[M3devel] A question for our language lawyers

[Dirk Muysers]

Daniel, with my apologies, sometimes I wonder if you do it on purpose.


From: Daniel Alejandro Benavides D. 
Sent: Friday, July 06, 2012 9:17 PM
To: m3devel at elegosoft.com ; Rodney M. Bates 
Subject: Re: [M3devel] A question for our language lawyers


      Hi all:
      English men say array is a sequence of elements (of a common type), and a BOOLEAN is an enumeration so you might attack that distinction to define what is an initialized boolean or array of boolean in common compilers, gcc javac, etc, which if is java-like is really undefined:

      http://www.drdobbs.com/architecture-and-design/the-humble-boolean-deserves-help/232900836?cid=DDJ_nl_upd_2012-05-02_h&elq=cef656ee4d6c4bca996b337620b98f85

      So I prefer non-uniform rules for records different of Sets, Arrays, and records as that, note that NEW expression doesn't allow constructors to be used, so the only thing you can use is array of uninitialized variables (but current gcc or javac, etc are really wrong in that)

      This means we need to address this by either a native backend (NT386) or by another language for that matter.

      Thanks in advance for any comments you may have

      --- El vie, 6/7/12, Rodney M. Bates <rodney_bates at lcwb.coop> escribió:


        De: Rodney M. Bates <rodney_bates at lcwb.coop>
        Asunto: Re: [M3devel] A question for our language lawyers
        Para: m3devel at elegosoft.com
        Fecha: viernes, 6 de julio, 2012 13:27




        On 07/06/2012 04:23 AM, Dirk Muysers wrote:
        > The report says (2.6.9)
        > "The values in the array will be arbitrary values of their type."

        > Now, ParseParams in its "init" method allocates an array of BOOLEANs
        > and relies on the fact that it is supposedly initialised with FALSE values.

        > At the other hand the report says (2.2.4)
        > "The constant |default| is a default value used when a record is constructed or allocated"

        > If I allocate an array of records, which statement is stronger:
        > - the array contains arbitray record values ?
        > - the array record fields will be initialised to their default values?

        Admittedly unclearly if not misleadingly worded.  Better wording might be
        to say each element is initialized as it would if it were a scalar variable
        of its type.

        I think the way to interpret this is that the array itself does not impose
        any initialization, but this fact will not eliminate initialization
        imposed by other rules, specifically, the type of the array's elements.

        This is a language quirk that I have always been deeply ambivalent about.
        The type safety would go down the drain if variables were not initialized
        to a bit pattern that represents some value of the type, so we have to pay
        the performance penalty of executing initialization code.  So why not define
        which value of the type is initialized-to and get behavioral predictability
        for free?  And further save redundant initialization in the likely event
        that the compiler's chosen arbitrary value happens to match what the
        programmer wants?

        (OK, a smart enough optimizer might figure this out, but we could have
        had it even with a naive compiler.)

        The contrary case is a type whose compiler-chosen representation happens
        to use every bit pattern in the allocated space for a value of the type.
        Here, no compiler-generated runtime initialization is needed.

        Also, the rule we have might sometimes encourage programmers to at least give a
        millisecond's thought to whether they need to do some explicit initialization.


        > The ParseParams "init" method is obviously erroneous and works only
        > by virtue of a happy combination of circumstances.
        > But how is the report to be interpreted in the second case?
     
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