same linear variable in an argument

[chotu s]

Hello,

Can we pass same linear var in argument of a fun , for example :


#include "share/atspre_define.hats"
#include "share/atspre_staload.hats"


extern
fun{a:t@ype}
some_fun{n:nat} (v1 : !arrayptr(a,n) , v2 : !arrayptr(a,n)) : void
 

implement main0 () = {
  val v = arrayptr($arrpsz{int}(1,2,3,4,5))
  val () = some_fun<int> (v,v)
  val () = arrayptr_free (v)
}

Above code does not compile , and complain for 2nd arg , which is no more available

How to write above code in ATS2

Thanks

[Brandon Barker]

I think it makes sense that you can not pass (by value) the same linear argument more than once in the same function call (you would essentially have two "active views" for the same memory location); probably what you want is to make an explicit copy when needed and pass it as well.

Brandon Barker
brandon...@gmail.com

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[chotu s]

I thought so  . Is there any other way , without copy , say if copy is costly.

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[Brandon Barker]

I can't think of a trivial (and safe) way to do it; one way might be to use an option type for the vectors, and v2 is none, just access it as v1 inside the function. I admit this is a rough idea and I have not tested it.

An unsafe way might be to just pass pointers and use unsafe functions to cast.

Brandon Barker
brandon...@gmail.com

[Brandon Barker]

Another thing that should work in principle is if we specify both vectors are read-only, and while possible, I think this is a little more involved in ATS (and I don't think I've tried it). 

Brandon Barker
brandon...@gmail.com

[Brandon Barker]

Here (I believe) is some not so light reading on the topic: http://www.cs.bu.edu/~hwxi/academic/papers/ictac10.pdf

It does have a few examples.

Unfortunately I can't recall if I've seen any other example code.

Brandon Barker
brandon...@gmail.com

[chotu s]

Yes , they are read only in my original code they are not modified by the function.

[chotu s]

Thanks Brandon , I will take a look at it.

[gmhwxi]

Here is a way to do it. It is not type-safe but safe enough :)

//

extern
fun{
a:t@ype
} some_fun{n:nat}

  (!arrayptr (a, n), !arrayptr (a, n)): void
//
(* ****** ****** *)

implement
main0 ((*void*)) =
{
//
val A =
(arrayptr)$arrpsz{int}(0, 1)
//
val (fpf | A2) =
  decode ($vcopyenv_vt(A))
val () = some_fun<int> (A, A2)
prval ((*void*)) = fpf (A2)
//
val () = arrayptr_free (A)
//
} (* end of [main0] *)

[gmhwxi]

I wrote plenty code involving read-only views.

However, it is very cumbersome to program with read-only views,
and the benefits from doing it seem rather limited. So there is no
longer built-in support for read-only views in ATS2.

However, at a higher-level, read-only views (and other data protection
concepts) can be very useful in system design.

[Brandon Barker]

Ah OK, I hadn't realized they were removed. I'm not sure what is meant by "at a higher level". 

Brandon Barker
brandon...@gmail.com

[gmhwxi]

Say you want to implement a read/write buffer shared by multiple threads.

[Brandon Barker]

So in ATS2, I presume this wouldn't be done with specialized  views, but rather read/write functions?

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[gmhwxi]

One can define read-only views on one's own.