Up

docs/fsharp/language-reference/active-patterns.md

@@ -181,7 +181,7 @@ The attribute must be specified, because the use of a struct return is not infer
 
 In F# 9, nullability related active patterns were added.
 
-The first one is `| Null | NonNull x |`, which is a recommended way to handle possible nulls. In the following example, parameter `s` is inferred nullable via this active pattern usage: 
+The first one is `| Null | NonNull x |`, which is a recommended way to handle possible nulls. In the following example, parameter `s` is inferred nullable via this active pattern usage:
 
 ```fsharp
  let len s =

docs/fsharp/language-reference/compiler-directives.md

@@ -52,6 +52,31 @@ let str = "Debugging!"
 #endif
 ```
 
+## NULLABLE directive
+
+Starting with F# 9, you can enable nullable reference types in the project:
+
+```xml
+<Nullable>enable</Nullable>
+```
+
+This automatically sets `define:NULLABLE` directive to the build. It's useful while initially rolling out the feature, to conditionally change conflicting code by `#if NULLABLE` hash directives:
+
+```fsharp
+#if NULLABLE 
+let checkNonNull argName (arg: obj) =
+    match arg with
+    // nullness warning: the type 'obj' does not support 'null'
+    | null -> nullArg argName
+    | _ -> ()
+#else
+let checkNonNull argName (arg: obj) =
+    match arg with
+    | null -> nullArg argName
+    | _ -> ()
+#endif
+```
+
 ## Line Directives
 
 When building, the compiler reports errors in F# code by referencing line numbers on which each error occurs. These line numbers start at 1 for the first line in a file. However, if you are generating F# source code from another tool, the line numbers in the generated code are generally not of interest, because the errors in the generated F# code most likely arise from another source. The `#line` directive provides a way for authors of tools that generate F# source code to pass information about the original line numbers and source files to the generated F# code.

docs/fsharp/language-reference/compiler-options.md

@@ -18,6 +18,7 @@ The following table shows compiler options listed alphabetically. Some of the F#
 |`--allsigs`|Generates a new (or regenerates an existing) signature file for each source file in the compilation. For more information about signature files, see [Signatures](signature-files.md).|
 |`-a filename.fs`|Generates a library from the specified file. This option is a short form of `--target:library filename.fs`.|
 |`--baseaddress:address`|Specifies the preferred base address at which to load a DLL.<br /><br />This compiler option is equivalent to the C# compiler option of the same name. For more information, see [&#47;baseaddress &#40;C&#35; Compiler Options&#41;](../../csharp/language-reference/compiler-options/advanced.md#baseaddress).|
+|<code>--checknulls[+&#124;-]</code>|Enables [nullable reference types](./values/null-values.md#null-values-starting-with-f-9), added in F# 9.|
 |`--codepage:id`|Specifies which code page to use during compilation if the required page isn't the current default code page for the system.<br /><br />This compiler option is equivalent to the C# compiler option of the same name. For more information, see [&#47;code pages &#40;C&#35; Compiler Options&#41;](../../csharp/language-reference/compiler-options/advanced.md#codepage).|
 |`--consolecolors`|Specifies that errors and warnings use color-coded text on the console.|
 |`--crossoptimize[+ or -]`|Enables or disables cross-module optimizations.|

docs/fsharp/language-reference/fsharp-interactive-options.md

@@ -27,6 +27,7 @@ Where lists appear in F# Interactive option arguments, list elements are separat
 |------|-----------|
 |**--**|Used to instruct F# Interactive to treat remaining arguments as command-line arguments to the F# program or script, which you can access in code by using the list **fsi.CommandLineArgs**.|
 |**--checked**[**+**|**-**]|Same as the **fsc.exe** compiler option. For more information, see [Compiler Options](compiler-options.md).|
+|**--checknulls**[**+**|**-**]|Same as the **fsc.exe** compiler option. For more information, see [Compiler Options](compiler-options.md).|
 |**--codepage:<int>**|Same as the **fsc.exe** compiler option. For more information, see [Compiler Options](compiler-options.md).|
 |**--consolecolors**[**+**|**-**]|Outputs warning and error messages in color.|
 |**--compilertool:<extensionsfolder>|Reference an assembly or directory containing a design time tool (Short form: -t).|

docs/fsharp/language-reference/values/null-values.md

@@ -39,6 +39,8 @@ In F# 9, extra capabilities are added to the language to deal with reference typ
 <Nullable>enable</Nullable>
 ```
 
+This passes the `--checknulls+` [flag](../compiler-options.md) to the F# compiler and sets a `define:NULLABLE` [preprocessor directive](../compiler-directives.md#nullable-directive) for the build.
+
 To explicitly opt-in into nullability, a type declaration has to be suffixed with the new syntax:
 
 ```fsharp