docs: Blog anouncing Fable 5 RC

docs/blog/2026/2026-02-27-Fable_5_release_candidate.md

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+---
+layout: fable-blog-page
+title: Announcing Fable 5 Release Candidate
+author: Mangel Maxime
+date: 2026-02-27
+author_link: https://twitter.com/MangelMaxime
+author_image: https://github.com/MangelMaxime.png
+# external_link:
+abstract: |
+    Fable 5 RC is Here: New Targets, Better Tooling, and Nullness Support
+---
+
+More than 1 year after the first alpha release, we are happy to announce that Fable 5 is now in release candidate stage!
+
+You can install Fable 5 by running the following command:
+
+```bash
+# This is only necessary if you haven't installed any dotnet tool yet in the directory
+dotnet new tool-manifest
+
+dotnet tool install fable --prerelease
+```
+
+If you already have Fable installed, you can update it by running:
+
+```bash
+dotnet tool update fable --prerelease
+```
+
+:::info
+When upgrading, make sure to upgrade your dependencies as well.
+:::
+
+## Compatibility with Fable 4
+
+Fable 5 is compatible with Fable 4 projects, except that it is now a `net10.0` tool.
+
+## Project cracking
+
+We remove support for the old project cracker using Buildalyzer. It is now replaced by invoking MSBuild directly, which should be more robust for the future.
+
+## `WarnAsError` support
+
+It has been a long-standing request to support `<TreatWarningsAsErrors>true</TreatWarningsAsErrors>` in Fable, and we are happy to announce that it is now supported in Fable 5.
+
+You can enable it in your project file like this:
+
+```xml
+<PropertyGroup>
+  <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+</PropertyGroup>
+```
+
+Fable will now treat all warnings from **your** code as errors, but it will still allow warnings from dependencies.
+
+This behavior should mimic how standard F# compiler works.
+
+## .NET 10 and F# 10 support
+
+Fable 5 add support for the following F# features:
+
+* [Nullable reference types](https://learn.microsoft.com/en-us/dotnet/fsharp/whats-new/fsharp-9#nullable-reference-types)
+* [Discriminated union <code>.Is*</code> properties](https://learn.microsoft.com/en-us/dotnet/fsharp/whats-new/fsharp-9#discriminated-union-is-properties)
+* [Partial active patterns can return <code>bool</code> instead <code>unit option</code>](https://learn.microsoft.com/en-us/dotnet/fsharp/whats-new/fsharp-9#partial-active-patterns-can-return-bool-instead-of-unit-option)
+* [Empty-bodied computation expressions](https://learn.microsoft.com/en-us/dotnet/fsharp/whats-new/fsharp-9#empty-bodied-computation-expressions)
+* [Updates to the standard library (FSharp.Core)](https://learn.microsoft.com/en-us/dotnet/fsharp/whats-new/fsharp-9#updates-to-the-standard-library-fsharpcore)
+
+## JavaScript / TypeScript
+
+JavaScript target is still the most used and stable target, and we kept improving it during the last year.
+
+### Supports direct nested types when using `jsOptions`
+
+```fs
+    let opts =
+        jsOptions<Level1> (fun o ->
+            o.level2.level3.valueA <- 10
+            o.level2.level3.valueB <- 20
+            o.topValueA <- 20
+        )
+```
+
+generates
+
+```js
+export const opts = {
+    level2: {
+        level3: {
+            valueA: 10,
+            valueB: 20,
+        },
+    },
+    topValueA: 20,
+};
+```
+
+### Simplify `Pojo` bindings
+
+Fable has a number of ways to create [Plain Old JavaScript Objects](https://fable.io/docs/javascript/features.html#plain-old-javascript-objects) (POJOs).
+
+In Fable 4, people discovered a way of abusing a combination of attributes to write POJO bindins in a F# natural way.
+
+```fs
+open Fable.Core
+
+[<AllowNullLiteral>]
+[<Global>]
+type Options
+    [<ParamObject; Emit("$0")>]
+    (
+        searchTerm: string,
+        ?isCaseSensitive: bool
+    ) =
+    member val searchTerm: string = jsNative with get, set
+    member val isCaseSensitive: bool option = jsNative with get, set
+
+let options1 = new Options("foo")
+
+let options2 = new Options("foo", isCaseSensitive = true)
+```
+
+Improving on this, Fable 5 introduce a new attribute, `Pojo`, that can be used to write the same code with only one attribute:
+
+```fs
+open Fable.Core
+
+[<AllowNullLiteral>]
+[<JS.Pojo>]
+type Options
+    (
+        searchTerm: string,
+        ?isCaseSensitive: bool
+    ) =
+    member val searchTerm: string = jsNative with get, set
+    member val isCaseSensitive: bool option = jsNative with get, set
+
+let options1 = new Options("foo")
+
+let options2 = new Options("foo", isCaseSensitive = true)
+```
+
+Both examples generate the same JavaScript code:
+
+```js
+export const options1 = {
+    searchTerm: "foo",
+};
+
+export const options2 = {
+    searchTerm: "foo",
+    isCaseSensitive: true,
+};
+```
+
+## Python
+
+If you have been following the Fable 5 alpha releases, you know the Python target has received a staggering amount of love.
+
+This is all thanks to Dag and early adopters who have been testing it.
+
+* **Python 3.12-3.14 support** (3.10/3.11 are deprecated)
+* **fable-library via PyPI** - No more bundled runtime files
+* **Modern type parameter syntax** - Better type hinting in generated code
+* **`Py.Decorate` attribute** - Add Python decorators from F#
+* **`Py.ClassAttributes` attribute** - Fine-grained class generation control
+* **Improved Pydantic interop** - First-class support for data validation
+
+### Rust Core with PyO3
+
+One of the biggest changes is that the core of fable-library is now written in Rust using PyO3.
+The motivation is **correctness**, not performance:
+
+**Why Rust?**
+
+* **Correct .NET semantics** - Sized/signed integers (int8, int16, int32, int64, uint8, etc.)
+* **Proper overflow behavior** - Matches .NET exactly
+* **Fixed-size arrays** - No more Python list quirks for byte streams
+* **Reliable numerics** - Fable 4's pure Python numerics were a constant source of bugs
+
+### fable-library via PyPI
+
+Before Fable v5, the runtime was bundled in the NuGet package and copied to your output directory.
+
+Now it's a simple pip/uv dependency:
+
+```bash
+# Install with pip
+pip install fable-library
+
+# Or with uv (recommended)
+uv add fable-library
+```
+
+:::info
+You can learn more about Fable.Python in general on our [documentation](https://fable.io/docs/python/build-and-run.html) or this [blog post](https://cardamomcode.dev/fable-python#heading-introduction-to-fablepython)
+:::
+
+## Rust
+
+Rust target kept improving as well, and is now using Rust 2024 language edition.
+
+## Hello Erlang/BEAM !
+
+Dag Brattli added a new target for Erlang/BEAM, which is still in early stages.
+
+He has been testing is using [Fable.Giraffe](https://github.com/dbrattli/Fable.Giraffe) and the first results are promising:
+
+> ## Benchmarks
+>
+> Simple `/ping` endpoint returning "pong", 10,000 requests with 100 concurrent
+> connections (oha):
+>
+> | Metric | BEAM | .NET | Python |
+> |---|---|---|---|
+> | Requests/sec | 124,256 | 70,375 | 4,006 |
+> | Avg latency | 0.79 ms | 1.40 ms | 24.9 ms |
+> | P99 latency | 2.49 ms | 3.50 ms | 34.2 ms |
+
+Indeed, it seems like Erlang/BEAM is a great target for Fable, and we are excited to see how it evolves in the future.
+
+## Conclusion
+
+Fable 5 would not have been possible without the help of many people who contributed to it in different ways, from testing, to reporting issues, to contributing code.
+
+Thank you all for your help and support during the last year, and we hope you will continue to help us in the future to make Fable even better!

package.json

@@ -1,4 +1,5 @@
 {
+    "name": "fable-compiler.github.io",
     "private": true,
     "engines": {
         "node": ">=14.0.0",