Pretty

Introduction

pretty is an advanced pretty printer for Lua. It attempts to understand your datastructures and return a pretty printed string representation. It's primary purpose is to be a good human-readable debugging tool, not a speedy serialization tool.

Humans are flexible in their understanding of data, as long as certain underlying structural patterns can be found. pretty attempts to find those patterns and highlight them, often after performing huge analytical tasks.

Sometimes just aligning elements are enough to easy the burden on the eyes. Contrast the two following pieces of code:

	bad = {
		a = 'hello world',
		hello = 'hi'
	}
	good = {
		a      =  'hello world',
		hello  =  'hi'
	}

This project came out of the frustration with existing pretty printers, which would often employ simpler heuristics, each good at displaying some specific structure, but bad at others. See below for other pretty printers.

Another aspect where pretty shines is in exploratory programming, when attempting to avoid reliance on outside documentation.

Features

• Written in good-old pureblood Lua, with support for PUC Lua 5.0 - 5.3 and LuaJIT 2.0 and up.
• Redefining what it means to be "human readable":
  • Is multi-line centric, to aid readablitiy.
  • Indention and alignment of keys-value pairs.
  • Keys-value pairs are properly sorted by key type and thereafter alphabetically.
  • The format and structure of output changes depending upon the input. Maps are displayed differently to deeply nested tables to long sequences with short strings to short lists.
  • Uses the standard debug library to gain information about functions and other advanced structures.

Performance

As specified in the introduction, pretty is not a performance oriented library. Expected usage is in error conditions and debugging, not in tight inner loops.

Don't use pretty.lua if you want fast serialization. Use one of the pretty printers specified below.

TODO

I'm looking into implementing following features:

• Improve display of medium-long lists with short elements better. One option would be to implement something analog to the default results of ls on Linux.
• Add support for setmetatable, and exploring the values accessible through it.
• Nice formatting for cdata datatype in LuaJIT.
• Add possibility of comments in output, for stuff like __tostring methods, and global namespaces like io or math.
• Better support upvalues in functions. Complete support is impossible without traversing the original code or inspecting the intermediate representation, due to lexical scoping. (Pluto does it, but it's written in C.)
• Look more into string.dump in the core library.
• Look into using concat operation to improve appearance of overly long non-breaking strings. Maybe even attempt to break near whitespace.
• Attempt to fit output within a predefined width limit. Default to 80(?)

Other pretty printers

pretty is a large and slow library, and is not designed for serialization purposes, nor is pretty concerned with offering the same level of stabilizability as other libraries do.

Luckily Lua has a large library of pretty printers and serialization libraries:

• inspect.lua: One of the classic debugging pretty printers.
• pprint.lua: Reimplementation of inspect.lua
• serpent: Advanced and fast pretty printer.
• pluto: Can serialize arbitrary parts of Lua, including functions, upvalues, and proper lexical scoping. Not written in native Lua.
• binser: Library for special purpose serialization.

Others can be found at the lua-users wiki.