I’ve written about Haskell x Swift interoperability before. Calling Haskell from Swift is about marshalling and the foreign function interface. But Creating a macOS app with Haskell and Swift tells the much messier tale of hijacking XCode to vodoo together the Haskell library, its headers, and two handfuls of other magic ingredients into one buildable SwiftUI application.

Stop! Don’t click on the last link. No, it turns out that my XCode sallies strayed very far from the yellow brick road. The IDE is confused. Recompilation bugs abound. Complexity is through the roof juggling .modulemaps, .xcconfig dynamic settings, and sketchy .sh scripts.

Let’s walk the happy path.

1 Announcing: xcframework

Perhaps obvious in retrospect, the demon-less way to add a Haskell library to the dependencies of a Swift application is to build an independent Swift Package wrapping the Haskell library – something that can be done without XCode in sight. Easy peasy:

1. Build the Haskell library using Cabal
2. Create a Swift package from the Haskell artifacts
3. Add the Swift package as a dependency to the project

And it turns out that (1) and (2) can be merged together using Cabal SetupHooks!

Moreover, I’m happy to announce I’ve neatly packaged and released that build process automation as a Haskell library called xcframework on Hackage.

Onwards! – for what it does and how to use it.

1.1 XCFrameworks

Apple introduced XCFramework bundles back in a WWDC19 session. An XCFramework is a multiplatform binary framework bundle.

For our purposes, that means we can create a Swift Package just from a binary linkable artifact and a couple of header files. Then, any Swift project can depend on this binary Swift package and call the functions exposed to the headers and make sure the bundled library will be linked in with the final executable. Specifically, the xcframework Haskell library, for a given Haskell library, bundles:

• The foreign shared library (.dylib) resulting from building with GHC/Cabal
• The foreign export headers generated from the foreign export <haskell_function> declarations
• The RTS headers
  • which are needed to initialize the RTS from Swift
  • and because they are #included by the foreign export headers
• A .modulemap exporting the foreign exported functions and HsFFI.h
  • The module map basically turns the headers into a Swift module that can be transparently imported from other Swift modules.

And any Swift library or application can transparently depend on this .xcframework and use the foreign exported Haskell functions without further ado.

1.2 How to install xcframework

In your cabal file, change the build-type to Hooks (and set cabal-version: 3.14 if not set already):

- build-type:     Simple
+ build-type:     Hooks

And add a setup-depends stanza with a dependency on xcframework:

custom-setup
  setup-depends:
    base        >= 4.18 && < 5,
    xcframework >= 0.1

Finally, create a file called SetupHooks.hs in the root of your Cabal package with the following contents, substituting the _build/MyHaskellLib.xcframework string for the filepath to where the .xcframework should be written:

module SetupHooks ( setupHooks ) where

import Distribution.XCFramework.SetupHooks

setupHooks :: SetupHooks
setupHooks = xcframeworkHooks "_build/MyHaskellLib.xcframework"

Now, whenever you run cabal build, the built libraries will also be bundled into an .xcframework.

1.3 How to use the XCFramework in XCode

In XCode:

1. Navigate to the target settings of your project.
2. Find under “General” the “Frameworks, Libraries, and Embedded Content” (or similar) section.
3. Click the add button and add the .xcframework framework outputted at the specified path by Cabal

Now, in the entry Swift module, import the RTS and init/exit the RTS. For instance, in a sample SwiftUI app:

  import SwiftUI
+ import Haskell.Foreign.Rts
  
  @main
  struct MyExample: App {
+ 
+     init() {
+         hs_init(nil, nil)
+
+         NotificationCenter.default
+           .addObserver(forName: NSApplication.willTerminateNotification,
+                        object: nil, queue: .main) { _ in
+           hs_exit()
+         }
+     }
+ 
      var body: some Scene {
          WindowGroup {
              ContentView()
          }
      }
  }

Finally, in any Swift module, do import Haskell.Foreign.Exports. For now, the name Haskell.Foreign.Exports is fixed and exports all foreign-exported functions, but it could be improved in the future (perhaps it’s a good task to contribute a patch for!)

For example, if your Haskell module looked like:

module MyLib (doSomething) where

fib :: Integral b => Int -> b
fib n = round $ phi ** fromIntegral n / sq5
  where
    sq5 = sqrt 5 :: Double
    phi = (1 + sq5) / 2

doSomething :: IO Int
doSomething = do
  putStrLn "doing some thing"
  return $ fib 42

foreign export ccall doSomething :: IO Int

In your Swift module you can now

import Haskell.Foreign.Exports

  ...
  let x = doSomething()
  ...

1.4 Building simple Swift package

The .xcframework can also be easily used in a standalone swift package built with swift build.

In your Package.swift, add MyHaskellLib.xcframework as a binary target and make it a dependency of your main target. For instance, a simple library would look like:

// swift-tools-version: 6.1
import PackageDescription

let package = Package(
    name: "MySwiftLib",
    platforms: [
        .macOS(.v15)
    ],
    products: [
        .library(name: "MySwiftLib", targets: ["MySwiftLib"])
    ],
    targets: [
        .target(name: "MySwiftLib", dependencies: ["MyHaskellLib"], path: "Swift"),
        .binaryTarget(
            name: "MyHaskellLib",
            path: "haskell/_build/MyHaskellLib.xcframework"
        )
    ]
)

Now you can use the Haskell.Foreign.Exports import in any module in the package as explained above, for instance in Swift/MySwiftLib.hs:

import Foundation
import Haskell.Foreign.Exports

public struct Fib {
  var val: Int64
}

public func mkFib() -> Fib {
    let x = doSomething()
    return Fib(val: x)
}

Build the Swift package using swift build in the project root.

1.5 Must use Cabal Foreign Library stanza

Unfortunately, while I don’t figure out how to link the right amount of things into the .xcframework after building a normal library component in Cabal, the foreign exports must be exported from a foreign-library Cabal stanza:

foreign-library myexample
    type: native-shared
    options: standalone
    other-modules:    MyLib
    build-depends:    base ^>=4.20.0.0
    hs-source-dirs:   src
    default-language: GHC2021

To clarify the instructions, I put together a small demo project with a working setup – if you want to try it out. Remember to build the Cabal library first!

1.6 Conclusion

Building the Haskell library as an independent Swift Package is a much more robust way of adding a Haskell dependency to a Swift application.

The xcframework Haskell library makes it easy to create XCFrameworks from Haskell packages by leveraging the SetupHooks very nicely designed API.

While this work further lowers the bar for integrating Haskell and Swift, marshaling and sharing high-level datatypes remains challenging. Calling Haskell from Swift explored the basics of using more interesting types across the FFI, but I’m also working on a more automated approach using TH and GHC plugins.

Finally, I’m looking forward to bug reports if you try it out.