Line of code including a backward composition operator causes error

[thomasorton-i2o]

The following line within a block of code causes Fantomas to throw an error:

(SomeModule.doSomething << SomeModule.doSomethingElse) (fun x -> x)

The error starts with "System.Exception: cannot determine if synExpr Paren". I have reproduced it in the in the Fantomas online tool.

(I tried to use the "Create an issue" button but I got a GitHub "Your request URL is too long. " error.)

[nojaf]

Hello, thank you for reporting this issue.

I believe the problem is occurring around:

fantomas/src/Fantomas/CodePrinter.fs

Lines 2031 to 2039 in 2c6c967

	// functionName arg1 arg2 (fun x y z -> ...)
	| AppWithLambda (e, es, lpr, lambda, rpr, pr) ->
	let sepSpaceAfterFunctionName ctx =
	match List.tryHead es with
	| Some (SimpleExpr _) -> sepSpace ctx
	| _ ->
	match e with
	| UppercaseSynExpr -> onlyIf ctx.Config.SpaceBeforeUppercaseInvocation sepSpace ctx
	| LowercaseSynExpr -> onlyIf ctx.Config.SpaceBeforeLowercaseInvocation sepSpace ctx

We try to see if the (fun x -> x) is uppercase or lowercase. I think in this case, we should just add a space between both the function expression and the argument expression have parenthesis.

A potential fix might be something like:

            let sepSpaceAfterFunctionName ctx =
                match e with
                | Paren _ -> sepSpace ctx
                | _ ->
                    match List.tryHead es with
                    | Some (SimpleExpr _) -> sepSpace ctx
                    | _ ->
                        match e with
                        | UppercaseSynExpr -> onlyIf ctx.Config.SpaceBeforeUppercaseInvocation sepSpace ctx
                        | LowercaseSynExpr -> onlyIf ctx.Config.SpaceBeforeLowercaseInvocation sepSpace ctx

Are you interested in submitting a PR for this?

[thomasorton-i2o]

Hello,

No problem. I would be happy to submit a PR, I'm just a bit busy at the moment. I will try to submit one tomorrow if that is OK.

[nojaf]

Sure thing, take your time. Thanks!
Please review our Contribution Guidelines and don't hesitate to ask any questions you might have.
Good luck!

[nojaf]

I believe this issue was resolved by #2036.
To close this issue, we can add a regression test to be sure.

[knocte]

This can be closed now after the merge of #2060 FYI