Unexpected newline added before let bang

[nojaf]

Issue created from fantomas-online

Code

                promise {
                    setItems [||]
                    setSelectedItem None
                    setFetchingItems true
                    let! items = Api.fetchItems partNumber
                    setItems items
                    items |> Array.tryHead |> setSelectedItem
                    setFetchingItems false

                    Seq.tryHead items
                    |> Option.iter (Some >> setSelectedItem)
                }
                |> Promise.start

Result

promise {
    setItems [||]
    setSelectedItem None
    setFetchingItems true

    let! items = Api.fetchItems partNumber
    setItems items
    items |> Array.tryHead |> setSelectedItem
    setFetchingItems false

    Seq.tryHead items
    |> Option.iter (Some >> setSelectedItem)
}
|> Promise.start

Problem description

There should not be a newline added by let! items = .

Extra information

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

Options

Fantomas 4.6 branch at 10/27/2021 20:15:01 - 95833f5

Default Fantomas configuration

_{Did you know that you can ignore files when formatting from fantomas-tool or the FAKE targets by using a .fantomasignore file?}

[knocte]

AFAIU this should be marked as bug(stylistic) instead of bug(soundness)?

[ribeirotomas1904]

I will try to solve this one

[nojaf]

Hello @ribeirotomas1904, thank you for your interest.

This one is a bit trickier to solve, I believe a first step might be to revisit:

fantomas/src/Fantomas/SourceParser.fs

Lines 977 to 984 in b742103

	let rec (|CompExprBody|_|) expr =
	match expr with
	| SynExpr.LetOrUse (_, _, _, CompExprBody _, _)
	| SynExpr.LetOrUseBang _
	| SynExpr.Sequential (_, _, _, SynExpr.YieldOrReturn _, _)
	| SynExpr.Sequential (_, _, _, SynExpr.LetOrUse _, _)
	| SynExpr.Sequential (_, _, _, SynExpr.LetOrUseBang _, _) -> Some(collectComputationExpressionStatements expr id)
	| _ -> None

I believe this active pattern is a bit too strict and should be more something like:

let rec (|CompExprBody|_|) expr =
    match expr with
    | SynExpr.LetOrUse (_, _, _, CompExprBody _, _)
    | SynExpr.LetOrUseBang _
    | SynExpr.Sequential _ -> Some(collectComputationExpressionStatements expr id)
    | _ -> None

Because of this change, you might need to update

fantomas/src/Fantomas/CodePrinter.fs

Lines 1633 to 1643 in b742103

	| Paren (lpr, e, rpr, _pr) ->
	match e with
	| LetOrUses _
	| Sequential (_, LetOrUses _, _) ->
	sepOpenTFor lpr
	+> atCurrentColumn (genExpr astContext e)
	+> sepCloseTFor rpr
	| _ ->
	sepOpenTFor lpr
	+> genExpr astContext e
	+> sepCloseTFor rpr

accordingly to:

        | Paren (lpr, e, rpr, _pr) ->
            match e with
            | LetOrUses _
            | Sequential _ ->
                sepOpenTFor lpr
                +> atCurrentColumn (genExpr astContext e)
                +> sepCloseTFor rpr
            | _ ->
                sepOpenTFor lpr
                +> genExpr astContext e
                +> sepCloseTFor rpr

I hope this helps.