interest.fsx

type Compounding = 
    | TimesPerYear of float 
    | Constant 

type InterestCalculationOptions = {
    Compounding : Compounding
    Rate : float
    Principal : float
    TermInYears : float
}

let compound options = 
    let core r e = 
        options.Principal * (r ** e)
    match options.Compounding with 
    | Constant -> core System.Math.E (options.Rate * options.TermInYears)
    | TimesPerYear f -> let rate = options.Rate / f
                        core (1.0 + rate) (f * options.TermInYears)

let fry = compound { Compounding = TimesPerYear 1.0
                     Rate = 0.0225
                     Principal = 0.93
                     TermInYears = 1000.0 }