Optional parameters and option types using F#

user_249695 0
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Consider the following code:

type Test () =
  member o.fn1 (?bo) = 1
  member o.fn2 (?bo) = o.fn1 bo

  member o.fn3 (?bo) = 1 + bo.Value
  member o.fn4 (?bo) = o.fn3 bo

While fn1 and fn2 work just fine, fn4 produces the following error:

init.fsx(6,30): error FS0001: This expression was expected to have type int but here has type 'a option

MSDN states:

Optional parameters are interpreted as the F# option type, so you can query them in the regular way that option types are queried, by using a match expression with Some and None.

To me, optional parameters are not interpreted as the F# option type otherwise the code would compile. Moreover I do not understand why, when I hover over ?bo in fn3 the tooltip says val bo: int option but from outside expects only int. I would expect a behavior of accepting nothing, int, Some int and None. And as the last note, I do not understand why fn2 works but fn4 does not.

Thanks for clarification

option-type optional-parameters f# types
3 Answers

F# does not require matching to pass optional arguments. Using the plot-example:

type Plotter() =
    static member Plot(?x,?y,?color) = 
        printfn $"{x}, {y}, {color}" // Do something "plottish" here

type PlotUser() =
    static member MyPlotUser(?x, ?y, ?color) =
        Plotter.Plot(?x = x, ?y = y, ?color = color)

/// Perform calls
PlotUser.MyPlotUser(10) // => Some(10), , 
PlotUser.MyPlotUser(10, color = "Green") // => Some(10), , Some(Green)

The above lets you call plot with your own optional arguments. /JEE, Sharp#Soft

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