My impression is that polymorphic variants are often used in library interfaces to avoid having to use qualified names, and this always makes me feel funny—in the sense that it would be better to just use normal variants for the sake of keeping type errors simple for my limited brainspace. Is there…

A typical usecase is when you need many slight variations of a sum type and you want to avoid duplicating the whole sum type for each one (this is one of the original motivations, found in LablTk GitHub - garrigue/labltk: LablTk, an OCaml interface for Tcl/Tk ). For this and other use cases, you can…

My major use case is with the result type. All my functions that return a result look something like: val foo : something -> (r, [> err ]) result This combines well with monads over result. I know some people in the community disagree but I find this style really powerful. The other primary useca…

My own experience is to start with polymorphic variants first, especially if the data type is simple. For example, many functions need to return some result that may require just a little more than an option type - perhaps 3 or 4 possible states. It’s much easier not to have to define this type. La…

This is exactly the kind of use of polymorphic variants that makes me uncomfortable! :rofl: I’m very fond of clearly defined types and even more fond of namespaces, so using polymorphic variants as a way to avoid these things makes me squirm! I’m really looking for cases where it makes sense to de…

Poly vars are essential IMO for cases where what would otherwise be a monolithic variant type with many constructors can be the composition of many other smaller types, often defined in diverse modules. This lets those modules focus on their particular domains, rather than having to (usually poorly)…

(it was years ago but) Encoding this state-machine: https://wr.mondet.org/smondet/ocaml2015/img/target_graph.jpg (from these slides ) using polymorphic variants to enforce that “a given state can come only from certain previous states” → https://github.com/hammerlab/ketrew/blob/master/src/pure/ta…

I like this. Using polymorphic variants as a form of constraint is great. I often forget to thing in terms of constraints when programming, but I’m always happy when I do because it gives me much more of a feeling of security that my program is actually doing what I expect it to do.

The main ingredient polymorphic variants bring to the table is sub typing, which is not available for standard sum types. It is possible to write functions that act only on some of the variants defined. The variants don’t belong to a particular type but form an implicitly defined universe of variant…

One case where I was grateful that other libraries used polymorphic variants was in my textmate-language package. I was able to create a union polymorphic variant of various JSON and plist types from different libraries. Then, I wrote a single reader function that handled all of them at once and use…

I have some more usecases then the ones mentioned. Firstly I like to use the polymorphic variant types for phantom types in more typesafe library interfaces - here the subtyping is very useful and simple to understand vs GADTs. Phantom types also allow to wrap an efficient implementation with a type…

Is there any kind of guidline about when to use polymorphic variants?

Learning

Gopiandcode January 12, 2023, 2:33pm 21

Managed to get round to publishing the SQL library as a standalone artefact: [ANN] Petrol 1.0.0 - A high-level typed SQL API for OCaml designed to go fast! - #2 by Gopiandcode

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Is there any kind of guidline about when to use polymorphic variants?

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