Convert double to long if lossless

[drug]

On 1/19/21 6:50 PM, Ali Çehreli wrote:
> On 1/19/21 6:04 AM, drug wrote:
> 
>  > Another (low level) way is to shift mantissa left by exponent value.
> 
> Luckily, we already have a helper in Phobos:
> 
>    https://dlang.org/phobos/std_bitmanip.html#FloatRep
> 
> Ali
> 

That makes life simpler, thanks for sharing this:
```D
import std;

void main()
{
     auto valueRange = [
         10.000000000000001,
         10.0000000000000001, // in fact this is just 10.0
     ];

     foreach(value; valueRange)
     {
         auto dr = DoubleRep(value);
         const hasFractional = !!(dr.fraction << (dr.exponent-1023+12));
         writefln("has `%.18f` fractional part: %s", value, hasFractional);
     }
}
```

but I think that casting to long and comparing it to the value is easier 
to understand and more portable:
```D
import std;

void main()
{
     auto valueRange = [
         10.000000000000001,
         10.0000000000000001, // in fact this is 10.0
     ];

     foreach(value; valueRange)
     {
         const hasFractional = (value != cast(long)value);
         writefln("has `%.18f` fractional part: %s", value, hasFractional);
     }
}
```

P.S. shouldn't compiler emit the error if a literal can't be represented 
lossless?