Fantomas introduces meaningless match block

[Smaug123]

Issue created from fantomas-online

Code

module Foo =
    let bar () =
        {
            Foo =
                blah
                |> Struct.map (fun _ (a, _, _) -> filterBackings a)
        }

Result

module Foo =
    let bar () =
        { Foo =
              blah
              |> Struct.map
                  (fun _ (a, _, _) ->
                      match _arg2 with
                      | _ ->
                          match _arg2 with
                          | _ -> filterBackings a) }

Problem description

I honestly have not the faintest idea how this could have happened, but anyway, the additional match block is not meant to be there.

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 Master at 07/01/2021 18:56:12 - 3a12872

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?}

[auduchinok]

The parser does some desugaring for lambdas, and these nested matches are what the type checker is actually dealing with later.

@nojaf I've added the originally parsed lambdas to the tree in dotnet/fsharp#10166 to workaround this problem in our plugin, you can try using it too.

[nojaf]

Yep, that is exactly what I'm using and has been really helpful.
The challenge here seems to be finding the correct body expression.

fantomas/src/Fantomas/SourceParser.fs

Lines 1250 to 1269 in 3a12872

	let (|Lambda|_|) =
	function
	| SynExpr.Lambda (_, _, _, _, Some (pats, body), range) ->
	let maxDepth =
	match pats with
	| [ PatParen (PatTuple ts) ] -> List.length ts
	| _ -> List.length pats
	// find the body expression from the last lambda
	let rec visit (currentDepth: int) (e: SynExpr) : SynExpr =
	if currentDepth < maxDepth then
	match e with
	| SynExpr.Match (matchSeqPoint = NoDebugPointAtInvisibleBinding; clauses = [ Clause (_, expr, _) ])
	| SynExpr.Lambda (_, _, _, SynExpr.Match(clauses = [ Clause (_, expr, _) ]), _, _) ->
	visit (currentDepth + 1) expr
	| _ -> e
	else
	e
	Some(pats, visit 0 body, range)
	| _ -> None

[auduchinok]

I might've added some incorrect range or expression in the PR, since I've found I had also added a workaround for getting the body, see skipGeneratedMatch here. We should probably fix it in FCS. :)

[nojaf]

Many thanks for the pointer there. I'll check if I can fix this in FCS sometime.