Open and closed union types in Ocaml

Question

I\'m looking into OCaml for the first time, having a bit of background with F# and Haskell. As such, a lot is familiar-looking, but one thing that isn\'t is the concept of \"ope

Answer 1

You need to read Jacques Garrigue's "Code reuse with polymorphic variants":

http://www.math.nagoya-u.ac.jp/~garrigue/papers/fose2000.html

The problem with polymorphic variants is that they are so flexible that type inference cannot help you a lot with errors in polymorphic variants code. For example, if you mistype one constructor name, the compiler won't be able to flag an error, it will only infer a slightly different types with the usual constructors, plus the misspelled one. Errors will only be spotted later, when you try to combine the faulty code with a function with strict assumptions on the variants (closed pattern matching), with an unwieldy error message.

My advice for polymorphic variant users is to massively use annotations to control type-checking : every time you take a polymorphic variant as input, or output one, the function should be annotated with a precise type for the variant part. This will shield you from most inference issues, and force you to build an expressive set of type definitions that can be composed and help reasoning about your program.