Multiline type parameter arguments inside indentation

[piaste]

Issue created from fantomas-online

Code

module PoorlyIndented = 

 let findThing dependency thingId = 
  use cmd = 
   query SomeDatabase.CreateCommand<"
                    select name
                    from things
                    where id = :id
   "> dependency

  cmd.AsyncExecute(id = thingId)

Result

module PoorlyIndented =

    let findThing dependency thingId =
        use cmd =
            query
                SomeDatabase.CreateCommand<"
                    select name
                    from things
                    where id = :id
   ">
                dependency

        cmd.AsyncExecute(id = thingId)

Problem description

This one is tricky and very, very niche. The sample code above is how we often write one-off queries in our functions. The query function is a helper that lets us pass type-safe database connection and/or transactions instead of raw strings, and the first argument is the type-provider-generated command.

Because the argument has a multi-line string literal as a type parameter, the angle brackets need to end within the indentation rules for the containing declaration, or the F# compiler gets confused and doesn't compile anymore.

So now let's say the whole code tree is insufficiently indented (this actually happened). After Fantomas applies the wider indentation, the indentation requirement for the parameter increases, and the code breaks.

When we edit this code manually, we add the right amount of whitespace at the end of the query string, since placing the entire "> in the right place looks better. Fantomas, of course, should never change the content of a string literal, so what it should do is add space between the " and the >, like this:

module PoorlyIndented =

    let findThing dependency thingId =
        use cmd =
            query
                SomeDatabase.CreateCommand<"
                    select name
                    from things
                    where id = :id
   "            >
                dependency

        cmd.AsyncExecute(id = thingId)

which is ugly but will satisfy the compiler and won't change the code semantics.

Extra information

• The formatted result breaks my code.
• The formatted result gives compiler warnings.
• I or my company would be willing to help fix this.

Options

Fantomas Master at 04/05/2021 16:09:34 - 31922c0

Default Fantomas configuration

[nojaf]

Hello @piaste, thanks for reporting this issue!
This is quite the challenge at first glance.
I believe the function indentIfNeeded can be used to get around this problem.

fantomas/src/Fantomas/Context.fs

Line 774 in 31922c0

let internal indentIfNeeded f (ctx: Context) =

Add it to genGenericTypeParameters

fantomas/src/Fantomas/CodePrinter.fs

Lines 2878 to 2891 in 31922c0

	and genGenericTypeParameters astContext ts =
	match ts with
	| [] -> sepNone
	| ts ->
	!- "<"
	+> coli
	sepComma
	ts
	(fun idx ->
	genType
	{ astContext with
	IsFirstTypeParam = idx = 0 }
	false)
	-- ">"

so it becomes something like:

        !- "<"
        +> coli
            sepComma
            ts
            (fun idx ->
                genType
                    { astContext with
                          IsFirstTypeParam = idx = 0 }
                    false)
        +> indentIfNeeded sepNone
        -- ">"

the closing > won't really align with anything, but it will be indented far enough to make it compile.

Are you interested in submitting a PR for this?

[piaste]

Are you interested in submitting a PR for this?

Yes, that should be enough guidance to get me started, thank you 🙂 I'll take a crack at it this weekend.