[swift-evolution] Allow FloatLiteralType in FloatLiteralConvertible to be aliased to String

[Joe Groff]

> On May 6, 2016, at 2:24 AM, Morten Bek Ditlevsen via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Currently, in order to conform to FloatLiteralConvertible you need to implement
> an initializer accepting a floatLiteral of the typealias: FloatLiteralType.
> However, this typealias can only be Double, Float, Float80 and other built-in
> floating point types (to be honest, I do not know the exact limitation since I have
> not been able to read find this in the documentation).
> 
> These floating point types have precision limitations that are not necessarily
> present in the type that you are making FloatLiteralConvertible.
> 
> Let’s imagine a CurrencyAmount type that uses an NSDecimalNumber as the
> representation of the value:
> 
> 
> public struct CurrencyAmount {
>  public let value: NSDecimalNumber 
>  // .. other important currency-related stuff ..
> }
> 
> extension CurrencyAmount: FloatLiteralConvertible { 
>  public typealias FloatLiteralType = Double
> 
>  public init(floatLiteral amount: FloatLiteralType) {
>    print(amount.debugDescription) 
>    value = NSDecimalNumber(double: amount) 
>  } 
> }
> 
> let a: CurrencyAmount = 99.99
> 
> 
> The printed value inside the initializer is 99.989999999999995 - so the value
> has lost precision already in the intermediary Double representation.  
> 
> I know that there is also an issue with the NSDecimalNumber double initializer,
> but this is not the issue that we are seeing here.
> 
> 
> One suggestion for a solution to this issue would be to allow the
> FloatLiteralType to be aliased to a String.  In this case the compiler should
> parse the float literal token: 99.99 to a String and use that as input for the
> FloatLiteralConvertible initializer.
> 
> This would mean that arbitrary literal precisions are allowed for
> FloatLiteralConvertibles that implement their own parsing of a String value.
> 
> For instance, if the CurrencyAmount used a FloatLiteralType aliased to String we
> would have:
> 
> extension CurrencyAmount: FloatLiteralConvertible { 
>  public typealias FloatLiteralType = String
> 
>  public init(floatLiteral amount: FloatLiteralType) { 
>    value = NSDecimalNumber(string: amount) 
>  } 
> }
> 
> and the precision would be the same as creating an NSDecimalNumber from a
> String: 
> 
> let a: CurrencyAmount = 1.00000000000000000000000000000000001
> 
> print(a.value.debugDescription)
> 
> Would give: 1.00000000000000000000000000000000001
> 
> 
> How does that sound? Is it completely irrational to allow the use of Strings as
> the intermediary representation of float literals?
> I think that it makes good sense, since it allows for arbitrary precision.
> 
> Please let me know what you think.

Like Dmitri said, a String is not a particularly efficient intermediate representation. For common machine numeric types, we want it to be straightforward for the compiler to constant-fold literals down to constants in the resulting binary. For floating-point literals, I think we could achieve this by changing the protocol to "deconstruct" the literal value into integer significand and exponent, something like this:

// A type that can be initialized from a decimal literal such as
// `1.1` or `2.3e5`.
protocol DecimalLiteralConvertible {
  // The integer type used to represent the significand and exponent of the value.
  typealias Component: IntegerLiteralConvertible

  // Construct a value equal to `decimalSignificand * 10**decimalExponent`.
  init(decimalSignificand: Component, decimalExponent: Component)
}

// A type that can be initialized from a hexadecimal floating point
// literal, such as `0x1.8p-2`.
protocol HexFloatLiteralConvertible {
  // The integer type used to represent the significand and exponent of the value.
  typealias Component: IntegerLiteralConvertible

  // Construct a value equal to `hexadecimalSignificand * 2**binaryExponent`.
  init(hexadecimalSignificand: Component, binaryExponent: Component)
}

Literals would desugar to constructor calls as follows:

1.0 // T(decimalSignificand: 1, decimalExponent: 0)
0.123 // T(decimalSignificand: 123, decimalExponent: -3)
1.23e-2 // same

0x1.8p-2 // T(hexadecimalSignificand: 0x18, binaryExponent: -6)

-Joe