Formatting power operator in code quotation pattern match fails

[cgravill]

Issue created from fantomas-online

Code

open System
open FSharp.Quotations
open FSharp.Quotations.DerivedPatterns

let expr : Expr<float> = <@ 1.4 / 4.4 @>

match expr with
| SpecificCall <@@ ( / ) @@> (_, _, [s1; s2]) ->    
    printfn "Div"
| SpecificCall <@@ ( ** ) @@> (_, _, [s1; s2]) ->
    printfn "Pow"
| _ ->
    printfn "No match"

[<EntryPoint>]
let main argv =
    printfn "Hello world"
    0 // return an integer exit code

Result

open System
open FSharp.Quotations
open FSharp.Quotations.DerivedPatterns

let expr: Expr<float> = <@ 1.4 / 4.4 @>

match expr with
| SpecificCall <@@ (/) @@> (_, _, [ s1; s2 ]) -> printfn "Div"
| SpecificCall <@@ (**) @@> (_, _, [ s1; s2 ]) -> printfn "Pow"
| _ -> printfn "No match"

[<EntryPoint>]
let main argv =
    printfn "Hello world"
    0 // return an integer exit code

Problem description

Taken from a larger codebase where we're doing code transformations. The power operator ** is getting formatted such that it breaks the code - it looks like it's become a code comment which may explain the error.

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 11/01/2021 19:30:14 - c02b8d1

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?}

[nojaf]

Hey Colin! Thanks for reporting this issue.

I believe the problem might be centred around

fantomas/src/Fantomas/SourceParser.fs

Lines 136 to 155 in 735d58e

	let (|OpNameFullInPattern|) (x: Identifier) =
	let r = x.Ranges
	let s = x.Text
	let s' = DecompileOpName s
	(if IsActivePatternName s
	|| IsInfixOperator s
	|| IsPrefixOperator s
	|| IsTernaryOperator s
	|| s = "op_Dynamic" then
	/// Use two spaces for symmetry
	if String.startsWithOrdinal "*" s' && s' <> "*" then
	sprintf "( %s )" s'
	else
	sprintf "(%s)" s'
	else
	match x with
	| Id (Ident s)
	| LongId (LongIdent s) -> DecompileOpName s)
	|> fun s -> (s, r)

Are you interested in submitting a PR?

[cgravill]

I'd be happy to have a go. It looks like adding a special case then leaving extra space would work.

[nojaf]

Yup, I was thinking along those lines as well.