Unified Parsing and Printing with Prisms

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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).

In this talk, attendees will learn an alternative approach to unified parsers and printers based on a familiar abstraction: prisms. We begin with an abstract parser definition based on the Cons type class (part of the lens library). The underlying prism gives rise to the functions uncons, whose type matches that of a “typical” functional parser, and cons, the dual (a printer!) From there we will examine how the fresnel library1 uses these building blocks to implement a combinator library for building parser/printer prisms out of existing prisms and isos.

We will see how fresnel has been applied in an ASN.1/DER library to correctly handle some of DER’s “special” encoding requirements. Finally, I will discuss some shortcomings of prism-based parser/printers, including the lack of useful error reporting.

1 https://github.com/frasertweedale/hs-fresnel

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