Up
# Module Type_immediacy

#
Converting between values and ints

#
Faster *_exn functions and functions with boolean results

#
Example

#
N-ary types that are immediate independently of their type arguments

### Signature

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`

).

module
Always
: sig .. end

module
Sometimes
: sig .. end

module
Never
: sig .. end

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

val
int_as_value_exn : 'a t -> int -> 'a

val
int_is_value : 'a t -> int -> bool

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

val
value_as_int_exn : 'a t -> 'a -> int

val
value_is_int : 'a t -> 'a -> bool