在OCaml中指定记录字段的类型

Question

I have this code

class person name_init =
  object
    val name = name_init
    method get_name = name
end;;

let p1 = new person "Steven" 
and p2 = new person "John" in
print_endline p1#get_name;
print_endline p2#get_name;;

It complains that get_name and val name in my person object are of unbound type, which I realize is accurate. How would I specify that name_init (and therefore name and get_name) is of type string in OCaml?

Answer 1

Specify parameter types for functions (including constructors) like this:

class person (name_init : string) =
…

If you have multiple parameters, put them all in the parens.

Answer 2

OCaml requires all values in the class expression to be either concrete, or bound to a type parameter. As a consequence, when type system infers that the type of your expression is polymorphic you need to do something with it. You have two choices:

1. Constraint expression to have specific type
2. Constraint the expression to have type equal to the type parameters.

In the first case, the constraint can be put anywhere inside the class expression, given that this constraint will not allow the polymorphic expression to escape the class expression. A few examples, to demonstrate the idea:

Constraining at the instance variable specification:

class person name_init =
  object
    val name : string = name_init
    method get_name = name
end

Constraining at the method specification:

class person name_init =
  object
    val name = name_init
    method get_name : string = name
end

At your example, we have two more places where you can put the constraint, but I think that the idea is rather clear.