Witnesses that express whether a type's values are always, sometimes, or never immediate.

A value is immediate when it is internally represented unboxed, using up one word of memory, rather than a pointer to a heap-allocated block.

Some examples:

• All int values are by definition immediate, i.e. unboxed, and so int is always immediate.

• A 'a list is either [], which is internally represented as 0 (immediate), or a non-empty list, which is represented as a pointer to a heap block (boxed), which contains the first element and the pointer to the rest of the list. Therefore 'a list is sometimes immediate.

• All values of type 'a ref are represented as a pointer to a heap block, containing the actual values 'a. Therefore 'a ref is never immediate.

The witness values can be used to perform safe optimizations such as allowing a more efficient 'a array blit operations if 'a is always immediate. These witnesses can also be used to perform safe conversions between immediate values of type 'a and int instead of using Obj.magic.

Consider an arbitrary type 'a for which you have built a type-immediacy witness using this interface. Let's call it w : 'a t.

You can use the two following functions and w to cast back and forth values from the type 'a to the type int:

      val int_as_value : 'a t -> int -> 'a option
      val value_as_int : 'a t -> 'a  -> int option

For the rest of this section, we will assume int_as_value and value_as_int partially applied to w.

Consider the following cases:

• let v be an immediate value.

Let i be the int that internally represents v. Then, value_as_int v returns Some i.

We can also recover v by using the conversions that go the other way. In particular, int_as_value i returns Some v.

• let v be a boxed value that cannot be converted to an int.

value_as_int v returns None because there does not exist an int s.t. int_as_value i evaluates to Some v.

• let i be an int that does not represent any value of type 'a

int_as_value i returns None.

value_is_int v is a faster equivalent to Option.is_some (value_as_int v). value_as_int_exn v is a faster equivalent to Option.value_exn (value_as_int v).

int_is_value i is a faster equivalent to Option.is_some (int_as_value i). int_as_value_exn i is a faster equivalent to Option.value_exn (int_as_value i).

These are lightweight functions that avoid allocating the option. value_is_int (resp int_is_value) can be used with value_as_int_exn (resp int_as_value_exn) to avoid both allocation or using a try with statement, paying only some small amount of CPU time for calling value_is_int (resp int_is_value):

      match value_as_int v with
      | Some v -> some v
      | None -> none

VS

      if value_is_int v
      then some (value_as_int_exn v)
      else none

Consider the following type:

      type test =
        | A
        | B
        | C of int
      with typerep

Type test is sometimes immediate, as A is represented as 0, B as 1, and C is a boxed value. We can construct a witness of type test Sometimes.t by using Sometimes.of_typerep or of_typerep and extracting the witness. Let's call the witness w here. We can now use it to safely convert between values of test and int:

Sometimes.value_as_int w A evaluates to Some 0 Sometimes.value_as_int w B evaluates to Some 1 Sometimes.value_as_int w (C 1) evaluates to None

Sometimes.int_as_value w 0 evaluates to Some A Sometimes.int_as_value w 1 evaluates to Some B Sometimes.int_as_value w n evaluates to None for all other values n

Consider this other example:

      type test = bool with typerep

Type test is always immediate, since true is represented as 1 and false as 0. We can construct a witness of type test Always.t by using Always.of_typerep or of_typerep and extracting the witness. Let's call the witness w:

Always.value_as_int w false evaluates to Some 0 Always.value_as_int w true evaluates to Some 1

Always.value_as_int_exn w false evaluates to 0 Always.value_as_int_exn w true evaluates to 1

Always.int_as_value w 0 evaluates to Some false Always.int_as_value w 1 evaluates to Some true Always.int_as_value w (-1) evaluates to None

Always.int_as_value_exn w 0 evaluates to false Always.int_as_value_exn w 1 evaluates to true Always.int_as_value_exn w (-1) raises

We also provide For_all_parameters_S* functors. Those are useful when one has a type with type parameters, but knows that values of that type will always be immediate (for example) no matter what the actual parameter is. They can use Always.For_all_parameters_S* to obtain access to a polymorphic witness.

An exception is raised on functor application if such witness cannot be obtained. That happens either because the witness depends on the actual type parameter, or because the type has a different witness (e.g. Sometimes instead of Always).