Fable

2.1.0-alpha-001 verified Fri May 01 auth: no javascript

Fable is an F# to JavaScript/TypeScript/Python/Dart/Rust/Erlang compiler. The current stable release is 4.29.0 with 5.0.0 in release candidate (rc.7). It is under active development, releasing multiple alpha/rc versions per month. Key differentiators: single codebase compiles to multiple targets, supports F# quotations, integrates with .NET tooling via dotnet CLI. The compiler is distributed as a .NET tool via NuGet.

Common errors

error The type 'Fable.Cli' is not defined ↓

cause Missing reference to Fable.Cli NuGet package.

fix

Add '#r "nuget: Fable.Cli"' before using Fable.Cli module.

error Unknown language 'beam' ↓

cause Using an older Fable version that doesn't support BEAM/Erlang target.

fix

Upgrade to Fable >=5.0.0-rc.1 to use '--lang beam'.

error Fable.Core.Emit does not support this target ↓

cause Using [<Emit>] attribute with Python or BEAM target.

fix

Use platform-specific interop or Fable.Core.JsInterop.

error Fable: Could not find 'fable-library.js' ↓

cause Missing Fable library runtime files.

fix

Ensure you have added Fable.Library NuGet package and restored.

Warnings

breaking Fable 5.0.0 renames the 'fable' command to 'fable-cli'. Use 'dotnet fable'. ↓

fix Replace 'fable' with 'dotnet fable' in scripts and CI.

breaking Fable 5.0.0 changes default output format for Python/Beam targets; check generated files. ↓

fix Review output directory structure and imports.

deprecated Fable 3.x 'fable-splitter' is deprecated; use Fable.Cli directly. ↓

fix Migrate to Fable.Cli with 'dotnet fable'.

gotcha Fable.Core.Emit attribute does not work with Fable 5 Python/Beam targets; use Fable.Core.JsInterop instead. ↓

fix Replace [<Emit>] with platform-specific interop attributes.

gotcha ResizeArray equality uses reference equality in Fable 5 for all targets; .NET uses structural equality. ↓

fix Use FSharp.Core's List or Array for structural equality, or implement custom equality.

gotcha Fable does not support all .NET APIs; especially reflection and AppDomain. ↓

fix Test your F# code against Fable's supported API subset.

Install

npm install fable-compiler-dotnet

yarn add fable-compiler-dotnet

pnpm add fable-compiler-dotnet

Imports

Fable.Cli
wrong
```
#r "nuget: Fable"
```
correct
```
#r "nuget: Fable.Cli"
open Fable.Cli
```
Fable.Cli is the package for the dotnet tool; 'Fable' package is the compiler library.
Fable.Core
wrong
```
#r "nuget: Fable"
```
correct
```
#r "nuget: Fable.Core"
open Fable.Core
```
Fable.Core provides F# attributes like [<Emit>] and JS interop. Not needed for simple compilation.
__SOURCE_DIRECTORY__
```
let dir = __SOURCE_DIRECTORY__
```
Built-in F# value, not an import.

Quickstart

Compiles the current F# file to JavaScript using Fable.Cli NuGet package.

#r "nuget: Fable.Cli"
open Fable.Cli

let output = 
    {
        Fable.CompilerOptions.Create() with
            file = __SOURCE_FILE__
            outDir = "./output"
            lang = Fable.Language.JavaScript
    }
Fable.Cli.compile output |> Async.RunSynchronously
|> function
| Ok _ -> printfn "Compilation succeeded"
| Error exn -> printfn "Error: %s" exn.Message