README.md

Free

This repo is based on my blog post on Free Monads in swift.

TL;DR

import Free

struct PrintLine : Symbol {
  typealias Meaning = Void
  let message : String
}

struct GetLine : Symbol {
  typealias Meaning = String
}


let example = PrintLine(message: "What is the answer to life, the universe and everything?")
                          .flatMap(GetLine.init)
                          .map{(answer) -> PrintLine in 
                                PrintLine(message: Int(answer) == 42 ? "Yay!" : "Nope...")
                            }
                            
// now interpret

extension PrintLine : AsyncInterpretation {

   func runUnsafe(_ env: Dependencies) {
      print(message)
   }

}


extension GetLine : AsyncInterpretation {

  func runUnsafe(_ env: Dependencies) async -> String {
     
     // avoid overflow of UInt64 by looping...
     for _ in (0..<Int(1e9)) {
          await Task.sleep(UInt64(1e9 * 60 * 60 * 24 * 365.25 * 7.5))
          print("Pon Farr!")
     }
     
     return "42"
     
  }

}


func test() {

  Task.detached {
    try? await example.runUnsafe()
  }

}

Installation

• Step 1: Add this package as a dependency in XCode: in the Files menu, choose "Add Packages...", paste the link to this repo into the search bar and confirm.
• Step 2: Install Sourcery. With homebrew, all you need to do is to run brew install sourcery in the terminal.
• Step 3: Add a run script phase to your build phases right before compile sources. Paste the following script:

/opt/homebrew/bin/sourcery --templates ${BUILD_DIR%Build/*}SourcePackages/checkouts/Free/Templates --sources ${SRCROOT}/${PRODUCT_NAME} --output ${SRCROOT}/Generated

Attention: depending on your system, hombrew might have put sourcery elsewhere, so you may need to adjust this a little bit.

• Step 4: Uncheck the "Based on dependency analysis" box in your run script phase.
• Step 5: Compile for the first time. A Folder Generated should have appeared in the folder where your .xcodeproj project is located. Add this folder and its contents to the project.