Re: [swift-users] Decimal to Double?

[David Sweeris via swift-users]

> On Nov 28, 2016, at 13:43, Joe Groff via swift-users  
> wrote:
> 
> 
>> On Nov 28, 2016, at 11:42 AM, Nevin Brackett-Rozinsky 
>>  wrote:
>> 
>> On Mon, Nov 28, 2016 at 2:21 PM, Joe Groff via swift-users 
>>  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?
> 
> Fixing ExpressibleByFloatLiteral has come up several times, but nobody's 
> formally written up the proposal AFAIK.

I'm in the middle of moving, or I'd do it.

- Dave Sweeris 
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Re: [swift-users] Decimal to Double?

[Joe Groff via swift-users]

> On Nov 28, 2016, at 11:42 AM, Nevin Brackett-Rozinsky 
>  wrote:
> 
> On Mon, Nov 28, 2016 at 2:21 PM, Joe Groff via swift-users 
>  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?

Fixing ExpressibleByFloatLiteral has come up several times, but nobody's 
formally written up the proposal AFAIK.

-Joe
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Re: [swift-users] Decimal to Double?

[Nevin Brackett-Rozinsky via swift-users]

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

>
> > On Nov 28, 2016, at 9:22 AM, Alex Blewitt via swift-users <
> swift-users@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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Re: [swift-users] Decimal to Double?

[David Sweeris via swift-users]

Sent from my iPhone
> On Nov 28, 2016, at 13:21, Joe Groff  wrote:
> 
> 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.

That's what I was worried about... What's the "correct" way to express a 
literal as a Decimal, then? Use a string literal, `"2.1"`, instead of a float 
literal, `2.1`?

- Dave Sweeris 
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Re: [swift-users] Decimal to Double?

[Joe Groff via swift-users]

> On Nov 28, 2016, at 9:22 AM, Alex Blewitt via swift-users 
> <swift-users@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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Re: [swift-users] Decimal to Double?

[Alex Blewitt via swift-users]

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 would be relatively simple for the NSDecimal to add an extension initialiser 
to Double to allow construction directly from an NSDecimal, but it would have 
to be done for both the Linux and Darwin implementations.

Alex

> On 28 Nov 2016, at 17:15, David Sweeris  wrote:
> 
> 
> On Nov 28, 2016, at 09:48, Philippe Hausler via swift-users 
> > wrote:
> 
>> This might be a bit nicer since that is relying on NSNumber bridges. You can 
>> bridge to NSDecimalNumber directly like this: 
>> 
>> (Decimal(1.0) as NSDecimalNumber).doubleValue
>> 
>> (but perhaps we should consider adding initializers that follow the same 
>> pattern as Double that don’t have to bridge to solve the problem)
>> 
>>> On Nov 28, 2016, at 2:40 AM, Alex Blewitt via swift-users 
>>> > wrote:
>>> 
>>> You can wrap it with an NSDecimalNumber, and then cast it to a Double from 
>>> there:
>>> 
>>> Double(NSDecimalNumber(decimal:Decimal(1.0)))
>>> 
>>> Alex
>>> 
 On 28 Nov 2016, at 10:13, Rick Mann via swift-users > wrote:
 
 How do I get a Double from a Decimal?
 
 TIA,
 
 -- 
 Rick Mann
 rm...@latencyzero.com 
 
 
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>>> 
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> 
> Does NSDecimalNumber conform to the floating point literal whosywhatsit 
> protocol? If not, wouldn't both of those examples just covert a Double to a 
> decimal and back again? Would there be a purpose to that? Like maybe it'd get 
> rounded differently or something?
> 
> Once you have an NSDecimalNumber variable, though, I don't know of any better 
> way than that '.doubleValue' property you and Alex mentioned.
> 
> - Dave Sweeris

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Re: [swift-users] Decimal to Double?

[Philippe Hausler via swift-users]

This might be a bit nicer since that is relying on NSNumber bridges. You can 
bridge to NSDecimalNumber directly like this: 

(Decimal(1.0) as NSDecimalNumber).doubleValue

(but perhaps we should consider adding initializers that follow the same 
pattern as Double that don’t have to bridge to solve the problem)

> On Nov 28, 2016, at 2:40 AM, Alex Blewitt via swift-users 
>  wrote:
> 
> You can wrap it with an NSDecimalNumber, and then cast it to a Double from 
> there:
> 
> Double(NSDecimalNumber(decimal:Decimal(1.0)))
> 
> Alex
> 
>> On 28 Nov 2016, at 10:13, Rick Mann via swift-users > > wrote:
>> 
>> How do I get a Double from a Decimal?
>> 
>> TIA,
>> 
>> -- 
>> Rick Mann
>> rm...@latencyzero.com 
>> 
>> 
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