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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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40 changes: 40 additions & 0 deletions docs/fsharp/language-reference/compiler-directives.md
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Expand Up @@ -52,6 +52,46 @@ let str = "Debugging!"
#endif
```

### Automatically Defined Symbols

In addition to symbols explicitly defined via project settings or command-line options, **F# projects inherit a set of predefined symbols** based on the SDK, target framework, and build configuration.

#### Debug, Tracing, and Nullable Settings

| Symbol | Description |
|-----------|-------------|
| `DEBUG` | Defined in debug builds when compiling with the `Debug` configuration. |
| `TRACE` | Defined when tracing is enabled in the project settings. |
| `NULLABLE` | Defined when `<Nullable>enable</Nullable>` is set in the project file. |

#### Target Framework Symbols
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I am sorry for the misunderstanding.
Could those be instead solved by a link to the specific SDK documentation?

This is not being defined by F# and it does naturally evolve. If we keep a duplicate here, it would also require maintenace.

I think linking this very page https://learn.microsoft.com/en-us/dotnet/standard/frameworks#preprocessor-symbols would be best when it comes to completeness (see how many there are overall)


| Symbol | Description |
|--------|-------------|
| `NET`, `NETSTANDARD`, `NETCOREAPP` | General indicators of the .NET target type. |
| `NET5_0_OR_GREATER`, `NET6_0_OR_GREATER`, `NET7_0_OR_GREATER` | Defined for specific .NET versions. |
| `NETCOREAPP3_1_OR_GREATER`, `NETCOREAPP2_0_OR_GREATER` | Defined for .NET Core versions. |

#### Build Configuration Symbols

| Symbol | Description |
|-----------|-------------|
| `Release` | Defined in release builds. |
| `COMPILED` | Used in compiled projects. |

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

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

For a full list of predefined symbols, refer to the official documentation:
[.NET SDK Predefined Compilation Symbols](https://learn.microsoft.com/dotnet/fsharp/tools/fsharp-interactive/#interactive-and-compiled-preprocessor-directives).

## NULLABLE directive

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