Module Core_kernel.Type_immediacy
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 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 soint
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
.
Converting between values and ints
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 theint
that internally representsv
. Then,value_as_int v
returnsSome i
.We can also recover
v
by using the conversions that go the other way. In particular,int_as_value i
returnsSome v
.
let
v
be a boxed value that cannot be converted to anint
.value_as_int v
returnsNone
because there does not exist an int s.t.int_as_value i
evaluates toSome v
.
let
i
be an int that does not represent any value of type'a
int_as_value i
returnsNone
.
Faster *_exn functions and functions with boolean results
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
Example
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
N-ary types that are immediate independently of their type arguments
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 of_typerep : 'a Core_kernel__.Import.Typerep.t -> 'a t
type 'a dest
=
|
Always of 'a Always.t
|
Sometimes of 'a Sometimes.t
|
Never of 'a Never.t
val dest : 'a t -> 'a dest
val int_as_value : 'a t -> Core_kernel__.Import.int -> 'a Core_kernel__.Import.option
val int_as_value_exn : 'a t -> Core_kernel__.Import.int -> 'a
val int_is_value : 'a t -> Core_kernel__.Import.int -> Core_kernel__.Import.bool
val value_as_int : 'a t -> 'a -> Core_kernel__.Import.int Core_kernel__.Import.option
val value_as_int_exn : 'a t -> 'a -> Core_kernel__.Import.int
val value_is_int : 'a t -> 'a -> Core_kernel__.Import.bool