Fantomas keeps adding new lines between two interface member implementations

[ivan-brko]

Description

Fantomas adds a new line between two interface member implementations each time the file is formatted

Repro code

namespace Quartz.Fsharp

module Logging =
    open Quartz.Logging
    open System

    //todo: it seems that quartz doesn't use mapped and nested context,
    //however, check if this is the best implementation for this interface
    type private QuartzLoggerWrapper(f) =
        interface ILogProvider with

            member this.OpenMappedContext(_, _) =
                { new IDisposable with
                    member this.Dispose() = () }
            member this.OpenNestedContext _ =
                { new IDisposable with
                    member this.Dispose() = () }

            member this.GetLogger _name = new Logger(f)

    let SetQuartzLoggingFunction f =
        let loggerFunction level (func: Func<string>) exc parameters =
            let wrappedFunction = Helpers.nullValuesToOptions (fun (x: Func<string>) -> (fun () -> x.Invoke())) func
            let wrappedException = Helpers.nullValuesToOptions id exc
            f level wrappedFunction wrappedException (parameters |> List.ofArray)
        LogProvider.SetCurrentLogProvider(QuartzLoggerWrapper(loggerFunction))

    let SetQuartzLogger l = LogProvider.SetCurrentLogProvider(l)

The link to fantomas tool example.

You can check if you copy the formatted code from the right to the source and format that, it will add new line after line 17, and it keeps doing this forever.

[theimowski]

Minimal repro I could come up with: https://jindraivanek.gitlab.io/fantomas-ui/#?code=C4TwDgpgBAggFASigXgFCqpqBbC2BGEATlMABYCWAzgHQCyehRMKGW7FAZqUQK7TkIAOygAGKBAA2VaAEZ07XAWKlKtBsqIAhVuw7dgfAWWFQATBOnQAzAqxKmq6vUbEAwijGogA

Code

type A() =

    member this.MemberA =
        if true then 0 else 1

    member this.MemberB =
        if true then 2 else 3

    member this.MemberC = 0

Result

type A() =

    member this.MemberA =
        if true then 0 else 1


    member this.MemberB =
        if true then 2 else 3

    member this.MemberC = 0

Options

Fantomas 3.1.0

Name	Value
IndentOnTryWith	false
IndentSpaceNum	4
KeepNewlineAfter	false
MaxIfThenElseShortWidth	40
PageWidth	120
ReorderOpenDeclaration	false
SemicolonAtEndOfLine	false
SpaceAfterComma	true
SpaceAfterSemicolon	true
SpaceAroundDelimiter	true
SpaceBeforeArgument	true
SpaceBeforeColon	false
StrictMode	false

[nojaf]

The problem here is that Fantomas adds a newline between each found SynMemberDefn.Member.
However a blank line was found in the trivia and is also printed.

So you end up with an extra newline. We have encountered this issue on other places as well.
The fix is to use the sepNlnConsideringTriviaContentBeforeWithAttributes function instead of sepNln. I would need to debug to see where exactly this should be done...

[theimowski]

I started looking at that, but I'm completely new to the code so I can hardly understand anything yet.

What I found is that there are two pattern matches on MultilineMemberDefnL in CodePrinter:

• MultilineMemberDefnL(xs, [])
• MultilineMemberDefnL(xs, ys)

and for my above snippet I end up in the later one, while if I remove MemberC then I end up in the first one and the extra newline is not added

There are some differences in implementation between those two and I'm trying get my head around those

[theimowski]

Ok I think I got it - see PR #584