Add `Option.thenSome` and `Option.thenSomeWith` to create an option from a bool

[mattnewport]

I propose we ...

Add functions similar to then() and then_some() on bool in Rust which take a bool and return an option which is None if the bool is false and Some f() or Some x if the bool is true.

module Option

let thenSome (value: 'a) (cond: bool) : 'a option =
    if cond then Some value else None

let thenSomeWith (thunk: unit -> 'a) (cond: bool) : 'a option =
    if cond then Some (thunk()) else None

Alternative names: Option.fromBool / Option.fromBoolWith

The existing way of approaching this problem in F# is ...

let opt = if x < 10 then Some x else None

Which becomes:

let opt' = x < 10 |> Option.thenSome x

Pros and Cons

The advantages of making this adjustment to F# are ...

It makes it a bit nicer to write pipelined code that needs to convert a bool value to an option. An example:

let combine (a: string option) (b: string option) : string option =
    match a, b with
    | Some a', Some b' -> Some(b' + a')
    | None, _ -> b
    | _ -> a

let fizzBuzz n =
    n % 3 = 0
    |> Option.thenSome "Fizz"
    |> combine (n % 5 = 0 |> Option.thenSome "Buzz")
    |> Option.defaultWith (fun () -> string n)

The alternative way to write this using if ... then ... else:

let fizzBuzz' n =
    if n % 3 = 0 then Some "Fizz" else None
    |> combine (if n % 5 = 0 then Some "Buzz" else None)
    |> Option.defaultWith (fun () -> string n)

I suggest that the version using Option.thenSome emphasizes the important part of the logic and gets rid of the repetitive else None branches that are really just noise here.

Another use can be seen in this alternative approach to implementing fizzBuzz using active patterns, where Option.thenSome makes the active pattern function implementations a bit cleaner:

let (|Fizzy|_|) n = 
    n % 3 = 0 |> Option.thenSome "Fizz"

let (|Buzzy|_|) n = 
    n % 5 = 0 |> Option.thenSome "Buzz"

let fizzBuzz'' n =
    match n with
    | Fizzy f & Buzzy b -> f + b
    | Fizzy f -> f
    | Buzzy b -> b
    | _ -> string n

An example of using Option.thenSomeWith to avoid evaluation when the condition is false:

type Classification =
    | Perfect
    | Abundant
    | Deficient

let fromCompare =
    function
    | 1 -> Deficient
    | 0 -> Perfect
    | -1 -> Abundant
    | _ -> failwith "Invalid comparison result"

let classify n =
    n > 0
    |> Option.thenSomeWith (fun () ->
        let n = int64 n

        seq {
            for i in 1L .. n - 1L do
                if n % i = 0 then i
        }
        |> Seq.sum
        |> compare n
        |> fromCompare)

Here's classify with if ... then ... else:

let classify n =
    if n <= 0 then
        None
    else
        let n = int64 n

        seq {
            for i in 1L .. n - 1L do
                if n % i = 0 then i
        }
        |> Seq.sum
        |> compare n
        |> fromCompare
        |> Some

Again the difference is fairly minor but we eliminate some noise with Nones and Somes.

The disadvantages of making this adjustment to F# are ...

This is a minor improvement and easily implemented in user code rather than in the library.

Extra information

Estimated cost (XS, S, M, L, XL, XXL):

S

Related suggestions: (put links to related suggestions here)

1041

The Option.thenSome function is a simpler way to get some of the benefits of issue 1041 for active patterns that are based on a simple boolean condition.

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• [X] This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• [X] I have searched both open and closed suggestions on this site and believe this is not a duplicate
• [X] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:

• [X] This is not a breaking change to the F# language design
• [X] I or my company would be willing to help implement and/or test this

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[dsyme]

If you really want functions like this, just add them in an auxiliary module

module Option =
    let thenSome (value: 'T) (cond: bool) : 'T option =
        if cond then Some value else None

    let thenSomeWith (thunk: unit -> 'T) (cond: bool) : 'T option =
        if cond then Some (thunk()) else None