[swift-evolution] Pre-proposal: Safer Decimal Calculations

[Stephen Canon]

> On Mar 24, 2016, at 10:01 AM, Rainer Brockerhoff <rainer at brockerhoff.net> wrote:
> 
> There's the mismatch between decimal representation of binary formats,
> causing confusion for very common cases like 0.01. There's your work in
> upgrading the FloatingPoint protocol. There's the question of
> modernizing NSDecimalNumber or writing a new decimal type. The
> scientific community needs IEEE 754, the mathematical community needs
> exact-precision bignums, the financial community needs predictable but
> small decimal precision, the educators need simple decimal numbers for
> teaching and graphing.
> 
> IMHO the existing Double, Float and CGFloat types don't cover all those
> use cases.
> 
> Maybe we need a DecimalLiteralConvertible not as generic as
> FloatLiteralConvertible, so that we can have a built-in type - call it
> Decimal or SimpleDecimal - that would be inferred in a statement like
> `let x = 0.01`
> such that, thereafter, calculations with x are sufficient for 99% of
> real-world graphing and financial calculations, with exact comparisons
> and so forth, but with none of the IEEE 754 complications. (Of course if
> you need trigonometry and square root etc. just convert to Double.)

I believe that providing the IEEE 754 “Decimal128” type (plus support for decimal literals) would satisfy 99% of the requirements listed here (while also conforming to FloatingPoint).  It provides 34 significant digits (sufficient to represent the US nation debt in Zimbabwean dollars).

Arbitrary-precision integer arithmetic is likely better served by a dedicated big integer type.  Does that seem reasonable?

– Steve
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