[swift-users] Decimal to Double?

[Nevin Brackett-Rozinsky]

On Mon, Nov 28, 2016 at 2:21 PM, Joe Groff via swift-users <
swift-users at swift.org> wrote:

> ExpressibleByFloatLiteral uses a binary float initial value and so doesn't
> guarantee decimal accuracy


That seems like a flaw in ExpressibleByFloatLiteral that should be fixed.

Did anyone ever submit a proposal following the discussion in
“[swift-evolution] Allow FloatLiteralType in FloatLiteralConvertible to be
aliased to String” where somebody—*runs to check*…oh it was you!—described
the proper solution?

Nevin


On Mon, Nov 28, 2016 at 2:21 PM, Joe Groff via swift-users <
swift-users at swift.org> wrote:

>
> > On Nov 28, 2016, at 9:22 AM, Alex Blewitt via swift-users <
> swift-users at swift.org> wrote:
> >
> > NSDecimal conforms to the ExpressibleBy(Float|Integer)Literal, but that
> just means it can be created from a literal value, not the other way around:
> >
> > https://github.com/apple/swift-corelibs-foundation/blob/
> 108a5b0006912c6cf6b59e4954255237bf01b67a/Foundation/
> NSDecimal.swift#L319-L329
> >
> >
> >
> > extension Decimal : ExpressibleByFloatLiteral {
> >     public init(floatLiteral value: Double) {
> >         self.init(value)
> >     }
> > }
> >
> > extension Decimal : ExpressibleByIntegerLiteral {
> >     public init(integerLiteral value: Int) {
> >         self.init(value)
> >     }
> > }
>
> It really shouldn't be, since ExpressibleByFloatLiteral uses a binary
> float initial value and so doesn't guarantee decimal accuracy. I'd
> recommend using init(mantissa:exponent:isNegative:) instead, and
> minimizing any conversions between Double and Decimal.
>
> -Joe
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