@hackage isomorphism-class0.3.1

Conversion problem

How often do you import Data.Text.Lazy only to call fromStrict or toStrict? How about importing Data.ByteString.Builder only to call its Data.ByteString.Builder.toLazyByteString and then importing Data.ByteString.Lazy only to call its Data.ByteString.Lazy.toStrict? How often do you convert from DiffTime to NominalDiffTime and back?

These are all instances of one pattern. They are conversions between different representations of the same information. Codebases that don't attempt to abstract over this pattern tend to be sprawling with this type of boilerplate. It's noise to the code reader, it's a burden to implementors and maintainers.

Why another conversion library?

Many libraries exist that approach the conversion problem. However, most of them provide lawless typeclasses, leaving it up to the author of the instance to define what makes a proper conversion. This results in inconsistencies across instances, their behavior not being evident to the user and no way to check whether an instance is correct.

This library tackles this problem with a lawful typeclass, making it evident what any of its instances do, and it provides a property-test for you to validate your instances.

The insight

The key insight of this library is that if you add a requirement for the conversion to be lossless and to have a mirror conversion in the opposite direction, there usually appears to be only one way of defining it. That makes it very clear what the conversion does to the user and how to define it for the author of the conversion. It also gives clear criteria for validating whether the instances are correct, which can be encoded in property-tests.

That insight itself stems from an observation that almost all of the practical conversions in Haskell share a property: you can restore the original data from its converted form. E.g., you can get a text from a text-builder and you can create a text-builder from a text, you can convert a bytestring into a list of bytes and vice-versa, bytestring to/from bytearray, strict bytestring to/from lazy, list to/from sequence, sequence to/from vector, set of ints to/from int-set. In other words, it's always a two-way street with them and there are many instances of this pattern.

A few other accidental findings like encoding this property with recursive typeclass constraints and fine-tuning for the use of the TypeApplications extension resulted in a terse and clear API.

Other work and acknowledgements

There is "lawful-conversions", which is a sibling of this library expanding upon the ideas from this one to also cover the patterns of smart construction and canonicalization. It's more involved and has different tradeoffs. Both libraries are maintained, letting their designs compete.

Some ideas and concepts are also shared with the following libraries:

• control-iso

• type-iso

• injections