Value Preservation and Polysemy -> context dependent integer literals

Don nospam at
Fri Dec 5 00:40:03 PST 2008

Andrei Alexandrescu wrote:
> Sergey Gromov wrote:
>> Thu, 04 Dec 2008 09:54:32 -0800, Andrei Alexandrescu wrote:
>>> Fawzi Mohamed wrote:
>>>> On 2008-12-01 22:30:54 +0100, Walter Bright 
>>>> <newshound1 at> said:
>>>>> Fawzi Mohamed wrote:
>>>>>> On 2008-12-01 21:16:58 +0100, Walter Bright 
>>>>>> <newshound1 at> said:
>>>>>>> Andrei Alexandrescu wrote:
>>>>>>>> I'm very excited about polysemy. It's entirely original to D,
>>>>>>> I accused Andrei of making up the word 'polysemy', but it turns 
>>>>>>> out it is a real word! <g>
>>>>>> Is this the beginning of discriminating overloads also based on 
>>>>>> the return values?
>>>>> No. I think return type overloading looks good in trivial cases, 
>>>>> but as things get more complex it gets inscrutable.
>>>> I agreee that return type overloading can go very bad, but a little 
>>>> bit can be very nice.
>>>> Polysemy make more expressions typecheck, but I am not sure that I 
>>>> want that.
>>>> For example with size_t & co I would amost always want a stronger 
>>>> typechecking, as if size_t would be a typedef, but with the usual 
>>>> rules wrt to ptr_diff, size_t,... (i.e. not cast between them).
>>>> This because mixing size_t with int, or long is almost always 
>>>> suspicious, but you might see it only on the other platform (32/64 
>>>> bit), and not on you own.
>>>> Something that I would find nice on the other hand is to have a kind 
>>>> of integer literals that automatically cast to the type that makes 
>>>> more sense.
>>> Wouldn't value range propagation take care of that (and actually 
>>> more)? A literal such as 5 will have a support range [5, 5] which 
>>> provides enough information to compute the best type down the road.
>> It sounds very nice and right, except it's incompatible with Cee.
>> Well, you can safely reduce bit count so that assigning "1025 & 15" to
>> "byte" would go without both a cast and a warning/error.  But you cannot
>> grow bitcount beyond the C limits, that is, you cannot return long for
>> "1024 << 30."  You should probably report an error, and you should
>> provide some way to tell the compiler, "i mean it."
>> In the worst case, any shift, multiplication or addition will result in
>> a compiler error.  Do I miss something?
> Well any integral value carries:
> a) type as per the C rule
> b) minimum value possible
> c) maximum value possible
> The type stays the type as per the C rule, so there's no change there. 
> If (and only if) a *narrower* type is asked as a conversion target for 
> the value, the range is consulted. If the range is too large, the 
> conversion fails.
> Andrei

Any idea how hard this would be to implement?

Also we've got an interesting case in D that other languages don't have: 
CTFE functions.
I presume that range propagation would not apply during evaluation of 
the CTFE function, but when evaluation is complete, it would then become 
a known literal, which can have precise range propagation. But there's 
still some funny issues:

uint foo(int x) { return 5; }

int bar(int y)
     ubyte w = foo(7); // this is a narrowing conversion, generates 
compiler warning (foo is not called as CTFE).
     return 6;

enum ubyte z = foo(7); // this is range propagated, so narrowing is OK.
enum int q = bar(3); // still gets a warning, because bar() didn't compile.

int gar(T)(int y)
     ubyte w = foo(7);
     return 6;

enum int v = gar!(int)(3); // is this OK???

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