Another take on decimal data types

[rumbu]

On Thursday, 11 January 2018 at 11:19:41 UTC, Robert M. Münch 
wrote:
> On 2018-01-08 22:16:25 +0000, rumbu said:
>
>> This is my first D finalized project (+16k loc).
>
> Great stuff! Will this work in betterC mode?
>

It will not work without some refactory. Most of phobos/druntime 
dependencies are minimal an can be rewritten - in fact there are 
only 9 dependencies: bsr, bsf, addu, subu, adds, subs from 
druntime and isNaN, isinfinity, signbit from std.math.

The rest of the dependencies are simply traits that must work by 
default under betterC.

Once dependencies are solved, another problem will be the 
exception mechanism. 90% of arithmetic operations are meant to 
throw exceptions, but this can be overridden by the alternate 
exception handling - raising and setting flags.

BUT - a very big but - the most important issue is the formatting 
thing. Even there is no direct dependency on std.format or 
std.stdio, the formatting mechanism is designed to fit nicely 
into phobos formatting paradigm. That means that there is no way 
to output decimal values on the console without phobos. I confess 
that 30% of my development time was the formatting feature: 
displaying correctly any possible combination of width, padding, 
alignment in 4 different formats (%f, %g, %e, %a) was really a 
challenge and despite of hundreds of unit tests, I'm not 
convinced today that I covered all the corner cases. The f*cking 
%g took me 3 days to find out exactly what is meant to do. More 
than that, each compiler producer have a different idea about 
format specifiers. Linking your application against snn, msvcrt 
or glibc will render completely different results for 
printf("%g", ...). On top of that, neither the FormatSpec 
documentation from phobos is something complete and clear...

AFAIK, there is no way to plug a custom formatting mechanism into 
printf, not even in plain C (ok, I'm aware that gcc has something 
in this respect).

To sum things up, it is possible to use it in betterC mode if:
- all dependencies are rewritten;
- the exception handling is dropped out completely;
- a printf equivalent for decimal types is rewritten from scratch 
or - decimal values must be converted to binary float before 
printing them - but this will negate the main purpose of the 
decimal type - precision.