How to write optional parameters in F# with default values

Question: What’s the difference between

let f ( a : int option, b : string option ) = ...


let f ( ?a : int, ?b : string ) = ...

Answer: in the first example, even if you have no values for a and b, you must call

let ret = f ( None, None )

whereas in the second example, you can simplify matters:

let ret = f ()

But what if you want to supply default values for those optional parameters? Here’s an example of a type definition in which the constructor takes optional parameters and assigns them default values if no value has been supplied. defaultArg is an F# operator, not a locally defined function.

type T( ?a : int, ?b : string ) =
  let a = defaultArg a 42
  let b = defaultArg b "Hello, World"

Whilst this syntax seems rather clunky, it is effective. You can read more about it on MSDN (scroll down to the section “Optional Parameters”).

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