Re: [ats-lang-users] Difference between viewt@ype and viewt@ype+?

[Hongwei Xi <hw...@cs...>]

On Sat, 23 Oct 2010, Andrew Richards wrote:

>>On Sat, Oct 23, 2010 at 01:21:35PM +0600, Artyom Shalkhakov wrote:
>>> I believe that [absviewtype] is for abstract view types of size of one
>>> pointer, whereas [absviewt@ype] is for abstract view types of unknown
>>> (arbitrary) size and analogously for [abstype] and [abst@ype].
>>
>>This seems to be the case. Thank you! And now this leads to another
>>question. If absviewtype is always the size of a pointer, and
>>absviewt@ype can be of any arbitrary size, why is it possible to have
>>an empty value for an absviewt@ype declaration? For example..
>>
>>absviewtype foo1 (b:bool) = int // rejected
>>absviewtype foo2 (n:int) = ptr // OK
>>absviewtype foo1 (b:bool) // We know it is the size of a pointer
>>
>>absviewt@ype bar1 (b:bool) = int // OK, size of int
>>absviewt@ype bar2 (n:int) = int // OK, size of a pointer
>>absviewt@ype bar3 (b:bool) // OK, no size, proposition only?

Declared as such, bar3 has *unknown* size. Therefore, if you do something 
like:

let
  var x: bar3
in
  ...
end

The C compiler will complain as it cannot compute the size for [x].

Then what is the use of bar3? Well, you can have a function like foo:

fun foo (x: &bar3): int

which uses call-by-reference to pass a parameter of type [bar3].

>>I'm guessing it has no size (void)? Or maybe I have misunderstood?

According to C, sizeof(void) = 1. In ATS, there is a type [empty]
whose size is 0.

--Hongwei

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