Does OCaml have record syntax in disjoint unions?

Question

I can write standard disjoint unions like this:

type test = A of int * int | B of int * int;;

However, as the types get more complex, it becomes more and more painful to access a specific field or keep track of what each field represents. Haskell solves this problem with record syntax. That is, it would be nice to able to do something like:

type test = A of {x:int * y:int} | B of {x:int * y:int};;

I can't easily find something like this, but OCaml docs have seemed a bitty spotty in general, so it may well exist. Any suggestions?

EDIT: I'm aware of both disjoint unions and of record syntax, I was just hoping for nice syntax in combining them.

Answer 1

Unfortunately, you cannot put record fields in the arguments of variant constructor declarations. You must define it separately:

type xy = { x : int; y : int }
type test = A of xy | B of xy

This is slightly memory inefficient since you need another memory block for xy. Personally I do not mind it if the readability is greatly improved by the record.

However, I remember there was a proposal and a patch of having a record fields as variant constructors just like you have written:

type test = A of { x : int; y : int } | B of { x : int; y : int }

which does not require the extra block. But it is not in OCaml 4.01.0, and I am not sure the possibility of availability of this extension in future OCaml releases. Further comments are welcome.

Answer 2

OCaml has record types. You can read about them in Chapter 1.4 of the OCaml manual.

# type myrec = { a: int; b: string };;
type myrec = { a : int; b : string; }
# let z = { a = 44; b = "yes" };;
val z : myrec = {a = 44; b = "yes"}
# z.a + 14;;
- : int = 58
# type test = A of myrec | B of myrec;
type test = A of myrec | B of myrec

This might be a little more cumbersome than what you're asking for.