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update compiler directives page of FSharp reference #47130

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19badc5
update for RFC FS-1146 (scoped nowarn)
Martin521 Jul 8, 2025
102795d
Update docs/fsharp/language-reference/compiler-directives.md
Martin521 Jul 8, 2025
c6983fe
removed empty line (for the linter)
Martin521 Jul 8, 2025
8c9305c
more details on warn directives
Martin521 Jul 10, 2025
905f918
fixed link
Martin521 Jul 10, 2025
70b835e
fixed the fix
Martin521 Jul 10, 2025
d3fbed5
sorry, yet another correction
Martin521 Jul 10, 2025
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40 changes: 24 additions & 16 deletions docs/fsharp/language-reference/compiler-directives.md
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|Directive|Description|
|---------|-----------|
|`#if` *symbol*|Supports conditional compilation. Code in the section after the `#if` is included if the *symbol* is defined. The symbol can also be negated with `!`.|
|`#else`|Supports conditional compilation. Marks a section of code to include if the symbol used with the previous `#if` is not defined.|
|`#if` *if-expression*|Supports conditional compilation. Code in the section after the `#if` is included if the *if-eyxpression* evaluates to `defined` (see below).|
|`#else`|Supports conditional compilation. Marks a section of code to include if the symbol used with the previous `#if` does not evaluate to `defined`.|
|`#endif`|Supports conditional compilation. Marks the end of a conditional section of code.|
|`#`[line] *int*,<br/>`#`[line] *int* *string*,<br/>`#`[line] *int* *verbatim-string*|Indicates the original source code line and file name, for debugging. This feature is provided for tools that generate F# source code.|
|`#nowarn` *warningcode*|Disables a compiler warning or warnings. To disable multiple warning numbers on the same line, separate each string by a space. <br/> For example: `#nowarn 9 42`|
|`#nowarn` *warningcode*|Disables a compiler warning or warnings. To disable multiple warning numbers on the same line, separate each string by a space. <br/> For example: `#nowarn 9 42`<br/> The warning is disabled until eof or until a `#warnon` directive for that same warning number is foud.|
|`#warnon` *warningcode*|Enables a compiler warning (or warnings) that was disabled by a compiler option or by a `#nowarn` directive..<br/> The warning is enabled until eof or until a `#nowarn` directive for that same warning number is found.|
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Do we want to list also the #nowarn "42" option (with quotes), since it is what most codebases use?

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Maybe include pseudogrammar:

#nowarn (int list | string list) , where strings are without the FS prefix?

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I guess I will add a whole section then with more details. I will come back to it.

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I added a more detailed section on the warn directives.

In this context I also removed the redundant term "preprocessor directives" from the page. In the F# spec, the term "preprocessor directive" is used only for what in this document is called "conditional compilation directives". Also, the one and only compiler has no separate preprocessor, so "compiler directives" seems to be the right general term.


The effect of disabling a warning applies to the entire file, including portions of the file that precede the directive.|

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docs/fsharp/language-reference/compiler-directives.md:29 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2] https://github.com/DavidAnson/markdownlint/blob/v0.38.0/doc/md012.md
## Conditional Compilation Directives

Code that is deactivated by one of these directives appears dimmed in the Visual Studio Code Editor.

> [!NOTE]
> The behavior of the conditional compilation directives is not the same as it is in other languages. For example, you cannot use Boolean expressions involving symbols, and `true` and `false` have no special meaning. Symbols that you use in the `if` directive must be defined by the command line or in the project settings; there is no `define` preprocessor directive.

The following code illustrates the use of the `#if`, `#else`, and `#endif` directives. In this example, the code contains two versions of the definition of `function1`. When `VERSION1` is defined by using the [-define compiler option](./compiler-options.md), the code between the `#if` directive and the `#else` directive is activated. Otherwise, the code between `#else` and `#endif` is activated.

[!code-fsharp[Main](~/samples/snippets/fsharp/lang-ref-2/snippet7301.fs)]

There is no `#define` preprocessor directive in F#. You must use the compiler option or project settings to define the symbols used by the `#if` directive.

Conditional compilation directives can be nested. Indentation is not significant for preprocessor directives.

You can also negate a symbol with `!`. In this example, a string's value is something only when _not_ debugging:
The `#if` directive also accepts logical expressions:

```fsharp
#if !DEBUG
let str = "Not debugging!"
#else
let str = "Debugging!"
#if SILVERLIGHT || COMPILED && (NETCOREFX || !DEBUG)
#endif
```

The following expressions can be used.

| if-expr | evaluation |
| --- | --- |
| `if-expr1 \|\| if-expr2` | `defined` if `if-expr1` or `if-expr2` is `defined`. |
| `if-expr1 && if-expr2` | `defined` if `if-expr1` and `if-expr2` are `defined`. |
| `!if-expr1` | `defined` if `if-expr1` is not `defined`. |
| `( if-expr1 )` | defined if `if-expr1` is defined. |
| `symbol` | `defined` if it is flagged as defined by the `-define` compiler option. |

The logical operators have the usual logical precedence.

There is no `#define` preprocessor directive in F#. You must use the compiler option or project settings to define the symbols used by the `#if` directive.

Conditional compilation directives can be nested. Indentation is not significant for preprocessor directives.

## NULLABLE directive

Starting with F# 9, you can enable nullable reference types in the project:
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These tokens indicate that the F# code generated at this location is derived from some constructs at or near line `25` in `Script1`.

Note that `#line` directives do not influence the behavior of `#nowarn` / `#warnon`. These two directives always relate the the file that is being compiled.

## See also

- [F# Language Reference](index.md)
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