Extend custom numeric types to support floating point literals

[baronfel]

Currently you can define custom numeric types for integer values.
It would be nice to be able to handle floating point values too, e.g.

type complex = 
    | Complex of double * double

type imaginery = 
    | Imaginery of double
    static member (+) (lhs:double,Imaginery(rhs)) = Complex(lhs,rhs)

module NumericLiteralI =
    let FromZero () = Imaginery 0.0
    let FromOne () = Imaginery 1.0
    let FromInt32 (x) = Imaginery (double x)
    let FromInt64 (x:int64) = Imaginery (double x)
    let FromDouble (x:float) = Imaginery x // extension
    let polar = 1.5 + 2.5I

Original UserVoice Submission

[dsyme]

Now marking as planned, which means we can move to an RFC https://github.com/fsharp/FSharpLangDesign/tree/master/RFCs. Please submit one if we don't get to it first. The details remain to be worked out however

Don Syme
F# Language Evolution

[jbeeko]

Is there any reason to not support Decimal as well?

[Happypig375]

To match F# terminology, the new function should be called FromFloat.
That function should only be used when the literal has <=15 significant figures and its exponent is within -300 to 300.
For values outside of this range, a FromFloatString should be added.