Add a Variant module, for construction of and destructuring of general ADTS

🚀 Feature request

Desired Behavior

I think it would be good to have a module for helpers to construct Variant types (aka ADTs / Sum types / Discriminated unions) - the dual to Records. The advantage of such a module is that it reduces boiler plate and increases consistency when constructing algebraic data types.

The module would also have functions to apply functional pattern matching that doesn’t rely on switch statements or the internals of the variant implementation.

Suggested Solution

I currently make use of my own library for this @rachel-barrett/variant-ts that I use alongside fp-ts, in both personal projects and at work. I think it could make sense for it to be a part of fp-ts. As far as I am aware there is not anything similar already in the fp-ts ecosystem?

I have also written a pull request against fp-ts to illustrate more directly how it could fit in to fp-ts, which I will link.

Who does this impact? Who is this for?

This would be beneficial for all typescript users wanting to use algebraic data types in a functional style.

[rachel-barrett]

Some of my thoughts on the advantages and disadvantages of Variant (or equivalent) being part of fp-ts:

On the one hand I’m not sure that it fits in fp-ts since there’s something slightly different about it as it doesn’t follow the pattern of other modules. Also with variants not being native to typescript, it is harder to agree on a canonical way to implement them.

On the other hand, things that make me think it makes sense in fp-ts:

• Variant is dual to Record which does have a module in fp-ts, so it is at the same “level”
• Since some way of building variants is core to functional programming, it would be good to have a consistent “default” approach
• If Variant was in fp-ts, the encoders and decoders could be in io-ts without needing a separate binding library.

[samhh]

NB there’s also sum-types.