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Document Automatically Defined Compiler Symbols in F# Compiler Directives #45607

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Document Automatically Defined Compiler Symbols in F# Compiler Directives #45607

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37e75ba
Fixed bug 45478.
Apr 2, 2025
1818a9c
More updates.
Apr 3, 2025
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21 changes: 21 additions & 0 deletions docs/fsharp/language-reference/compiler-directives.md
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Expand Up @@ -52,6 +52,27 @@ let str = "Debugging!"
#endif
```

### Automatically Defined Symbols

The F# compiler automatically defines certain symbols based on the build configuration. These symbols can be used with `#if` directives for conditional compilation.

| Symbol | Description |
|---------|-------------|
| `DEBUG` | Automatically defined in debug builds when compiling with the `Debug` configuration. |
| `TRACE` | Defined when tracing is enabled in the project settings. |

For example, you can use these symbols in conditional compilation:

```fsharp
#if DEBUG
printfn "Debugging mode is enabled."
#else
printfn "Release mode."
#endif
```

These symbols are typically set by the compiler and do not require explicit definition in the source code.

## NULLABLE directive

Starting with F# 9, you can enable nullable reference types in the project:
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