ocaml记录类型和null

Question

I'm trying to define type like:

type aaa = NULL | {a: int; b: int};;

But the compiler does not allow to do it. I'm not sure of the reason why we can't mix record type with anything else.

I need to match a value if it is the record type or null record and I'm tired of creating dummy record like {a = -999; b = -999}.

is there a better way ?

Answer 1

The "record" part of the definition has to be done in a separate type. Then you can wrap that in an "option" type if you want to express "None" or "Some value".

type aaa = {a: int; b: int}
type bbb = aaa option

Answer 2

First problem: in the Objective Caml approach, you can't have clean union types without constructors. Consider the following conundrum :

type test = Null | {a : int ; b: int }

let value = { a: 0 ; b : 42 } in print_int value.a

The second line is incorrect, because value is of an union type, and might therefore be Null , which has no member a . This would introduce an implicit assumption about the value of an union type, which Objective Caml avoids at all costs. This means you need a constructor.

But even that would be enough, because then you'd have an anonymous record type:

type test = Null | Pair of { a : int ; b : int }

match Pair { a : 0 ; b : 42 } with
  | Null -> 0
  | Pair p -> p.a

What would be the type of p here? This could certainly be solved by allowing anonymous record types into the language, but it's not a simple addition, as such types are notoriously difficult to handle with a type unification approach and require a lot of additional constructs for support (for instance, the < ; ... > , #type and value :> type constructs for handling objects).

Language designers took the easy way out, and required that all record types receive a name. Of course, if you have an exceedingly simple record, you can use a tuple:

type test = Null | Pair of int * int