// Document<T> is intended to be imported under an alias.
// If you're not using annotations (an advanced feature),
// you can set `T` to `object`.
using Doc = Gutenberg.Document<object>;

var doc = Doc.FromString("Hello world!");

concatenate documents,

var doc2 = doc + " My name is Johannes Gutenberg.";  // strings can be implicitly cast to documents

insert line breaks,

var doc3 = doc2 + Doc.LineBreak + "Pleased to meet you!";

and render the document as a string.

Console.WriteLine(doc3.ToString());
// Output:
// Hello world! My name is Johannes Gutenberg.
// Pleased to meet you!

You can instruct Gutenberg to try alternative layouts for a document using the Grouped method. Grouped tells Gutenberg to attempt to flatten the document, collapsing any LineBreaks into a single space.

Console.WriteLine(doc3.Grouped());
// Output:
// Hello world! My name is Johannes Gutenberg. Pleased to meet you!

Gutenberg tries to use its available horizontal space (a default of 80 characters) efficiently. It tries to flatten the groups within a document as long as there is enough horizontal space to do so. If there's an overflow, it falls back on the un-flattened version.

// set the page width to 60 characters - too narrow to flatten the LineBreak
Console.WriteLine(doc3.Grouped().ToString(60));
// Output:
// Hello world! My name is Johannes Gutenberg.
// Pleased to meet you!

The Nested operator controls indentation. If a document gets rendered with line breaks, the line breaks are indented to the document's nesting level.

Console.WriteLine(doc3.Nested(4).Grouped().ToString(60));
// Output:
// Hello world! My name is Johannes Gutenberg.
//     Pleased to meet you!

You can place groups inside each other, to give Gutenberg multiple options to layout a document as efficiently as possible. Here's an example of a recursive function to pretty-print a tree which uses nested groups to produce a flexible layout.

record Tree(string Name, params Tree[] Children) : IPrettyPrintable<object>
{
    public Doc PrettyPrint()
    {
        var children = Children.Length == 0
            ? Doc.Empty
            : Doc.ZeroWidthLineBreak
                .Append(Doc.Concat(
                    Children
                        .Select(c => c.PrettyPrint())
                        .Separated("," + Doc.LineBreak)
                ))
                .Nested(2)
                .Between("[", Doc.ZeroWidthLineBreak + "]");
        return Doc.Concat(Name).Append(children).Grouped();
    }
}
Console.WriteLine(exampleTree.PrettyPrint().ToString(20))
// Output:
// aaa[
//   bbbbb[ccc, dd],
//   eee,
//   ffff[gg, hhh, ii]
// ]

You can read about Gutenberg's internals on my website.