Invertible Parsing: The Solution, Part 2

Episode #180 • Mar 7, 2022 • Subscriber-Only

We will chip away at more and more parser printer conformances, some of which will truly stretch our brains, but we will finally turn our complex user CSV parser into a printer!

The Solution, Part 2
Introduction
00:05
Map parser-printing
00:34
One of parser-printing
03:30
Skip parser-printing
11:22
Int parser-printing
18:09
FromUTF8View
22:51
Bool parser-printing
25:56
User parser-printing
27:10
Many parser-printing
31:26
Next time: generalized printing
37:07

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Introduction

We are getting very close to having our first moderately complex printer.

We are now on the precipice of something really amazing. Let’s take a look at the field parser, which is responsible for first trying to parse a quoted field, and if that fails it just parses a regular field up until the next comma.

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References

Invertible syntax descriptions: Unifying parsing and pretty printing

Tillmann Rendel and Klaus Ostermann • Thursday Sep 30, 2010

Parsers and pretty-printers for a language are often quite similar, yet both are typically implemented separately, leading to redundancy and potential inconsistency. We propose a new interface of syntactic descriptions, with which both parser and pretty-printer can be described as a single program using this interface. Whether a syntactic description is used as a parser or as a pretty-printer is determined by the implementation of the interface. Syntactic descriptions enable programmers to describe the connection between concrete and abstract syntax once and for all, and use these descriptions for parsing or pretty-printing as needed. We also discuss the generalization of our programming technique towards an algebra of partial isomorphisms.

This publication (from 2010!) was the initial inspiration for our parser-printer explorations, and a much less polished version of the code was employed on the Point-Free web site on day one of our launch!

Unified Parsing and Printing with Prisms

Fraser Tweedale • Friday Apr 29, 2016

Parsers and pretty printers are commonly defined as separate values, however, the same essential information about how the structured data is represented in a stream must exist in both values. This is therefore a violation of the DRY principle – usually quite an obvious one (a cursory glance at any corresponding FromJSON and ToJSON instances suffices to support this fact). Various methods of unifying parsers and printers have been proposed, most notably Invertible Syntax Descriptions due to Rendel and Ostermann (several Haskell implementations of this approach exist).

Another approach to the parsing-printing problem using a construct known as a “prism” (a construct Point-Free viewers and library users may better know as a “case path”).

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