Create comprehensive F# 8 What's New documentation from blog post

docs/fsharp/whats-new/fsharp-8.md

@@ -3,7 +3,393 @@ title: What's new in F# 8 - F# Guide
 description: Find information on the new features available in F# 8.
 ms.date: 11/17/2023
 ms.topic: whats-new
+ai-usage: ai-assisted
 ---
 # What's new in F# 8
 
-For information on F# 8, please see [Announcing F# 8](https://devblogs.microsoft.com/dotnet/announcing-fsharp-8).
+F# 8 brings in many features to make F# programs simpler, more uniform, and more performant. This article highlights the major changes in F# 8, developed in the [F# open source code repository](https://github.com/dotnet/fsharp).
+
+F# 8 is released as part of .NET 8. You can download the latest .NET SDK from the [.NET downloads page](https://dotnet.microsoft.com/download/dotnet/8.0).
+
+For the full announcement with in-depth details and community contributions, see [Announcing F# 8](https://devblogs.microsoft.com/dotnet/announcing-fsharp-8).
+
+## F# language changes
+
+### `_.Property` shorthand for simple lambda functions
+
+F# 8 introduces a succinct notation for simple lambda functions that access properties or members. Instead of writing `(fun x -> x.Property)`, you can now write `_.Property`.
+
+**Before F# 8:**
+
+:::code language="fsharp" source="snippets/fsharp-8/PropertyShorthand.fsx" id="Types":::
+
+:::code language="fsharp" source="snippets/fsharp-8/PropertyShorthand.fsx" id="Before":::
+
+**With F# 8:**
+
+:::code language="fsharp" source="snippets/fsharp-8/PropertyShorthand.fsx" id="After":::
+
+This shorthand works for property access, nested property access, method calls, and indexers. You can also define standalone lambda functions:
+
+:::code language="fsharp" source="snippets/fsharp-8/PropertyShorthand.fsx" id="Standalone":::
+
+### Nested record field copy and update
+
+F# 8 extends the copy-and-update syntax for nested records, eliminating the need for multiple nested `with` keywords.
+
+**Before F# 8:**
+
+:::code language="fsharp" source="snippets/fsharp-8/NestedRecordUpdate.fsx" id="Types":::
+
+:::code language="fsharp" source="snippets/fsharp-8/NestedRecordUpdate.fsx" id="Before":::
+
+**With F# 8:**
+
+:::code language="fsharp" source="snippets/fsharp-8/NestedRecordUpdate.fsx" id="After":::
+
+This syntax also works for anonymous records:
+
+:::code language="fsharp" source="snippets/fsharp-8/NestedRecordUpdate.fsx" id="Anonymous":::
+
+### while!
+
+The `while!` (while bang) feature simplifies computation expressions by eliminating boilerplate when looping over a boolean condition that must be evaluated by the computation expression.
+
+**Before F# 8:**
+
+```fsharp
+let mutable count = 0
+let asyncCondition = async {
+    return count < 10
+}
+
+let doStuffBeforeThisFeature = 
+    async {
+       let! firstRead = asyncCondition
+       let mutable read = firstRead
+       while read do
+         count <- count + 2
+         let! nextRead = asyncCondition
+         read <- nextRead
+       return count
+    }
+```
+
+**With F# 8:**
+
+```fsharp
+let doStuffWithWhileBang =
+    async {
+        while! asyncCondition do
+            count <- count + 2
+        return count
+    }
+```
+
+For more details, see [Simplifying F# computations with the new while! keyword](https://devblogs.microsoft.com/dotnet/simplifying-fsharp-computations-with-the-new-while-keyword/).
+
+### Extended string interpolation syntax
+
+F# 8 improves support for interpolated strings, taking inspiration from C# interpolated raw string literals. You can now use multiple `$` signs at the beginning of an interpolated string to define how many braces are needed to enter interpolation mode.
+
+**Before F# 8:**
+
+:::code language="fsharp" source="snippets/fsharp-8/StringInterpolation.fsx" id="Before":::
+
+**With F# 8:**
+
+:::code language="fsharp" source="snippets/fsharp-8/StringInterpolation.fsx" id="After":::
+
+This is especially useful for HTML templating languages:
+
+:::code language="fsharp" source="snippets/fsharp-8/StringInterpolation.fsx" id="Template":::
+
+For more details, see [New syntax for string interpolation in F# 8](https://devblogs.microsoft.com/dotnet/new-syntax-for-string-interpolation-in-fsharp/).
+
+### Use and compose string literals for printf
+
+String literals can now be used and composed for `printf` and related functions. Format specifiers defined as literals can be reused as patterns.
+
+```fsharp
+[<Literal>] 
+let formatBody = "(%f,%f)"
+[<Literal>] 
+let formatPrefix = "Person at coordinates"
+[<Literal>] 
+let fullFormat = formatPrefix + formatBody
+
+let renderedCoordinates = sprintf formatBody 0.25 0.75
+let renderedText = sprintf fullFormat 0.25 0.75
+```
+
+### Arithmetic operators in literals
+
+Numeric literals can now be expressed using existing operators and other literals. The compiler evaluates the expression at compile time.
+
+:::code language="fsharp" source="snippets/fsharp-8/LiteralArithmetic.fsx" id="Literals":::
+
+Supported operators:
+
+- Numeric types: `+`, `-`, `*`, `/`, `%`, `&&&`, `|||`, `<<<`, `>>>`, `^^^`, `~~~`, `**`
+- Booleans: `not`, `&&`, `||`
+
+The feature also works for enum values:
+
+:::code language="fsharp" source="snippets/fsharp-8/LiteralArithmetic.fsx" id="Enums":::
+
+### Type constraint intersection syntax
+
+F# 8 simplifies defining multiple intersected generic constraints using flexible types with the `&` character.
+
+**Before F# 8:**
+
+```fsharp
+let beforeThis(arg1 : 't 
+    when 't:>IDisposable 
+    and 't:>IEx 
+    and 't:>seq<int>) =
+    arg1.h(arg1)
+    arg1.Dispose()
+    for x in arg1 do
+        printfn "%i" x
+```
+
+**With F# 8:**
+
+```fsharp
+let withNewFeature (arg1: 't & #IEx & #IDisposable & #seq<int>) =
+    arg1.h(arg1)
+    arg1.Dispose()
+    for x in arg1 do
+        printfn "%i" x
+```
+
+### Extended fixed bindings
+
+The `fixed` keyword, used for pinning memory in low-level programming, now supports:
+
+- `byref<'t>`
+- `inref<'t>`
+- `outref<'t>`
+- Types with `GetPinnableReference` method returning `byref<'t>` or `inref<'t>`
+
+This is especially relevant for types like `ReadOnlySpan<'T>` and `Span<'T>`.
+
+```fsharp
+open System
+open FSharp.NativeInterop
+
+#nowarn "9"
+
+let pinIt (span: Span<char>, byRef: byref<int>, inRef: inref<int>) =
+    use ptrSpan = fixed span
+    use ptrByRef = fixed &byRef
+    use ptrInref = fixed &inRef
+
+    NativePtr.copyBlock ptrByRef ptrInref 1
+```
+
+### Easier `[<Extension>]` method definition
+
+The `[<Extension>]` attribute no longer needs to be applied to both the type and the member. The compiler automatically adds the type-level attribute.
+
+**Before F# 8:**
+
+```fsharp
+open System.Runtime.CompilerServices
+[<Extension>]
+type Foo =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+```
+
+**With F# 8:**
+
+```fsharp
+open System.Runtime.CompilerServices
+type Foo =
+    [<Extension>]
+    static member PlusOne (a:int) : int = a + 1
+```
+
+## Making F# more uniform
+
+### Static members in interfaces
+
+Interfaces can now declare and implement concrete static members (not to be confused with static abstract members from F# 7).
+
+**With F# 8:**
+
+:::code language="fsharp" source="snippets/fsharp-8/StaticInInterfaces.fsx" id="Interface":::
+
+### Static let in discriminated unions, records, structs, and types without primary constructors
+
+F# 8 enables `static let`, `static let mutable`, `static do`, and `static member val` in:
+
+- Discriminated unions
+- Records
+- Structs (including `[<Struct>]` unions and records)
+- Types without primary constructors
+
+:::code language="fsharp" source="snippets/fsharp-8/StaticLetInDU.fsx" id="StaticLet":::
+
+### `try-with` within `seq{}`, `[]`, and `[||]` collection expressions
+
+Exception handling is now supported within collection builders.
+
+:::code language="fsharp" source="snippets/fsharp-8/TryWithInCollections.fsx" id="TryWith":::
+
+This yields the list `[1;1000;1;10;100]`, which sums to 1112.
+
+## New diagnostics
+
+F# 8 adds many new and improved diagnostic messages.
+
+### TailCall attribute
+
+Use the `[<TailCall>]` attribute to explicitly state your intention of defining a tail-recursive function. The compiler warns you if your function makes non-tail-recursive calls.
+
+:::code language="fsharp" source="snippets/fsharp-8/TailCallAttribute.fsx" id="Classic":::
+
+:::code language="fsharp" source="snippets/fsharp-8/TailCallAttribute.fsx" id="Accumulator":::
+
+### Diagnostics on static classes
+
+New warnings detect invalid scenarios on sealed abstract classes (static classes):
+
+- Instance let bindings aren't allowed
+- Implementing interfaces isn't allowed
+- Explicit field declarations aren't allowed
+- Constructor with arguments isn't allowed
+- Additional constructor isn't allowed
+- Abstract member declarations aren't allowed
+
+### Diagnostics on `[<Obsolete>]` usage
+
+Detection of obsolete members has been improved:
+
+- Enum value usage
+- Event usage
+- Record copy-and-update syntax when the obsolete field is part of the update
+
+```fsharp
+open System
+type Color =
+    | [<Obsolete("Use B instead")>] Red = 0
+    | Green = 1
+
+let c = Color.Red // warning at this line
+```
+
+### Optional warning when `obj` is inferred
+
+A new information-level diagnostic (FS3559) warns when type inference infers `obj` instead of a generic type, which might be unintended.
+
+```fsharp
+([] = [])  // triggers the warning
+```
+
+Enable with `<WarnOn>FS3559</WarnOn>` in your project file.
+
+### Optional warning when copy and update changes all fields
+
+A warning (FS3560) detects when copy-and-update syntax changes all fields of a record. It's more efficient to create a new record directly.
+
+Enable with `<WarnOn>FS3560</WarnOn>` in your project file.
+
+## Quality of life improvements
+
+### Trimmability for compiler-generated code
+
+F# 8 improves support for [.NET trimming](../../core/deploying/trimming/trim-self-contained.md):
+
+- Discriminated unions are now trimmable
+- Anonymous records are now trimmable
+- Code using `printfn "%A"` for trimmed records is now trimmable
+
+### Parser recovery
+
+Parser recovery has been vastly improved. IDE features like coloring and navigation now continue working even when code has syntactical errors or is incomplete.
+
+For more details, watch [Parser Recovery in F#](https://amplifying-fsharp.github.io/sessions/2023/07/07/).
+
+### Strict indentation rules
+
+F# 8 turns on strict indentation mode by default for projects targeting F# 8 and newer. This mode respects language rules for indentation and reports errors instead of warnings.
+
+Configure using `--strict-indentation[+|-]` or in your project file:
+
+```xml
+<OtherFlags>--strict-indentation-</OtherFlags>
+```
+
+### Autocomplete improvements
+
+Autocomplete scenarios have been improved:
+
+- Record completions in patterns
+- Union fields in patterns
+- Return type annotations
+- Method completions for overrides
+- Constant value completions in pattern matching
+- Expressions in enum values
+- Names based on labels of union-case fields
+- Collections of anonymous records
+- Settable properties in attribute completions
+
+### `[<Struct>]` unions can now have > 49 cases
+
+The limitation on struct unions with more than 49 cases has been removed, making them suitable for longer definitions.
+
+## Compiler performance
+
+### Reference assemblies
+
+[Reference assemblies](../../standard/assembly/reference-assemblies.md) are now better utilized for F# projects, improving incremental build times by reducing unnecessary rebuilds when only implementation details change.
+
+### Switches for compiler parallelization
+
+Three experimental features enable parallelization of different compilation stages:
+
+- `--test:GraphBasedChecking` - Enables graph-based type checking for parallel processing
+- `--test:ParallelOptimization` - Parallelizes the optimization phase
+- `--test:ParallelIlxGen` - Parallelizes IL code generation
+
+Read more in [Graph-based type-checking](https://devblogs.microsoft.com/dotnet/a-new-fsharp-compiler-feature-graphbased-typechecking/).
+
+Enable all features using the `FSHARP_EXPERIMENTAL_FEATURES` environment variable:
+
+```powershell
+$env:FSHARP_EXPERIMENTAL_FEATURES = '1'
+```
+
+## Enhancements to FSharp.Core
+
+### Inlining improvements
+
+F# 8 adds inlining optimizations to the `Option` and `ValueOption` modules, reducing allocations and improving performance. For example, mapping `None` now takes 0.17ns instead of 2.77ns (16× improvement).
+
+### `Array.Parallel.*` APIs
+
+The `Array.Parallel` module has been expanded with many new functions:
+
+- `exists`, `forAll`, `tryFindIndex`, `tryFind`, `tryPick`
+- `reduceBy`, `reduce`
+- `minBy`, `min`, `sumBy`, `sum`, `maxBy`, `max`, `averageBy`, `average`
+- `zip`, `groupBy`, `filter`
+- `sortInPlaceWith`, `sortInPlaceBy`, `sortInPlace`
+- `sortWith`, `sortBy`, `sort`, `sortByDescending`, `sortDescending`
+
+These functions are useful for CPU-intensive processing on large arrays and utilize all available logical processors.
+
+### `async` improvements
+
+- `Bind` of `Async<>` within `task{}` now starts on the same thread
+- `MailBoxProcessor` has a public `.Dispose()` member
+- `MailBoxProcessor` now has `StartImmediate` to start on the calling thread
+
+## See also
+
+- [F# Language Reference](../language-reference/index.md)
+- [What's new in F# 9](fsharp-9.md)
+- [What's new in F# 7](fsharp-7.md)