Styjun

I want to write a quantum program in F# but I don't know how to call Q# operations from F#. How exactly would I do this?

I've tried reading the C# version first but it doesn't seem to translate well to F#.

TL;DR: You have to create a Q# library project (which will yield a .csproj project with only Q# files in it) and to reference it from a purely F# application.

You can not mix F# and Q# in the same project, because it won't compile: Q# compiles to C# for local simulation, and you can't have C# and F# in the same projects. However, you can have two separate projects in different languages which both compile to MSIL and can reference each other.

The steps are:

1. 
   

   Create Q# library QuantumCode and write your code in it.

   
   
   

   Let's say your code has an entry point with the signature operation RunAlgorithm (bits : Int[]) : Int[] (i.e., it takes an array of integers as a parameter and returns another array of integers).

   
   



2. Create an F# application (for simplicity let's make it a console app targeting .NET Core) FsharpDriver.




3. Add a reference to the Q# library to the F# application.



4. 
   

   Install the NuGet package Microsoft.Quantum.Development.Kit which adds Q# support to the F# application.

   
   
   

   You will not be writing any Q# code in FsharpDriver, but you will need to use functionality provided by the QDK to create a quantum simulator to run your quantum code on, and to define data types used to pass the parameters to your quantum program.

   
   


5. 
   

   Write the driver in F#.

   
   
   

   // Namespace in which quantum simulator resides
   open Microsoft.Quantum.Simulation.Simulators
   // Namespace in which QArray resides
   open Microsoft.Quantum.Simulation.Core
   
   [<EntryPoint>]
   let main argv =
    printfn "Hello Classical World!"
    // Create a full-state simulator
    use simulator = new QuantumSimulator()
    // Construct the parameter
    // QArray is a data type for fixed-length arrays
    let bits = new QArray<int64>([| 0L; 1L; 1L |])
   
    // Run the quantum algorithm
    let ret = QuantumCode.RunAlgorithm.Run(simulator, bits).Result
   
    // Process the results
    printfn "%A" ret
   
    0 // return an integer exit code
   

   
   

I posted a full example of the project code here (originally that project dealt with using Q# from VB.NET, but for F# all the steps are the same).