to!string(double) at compile time

[cal]

On Tuesday, 21 August 2012 at 04:43:09 UTC, Bobby Bingham wrote:
> I'm just getting started with D, and was playing around with 
> string
> mixins.  I've hit a snag, and have reduced it to a minimal test 
> case:
>
>     import std.conv;
>
>     string test()
>     {
>         return to!string(0.0);
>     }
>
>     immutable auto testvar = mixin(test());
>
> This gives this result when compiling:
>
> /usr/include/phobos2/std/format.d(1479): Error: snprintf cannot 
> be interpreted at compile time, because it has no available 
> source code
> /usr/include/phobos2/std/conv.d(99):        called from here: 
> formatValue(w,src,f)
> /usr/include/phobos2/std/conv.d(824):        called from here: 
> toStr(value)
> /usr/include/phobos2/std/conv.d(268):        called from here: 
> toImpl(_param_0)
> test.d(6):        called from here: to(0)
> test.d(9):        called from here: test()
> test.d(9): Error: argument to mixin must be a string, not 
> (test())
>
> I guess converting a double to a string can't be done at 
> compile time
> because it requires calling the C snprintf function?  It 
> compiles fine
> if I replace the 0.0 with an int literal.  Is there any way 
> around
> this limitation?

I have had to work around this also. One way is to first multiply 
your float by a large factor (say 10000 depending on what 
precision you want) and then adding a decimal point back in to 
the string. Kinda hacky, but... Here is an example:

int fx = cast(int) (cos(angle)*1000000.);
string fsx = fx.to!string;

string xprefix;
if (fsx[0] == '-')
{
   xprefix = "-";
   fsx = fsx[1..$];
}

if (fsx.length == 7)
   sx = xprefix ~ fsx[0] ~ "." ~ fsx[1..$];
else
   sx = xprefix ~ "0." ~ fsx;

sx is then the string you want to mixin.