\t in string replaced by ASCII 9

[stanciuadrian]

Issue created from fantomas-online

Code

printfn "Hello\tWorld"

Result

printfn "Hello	World"

Problem description

The string is not recognized as trivia so the string from the AST is printed back out (thx @nojaf for quick analysis).
Thus, the \t in the string is replaced by ASCII 9 (TAB).

via ionide/ionide-vscode-fsharp#1534

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 05/01/2021 14:37:19 - d8e76d4

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?}

Edit: credits

[nojaf]

I believe extending this regex with t with resolve the problem:

fantomas/src/Fantomas/TokenParser.fs

Lines 532 to 533 in d8e76d4

	let escapedCharacterRegex =
	System.Text.RegularExpressions.Regex("(\\\\(a|b|f|n|r|u|v|x|'|\\\"|\\\\))+")

Are you interested in submitting a PR?

[stanciuadrian]

Yes.

[nojaf]

Great, please read our Contribution Guidelines. You can add you unit test to SynConstTests.fs.
Thank you for your investment.

[stanciuadrian]

I went with idiomatic TDD on this one.
After updating the hex escape in string literal should be preserved, 1508 test by adding a \t to the input string the test passed.

So it seems the current code (with no change to escapedCharacterRegex) works fine when formatting "\a\t" but fails with "\t".

Updating escapedCharacterRegex as suggested fixes the issue but there may be a lurking bug around.