Lambda functions passed into HOFs

[nojaf]

I propose we add an alternative way of formatting lambda functions passed into HOFs as describe by the G-Research style guide.

The existing way of Fantomas deals with this problem is following the Microsoft style guide without any options.

Pros and Cons

The advantages of making this adjustment to Fantomas are following both style guides,

The disadvantages of making this adjustment to Fantomas are introducing a new setting and maintaining both choices.

Examples

List.collect (fun (a, element) ->
    let path' =
        path
        |> someFunctionToCalculateThing

    innerFunc<'a, 'b>
        path'
        element
        (foo >> bar value >> List.item a)
        shape
)

let mySuperFunction a =
    someOtherFunction a (fun b ->
        // doing some stuff her
       b * b
    )

Things to keep in mind:

• This seems to only have an impact on multiline constructs
• There can be arguments before and after the lambda.
• What with multiple lambdas?
• Desugared lambdas?
• Setting name?

Extra information

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

Related suggestions: #1189

I think this feature cannot be implemented in one go. It will be a back and forward collaboration between trying things out in Fantomas and further polishing the style guide.

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• I have read the Contribution Guidelines.
• I have searched both open and closed suggestions on this site and believe this is not a duplicate

Please tick all that apply:

• This is not a breaking change to Fantomas
• I or my company would be willing to help implement and/or test this
• This suggestion is part of the Microsoft style guide (please add a link to section if so)
• This suggestion is part of the G-Research style guide: Clarify lambda positioning G-Research/fsharp-formatting-conventions#10

[knocte]

Doesn't this github issue overlap a bit with #815 ?

[knocte]

(If yes, maybe a better name for the setting is MultilineLambdaParensOnSameColumn, for consistency with the other Multiline* setting.)

[nojaf]

Closing parenthesis does not align in GR guide.
#815 does something slightly different.

[bamarsha]

@nojaf In Fantomas 4.3.0-beta-001, it looks like setting fsharp_multi_line_lambda_closing_newline to either true or false doesn't restore the style that was possible in Fantomas 4.2:

let printListWithOffset a list1 =
    list1
    |> List.iter (fun elem ->
        // print stuff
        printfn "%d" (a + elem))

Is there any way to keep using this style in the latest version?

[nojaf]

Hello @SamarSha, in #1201 the default style was updated to match the MS F# style guide.
It is no longer possible to have the style in 4.2.

[bamarsha]

Hi, I see in the MS style guide that it shows the lambda on a new line when a function is being applied with multiple arguments:

let printListWithOffset a list1 =
    List.iter
        (fun elem ->
            printfn $"%d{a + elem}")
        list1

which is OK, but I was not able to find where it says the lambda cannot start on the same line as the function name when it's the only argument applied. I found examples in other sections that use the 4.2 style instead:

https://docs.microsoft.com/en-us/dotnet/fsharp/style-guide/formatting#formatting-records

let foo a =
    a
    |> Option.map (fun x ->
        {
            MyField = x
        })

https://docs.microsoft.com/en-us/dotnet/fsharp/style-guide/formatting#pattern-matching-constructs

lambdaList
|> List.map (function
    | Abs(x, body) -> 1 + sizeLambda 0 body
    | App(lam1, lam2) -> sizeLambda (sizeLambda 0 lam1) lam2
    | Var v -> 1)

[nojaf]

Please open an issue at the MS style guide to clear this out.
The guide itself is a living document, not everything is always clearly defined.

[knocte]

@nojaf In Fantomas 4.3.0-beta-001

How did you install 4.3.0-beta-001? I don't find it in MyGet