The missing comment (#2491)

* Debugging CodePrinter.

* The Missing Comment

* Apply suggestions from code review

Co-authored-by: dawe <dawedawe@posteo.de>

Co-authored-by: dawe <dawedawe@posteo.de>

docs/docs/contributors/How Can I Contribute.md

@@ -192,4 +192,4 @@ This update to the tree made it very straightforward to fix the original bug in
 
 Another example of code that could benefit from a better representation is [extern](https://github.com/fsprojects/fantomas/issues?q=is%3Aissue+is%3Aopen+extern).
 
-<fantomas-nav previous="./Conditional%20Compilation%20Directives.html"></fantomas-nav>
+<fantomas-nav previous="./Conditional%20Compilation%20Directives.html" next="./The%20Missing%20Comment.html"></fantomas-nav>

docs/docs/contributors/Print AST with Context.md

@@ -68,4 +68,23 @@ Instead we can use various helper functions that take the `Context` as parameter
 Please take a moment to debug the unit tests in [CodePrinterHelperFunctionsTests.fs](https://github.com/fsprojects/fantomas/blob/master/src/Fantomas.Core.Tests/CodePrinterHelperFunctionsTests.fs).  
 This will give you a better understanding of how we capture events in `CodePrinter`.
 
+### Debugging CodePrinter
+
+One thing that is a bit harder to grasp initially, is what is happening when you put a breakpoint in `CodePrinter.fs`.  
+In `CodePrinter.fs` we compose a format function that takes a `Context` and returns a `Context`.
+We do this by traversing the syntax tree, and when you put a breakpoint in `genTypeDefn` for example:
+
+![Breakpoint in genTypeDefn](../../images/debugging-code-printer-1.png)
+
+we are still in the process of composing the format function.  
+**The Context has not been going through our format function yet!**
+
+If we want to debug when the `Context` is traveling through the format function, we can easily, temporarily, insert an additional function to inspect the `Context` content:
+
+![Breakpoint in Context -> Context](../../images/debugging-code-printer-2.png)
+
+The `dumpAndContinue` helper function can be used to inspect the `Context`.  
+Please remove all usages when submitting a PR 😸.
+
+
 <fantomas-nav previous="./Prepare%20Context.html" next="./Formatted%20Code.html"></fantomas-nav>

docs/docs/contributors/The Missing Comment.md

@@ -0,0 +1,124 @@
+---
+category: Contributors
+categoryindex: 2
+index: 10
+---
+# The Missing Comment
+
+Code comments can literally exist between every single F# token. I'm looking at you block comment `(* ... *)`.  
+As explained in [Detecting trivia](./Prepare%20Context.html#Detecting-trivia), we need to do quite some processing to restore code comments.  
+In this guide, we would like to give you seven tips to restore a missing comment!
+
+## Breathe
+
+We understand it very well that losing a code comment after formatting can be extremely frustrating.  
+There is no easy fix that will solve all the missing comments overnight. Each case is very individual and can be complex to solve.  
+May these steps help towards fixing your problem!
+
+## Isolate the problem
+
+Before we can commence our murder mystery, it is best to narrow down our problem space.
+
+Example ([#2490](https://github.com/fsprojects/fantomas/issues/2490)):
+
+```fsharp
+let (* this comment disappears after formatting *) a = []
+
+type A = 
+    {
+        X : int
+        ...
+    }
+
+while true do ()
+```
+
+Using the [online tool](https://fsprojects.github.io/fantomas-tools/#/fantomas/preview), we can remove any code that isn't relevant.
+The `type` and `while` code can be trimmed and the problem still exists.
+
+## Check the syntax tree
+
+Every code comment should be present on the root level of the syntax tree.
+[ParsedImplFileInputTrivia](../../reference/fsharp-compiler-syntaxtrivia-parsedimplfileinputtrivia.html) or [ParsedSigFileInputTrivia](../../reference/fsharp-compiler-syntaxtrivia-parsedsigfileinputtrivia.html) should contain the comment.
+
+```fsharp
+ImplFile
+  (ParsedImplFileInput
+     ("tmp.fsx", true, QualifiedNameOfFile Tmp$fsx, [], [],
+      [ ... ], (false, false),
+      { ConditionalDirectives = []
+        CodeComments = [BlockComment tmp.fsx (1,4--1,50)] }))
+```
+
+If the comment is not there this means the F# lexer and parser didn't pick up the comment. In the unlikely event this happened, this should be fixed first over at [dotnet/fsharp](https://github.com/dotnet/fsharp).
+
+## Was the comment detected as trivia?
+
+Fantomas grabs the comments straight from the syntax tree, and transforms them as `Trivia`.  
+This is a fairly straightforward process, and you can easily visually inspect this using the [online tool](https://fsprojects.github.io/fantomas-tools/#/trivia).
+
+![Trivia in online tool](../../images/online-tool-trivia-1.png)
+
+If your comment does not show up there, it means there is a problem between getting the information from the syntax tree and constructing the `Trivia` in `Trivia.fs`.
+
+## Was the trivia assigned to a trivia node?
+
+The `Trivia` needs to be assigned to a `TriviaNode` to be stored in the `Context`.  
+Using the center tab in the tool, we can see the tree structure of all available `TriviaNodes`.
+
+![Root node in online tool](../../images/online-tool-trivia-2.png)
+
+Choosing the best suitable node can be quite tricky, there are different strategies for different `TriviaTypes`.  
+In this example `SynBindingKind_Normal` and `SynPat_Named` are good candidates as they appear right after the comment.  
+Once the `Trivia` is assigned to a `TriviaNode`, a `TriviaInstruction` will be created. This is the link between them and is what will be stored in the `Context`.
+
+The left-most tab in the tool shows us the result of the assigment:
+
+![Trivia Instructions in online tool](../../images/online-tool-trivia-3.png)
+
+In this example, at the time of writing, there are no `TriviaInstructions`. This means the assignment in `Trivia.fs` failed and we would need to investigate why.
+
+## Was the trivia assigned to the best possible node?
+
+Sometimes the selected `TriviaNode` isn't really the best possible candidate.
+In [#640](https://github.com/fsprojects/fantomas/issues/640), the `Directive` trivia are assigned to `SynBindingKind_Normal`.
+
+![Wrong node assignment in online tool](../../images/online-tool-trivia-4.png)
+
+Notice that `SynBindingKind_Normal` starts at line `6`, while the `Trivia` is wrapped around the `inline` keyword on line `4`.  
+It would be ideal if we could use the `inline` keyword as a `TriviaNode`, but looking at the tree, it doesn't appear to be present.
+Why is that? Well, the syntax tree does, at the time of writing, not provide a `range` for the keyword.  
+This would be a great addition as to [SyntaxTreeTrivia](../../reference/fsharp-compiler-syntaxtrivia.html), and can be done by submitting a pull request to [dotnet/fsharp](https://github.com/dotnet/fsharp).
+
+## Printing the TriviaInstruction
+
+The last piece of the puzzle is printing the actual `TriviaInstruction` in `CodePrinter`.
+If everything up to this point went well, and the comment is still missing after formatting, it means it was not printed.
+
+Every `TriviaInstruction` has a type (`FsAstType`) and a `range`. The `range` is taken from an actual AST node, so when we encounter this `range` in `CodePrinter`, we need to act upon it.  
+We typically do this by calling `genTriviaFor`, passing down the `type`, the `range` and a function `f` that should be composed in between.
+
+Example:
+
+```fsharp
+genTriviaFor Ident_ (* : FsAstType *) ident.idRange (* range : *) genIdent (* : Context -> Context *) 
+```
+
+If we look at the definition of `genTriviaFor`:
+
+```fsharp
+and genTriviaFor (mainNodeName: FsAstType) (range: Range) f ctx =
+    (enterNodeFor mainNodeName range +> f +> leaveNodeFor mainNodeName range) ctx
+```
+
+We can derive that the composition will be as follows:
+
+```fsharp
+enterNodeFor Ident_ ident.idRange
++> genIdent
++> leaveNodeFor Ident_ ident.idRange
+```
+
+`enterNodeFor` and `leaveNodeFor` will print the `TriviaInstruction` it finds for the `range` and `type` inputs in the `Context`.
+
+<fantomas-nav previous="./How%20Can%20I%20Contribute.html"></fantomas-nav>