[Caml-list] Question on private type abbreviations

[immanuel litzroth]

I have a question related to private type abbreviations
I'm interfacing C++ and ocaml and I want to make sure that the ranges of
integer types are correct and reflect them in the ocaml interface.

So I define
type uint8 = private int
and
type int8 = private int
same for the other sizes/signedness
and the appropriate functions to do range checking (those are external and
use
std::numeric limits)
external uint8 : int -> uint8 = "make_uint8"
..
this gives typesafety and avoids boxing/unboxing and makes sure that the
user can
only pass values that are range checked at the earliest opportunity.

Now I wanna check my code
for all the types I wanna use 1 checking function something like this:

let test_conversions (the_fun : int -> 't) (the_val : int) =
try
let the_t = the_fun the_val in
Printf.printf "Numbers are %d\n" (the_t : 't :> int)
with
| Invalid_argument str -> Printf.printf "Error: %s" str

let () = test_conversions uint8 1 -> will work
.
let () = test_conversions uint64 (-1) -> will print Error...

Now this doesn't typecheck because the type var 't in the signature is too
general.
what I need to put there is "a type coercible to int"
Is that possible? Is there some way to achieve this?
Thanks in advance,
Immanuel

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[Jacques Garrigue]

On 2015/07/24 23:04, immanuel litzroth wrote:
>
> I have a question related to private type abbreviations
> I'm interfacing C++ and ocaml and I want to make sure that the ranges of integer types are correct and reflect them in the ocaml interface.
>
> So I define
> type uint8 = private int
> and
> type int8 = private int
> same for the other sizes/signedness
> and the appropriate functions to do range checking (those are external and use
> std::numeric limits)
> external uint8 : int -> uint8 = "make_uint8"

> ...

> this gives typesafety and avoids boxing/unboxing and makes sure that the user can
> only pass values that are range checked at the earliest opportunity.
>
> Now I wanna check my code
> for all the types I wanna use 1 checking function something like this:
>
> let test_conversions (the_fun : int -> 't) (the_val : int) =
> try
> let the_t = the_fun the_val in
> Printf.printf "Numbers are %d\n" (the_t : 't :> int)
> with
> | Invalid_argument str -> Printf.printf "Error: %s" str
>
> let () = test_conversions uint8 1 -> will work

> ..

> let () = test_conversions uint64 (-1) -> will print Error...
>
> Now this doesn't typecheck because the type var 't in the signature is too general.
> what I need to put there is "a type coercible to int"
> Is that possible? Is there some way to achieve this?

I see no way to do that implicitly.
Namely, subtyping is only checked for coercions, so if you don’t write a coercion for
each of your types, this won’t work.
This means you need to add another parameter:

let test_conversions (coerce : ’t -> int) (the_fun : int -> 't) (the_val : int) =

try
let the_t = the_fun the_val in

Printf.printf "Numbers are %d\n” (coerce the_t)

with
| Invalid_argument str -> Printf.printf "Error: %s” str

let from_uint8 x : uint8 :> int = x
let from_uint64 x : uint64 :> int = x

let () = test_conversions from_uint8 uint8 1
.
let () = test_conversions from_uint64 uint64 (-1)

Jacques Garrigue

[immanuel litzroth]

Yes,
I found that explicitly adding the conversion function works works but then
I got
into some currying problem -- sorry I'm relatively new to ocaml:

let conversion_test = test_conversions (fun (x : uint8) -> (x:>int)) in
conversion_test uint8 1

I want to just define the conversion test inside the body of the let so I
can reuse the name
for the other types. That doesn't seem to work?
Immanuel