[M3devel] Correction: Re: [modula3/cm3] Packed set literal generation issue (#21)

[Rodney M. Bates]

On 09/23/2017 10:59 AM, Rodney M. Bates wrote:
> The following didn't come through the first time, at least not to me:
>
>
> Fixed on github, commit 12f50b4cde746056b943bd8aaa105c3fcb46b1a8
>
> Deleted the assertion, which is neither true nor needed.
>
> But see below:
>
> On 09/16/2017 11:54 PM, jcchu wrote:
>> See the fragment below.
>>
>> VAR r := SET OF [0..31] { 0 }; (* OK *)
>> s := BITS 32 FOR SET OF [0..31] { 0 }; (* -- assertion failure in “SetExpr.m3” *)
>
> Are you aware that the BITS 32 here has no effect on the size of the global variable?
>
> Quoting from 2.2.5, Packed types:
>
>   TYPE T = BITS n FOR Base
>
>   where Base is a type and n is an integer-valued constant expression.
>   The values of type T are the same as the values  of type Base, but variables of type T
>   that occur in records, objects, or arrays
>   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>   will occupy exactly n bits and be packed adjacent to the preceding field or element.
>
> s is not inside a record, object, or array, so the BITS specification has no effect on
> the memory allocated by the compiler.
>
> There are, however some more obscure differences:
>
> 1) Although T and BITS n FOR T are are assignable to each other,
>    BITS n FOR T and BITS m FOR T, (n#m) have no subtype or assignability
>    relationship, so to assign one to the other would require two assignment
>    steps, with a T as the intermediate type.

I had thought the above was true was true for a long time, but recently realized
I was mistaken about that. Since <: is transitive, these two packed types are subtypes
of each other, transitively through T, and thus mutually assignable.

I haven't checked whether we have this implemented correctly.

>
> 2) Anywhere type equality, as opposed to assignability, is required, T and
>    BITS n FOR T will not match. For example, you cannot pass one of these to
>    the other as a VAR parameter.
>
>

-- 
Rodney Bates
rodney.m.bates at acm.org