Strict mode strips literal strings

[Smaug123]

Issue created from fantomas-ui

Strict mode causes the @ to be stripped from strings, possibly causing code to stop compiling.

Code

let s = @"\"

Error

let s = "\"

Options

Fantomas Next - 3.0.1-11/13/2019

Name	Value
IndentOnTryWith	false
IndentSpaceNum	4
KeepNewlineAfter	false
PageWidth	120
ReorderOpenDeclaration	false
SemicolonAtEndOfLine	false
SpaceAfterComma	false
SpaceAfterSemicolon	false
SpaceAroundDelimiter	false
SpaceBeforeArgument	false
SpaceBeforeColon	false
StrictMode	true

[nojaf]

Hello @Smaug123 ,

Strict mode means it will only use the information found in the UnTyped AST to print out the formatted code.

The AST, in this case, looks like this:

ImplFile
  (ParsedImplFileInput
     ("Script.fsx",true,QualifiedNameOfFile Script$fsx,[],[],
      [SynModuleOrNamespace
         ([Script],false,AnonModule,
          [Let
             (false,
              [Binding
                 (None,NormalBinding,false,false,[],
                  PreXmlDoc ((1,5),FSharp.Compiler.Ast+XmlDocCollector),
                  SynValData
                    (None,SynValInfo ([],SynArgInfo ([],false,None)),None),
                  Named
                    (Wild Script.fsx (1,4--1,5) IsSynthetic=false,s,false,None,
                     Script.fsx (1,4--1,5) IsSynthetic=false),None,
                  Const
                    (String ("\",Script.fsx (1,8--1,12) IsSynthetic=false),
                     Script.fsx (1,8--1,12) IsSynthetic=false),
                  Script.fsx (1,4--1,5) IsSynthetic=false,
                  SequencePointAtBinding
                    Script.fsx (1,0--1,12) IsSynthetic=false)],
              Script.fsx (1,0--1,12) IsSynthetic=false)],PreXmlDocEmpty,[],None,
          Script.fsx (1,0--1,12) IsSynthetic=false)],(true, true)))

The @ sign is not part of Const(String ("\",Script.fsx (1,8--1,12) IsSynthetic=false), so it is inpossible to know without looking at the original source code.

This is a known limitation and your best bet to resolve this is to open an issue at the dotnet/fsharp repo.

[Smaug123]

I'm not convinced that Fantomas should just pass this off to the F# compiler. Isn't the "\" that you quoted just an unescaped string? It's possible to format an unescaped string canonically, by printing it with an @ in front of it and duplicating any double-quotes inside it, if I understand correctly.

[nojaf]

@Smaug123 as we currently don't have this issue in a non-strict mode, I made a suggestion to store this information in the Syntax tree compiler dotnet/fsharp#10209.

Yes, Fantomas could try and detect this based on the content of the string but the compiler at some point has this information, so it seems like a good case to get my feet wet in extended the compiler itself.

[Smaug123]

Good idea - this is certainly not a high-priority ticket, we don't expect to be using strict mode.

[nojaf]

Following up on this, with a lot of help of Chet, we were able to submit dotnet/fsharp#10769.
This could provide the missing information in the AST.

[nojaf]

FCS 40, has what we need for this. Looking forward!