• d-bugmail puremagic.com Dec 04 2009
• d-bugmail puremagic.com Dec 04 2009
• d-bugmail puremagic.com Dec 04 2009
• d-bugmail puremagic.com Dec 04 2009
• d-bugmail puremagic.com Feb 17 2010

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=3572

           Summary: declaring pure function with void return type should
                    be compile time error
           Product: D
           Version: 2.036
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: michal.minich gmail.com


--- Comment #0 from Michal Minich <michal.minich gmail.com> 2009-12-04 02:37:33
PST ---
In D specification is written: "Pure functions are functions that produce the
same result for the same arguments. To that end, a pure function has parameters
that are all immutable or are implicitly convertible to immutable

void function cannot produce any result, thus they are meaningless as pure
functions. Also when all parameters are implicitly immutable, there is no
possibility to modify "out" parameters. So the only way for function to produce
result, is by returning it (and void has no value).

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d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=3572


Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au


--- Comment #1 from Don <clugdbug yahoo.com.au> 2009-12-04 12:01:56 PST ---
(In reply to comment #0)
 In D specification is written: "Pure functions are functions that produce the
 same result for the same arguments. To that end, a pure function has parameters
 that are all immutable or are implicitly convertible to immutable
 
 void function cannot produce any result, thus they are meaningless as pure
 functions. Also when all parameters are implicitly immutable, there is no
 possibility to modify "out" parameters. So the only way for function to produce
 result, is by returning it (and void has no value).


I think the restriction on 'out' parameters will be removed. I don't think
there's any reason for it.
A void pure function that doesn't have any 'out' parameters does seem to be
pretty useless (you can use it as a metaprogramming test that anything inside
it is pure, but that's about all I can think of).

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d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=3572


David Simcha <dsimcha yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dsimcha yahoo.com


--- Comment #2 from David Simcha <dsimcha yahoo.com> 2009-12-04 15:52:01 PST ---
?????  Why would you allow out parameters in a pure function?  This seems
reasonable for simple value types (ints, floats, etc.), but when you start
passing objects in, you start allowing the modification of whole object
subgraphs from pure functions.  This makes no sense.

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--- Comment #3 from Don <clugdbug yahoo.com.au> 2009-12-04 18:28:11 PST ---
(In reply to comment #2)
 ?????  Why would you allow out parameters in a pure function?  This seems
 reasonable for simple value types (ints, floats, etc.), but when you start
 passing objects in, you start allowing the modification of whole object
 subgraphs from pure functions.  This makes no sense.


Why doesn't it make sense? Aren't you thinking of inout parameters? A pure
function  void foo(out A a); ought to be exactly the same as A foo(); together
with an assignment. I'm not seeing anything impure in that.
BTW, it works fine in CTFE. (CTFE isn't quite the same concept as pure, but
it's close).

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--- Comment #4 from David Simcha <dsimcha yahoo.com> 2010-02-17 18:38:36 PST ---
(In reply to comment #3)
 Why doesn't it make sense? Aren't you thinking of inout parameters? A pure
 function  void foo(out A a); ought to be exactly the same as A foo(); together
 with an assignment. I'm not seeing anything impure in that.
 BTW, it works fine in CTFE. (CTFE isn't quite the same concept as pure, but
 it's close).


You're right, I did get confused between out and ref.  Allowing out parameters
in pure functions makes sense.  I tend to forget how out parameters work
because I almost never use them.  I almost always just return a tuple or a
struct.

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