Q#: how to write an algorithm in modular form (I don't understand the syntax and the documentation is a bit obscure)

Question

I would like to make a resource estimate based on Azure resource estimator.

However, the circuits I am interested in depend on some classical parameters: for this reason, I would like to make a modular script with sub-portions of the quantum circuits in separated parts.

I tried to do the following code (which does not work).

import qsharp.azure
targets = qsharp.azure.connect(
            resourceId = "/subscriptions/MYSUBSCRIPTION",
            location = "westeurope")
qsharp.azure.target("microsoft.estimator")
qsharp.azure.target("microsoft.estimator")

%%qsharp

open Microsoft.Quantum.Intrinsic;
open Microsoft.Quantum.Diagnostics;
open Microsoft.Quantum.Math;
open Microsoft.Quantum.Arrays;

operation subCircuit(q: Qubit[], x: Array[], n: Int) {
    for index in 0 .. n
    {
       H(q[index]); // Apply Hadamard on qubit "index"
       Rx(2.0*x[index],q[index]); // After the Hadamard, apply a Rx rotation on the qubit "index" of angle 2.0*x[index]
    }
}

operation AlgoTest() : Result[] {
    let n=3; // Number of qubits
    let x=[0.,1.,2.]; // Vector x
    mutable resultArray = [Zero, size = 3];
    use q = Qubit[n];
    subCircuit(q,x,n);

    for i in IndexRange(q) {
                set resultArray w/= i <- M(q[i]);
            }
    return resultArray;
}

result = qsharp.azure.execute(AlgoTest)

My questions:

As you see, I wanted to separate a sub-portion of my example algorithm subCircuit inside another function AlgoTest. However, the script returns several errors.

I know that I am probably not using good syntax, in particular, to define operation subCircuit, but this is precisely why I am asking this question. I looked at the documentation of operations and functions but the explanations are very obscure. Having an example of a working script doing what I want would be helpful for me to understand the syntax.

The errors returned are below (but overall I am not surprized there are errors because I am not using the good syntax, my question is precisely to understand which syntax is expected by the program, which is not very clear from the documentations -- perhaps I am not looking at the good place --):

fail: Microsoft.Quantum.IQSharp.Snippets[0] QS3212 (/snippet_.qs:Microsoft.VisualStudio.LanguageServer.Protocol.Range): Expecting return type annotation. The argument tuple needs to be followed by a colon and the return type of the callable. fail: Microsoft.Quantum.IQSharp.Snippets[0] QS3035 (/snippet_.qs:Microsoft.VisualStudio.LanguageServer.Protocol.Range): Unexpected code fragment. fail: Microsoft.Quantum.IQSharp.Snippets[0] QS3036 (/snippet_.qs:Microsoft.VisualStudio.LanguageServer.Protocol.Range): An expression used as a statement must be a call expression. fail: Microsoft.Quantum.IQSharp.Snippets[0] QS6005 (/snippet_.qs:Microsoft.VisualStudio.LanguageServer.Protocol.Range): No type with the name "Array" exists in any of the open namespaces. /snippet_.qs(7,53): error QS3212: Expecting return type annotation. The argument tuple needs to be followed by a colon and the return type of the callable. /snippet_.qs(28,8): error QS3035: Unexpected code fragment. /snippet_.qs(28,1): error QS3036: An expression used as a statement must be a call expression. /snippet_.qs(7,37): error QS6005: No type with the name "Array" exists in any of the open namespaces.

Answer 1

(The errors you get are always helpful, so do include them whenever you can - for example, they can tell us the environment you're using to run the code, and that you're not putting Python and Q# code in the same code cell of a Jupyter Notebook.)

Expecting return type annotation. The argument tuple needs to be followed by a colon and the return type of the callable. 

When you define an operation or a function, it needs to have a return type. If it doesn't return anything, like in your case, the return type is Unit, so your operation signature should have : Unit after the closing bracket after the list of arguments.

No type with the name "Array" exists in any of the open namespaces.

There is indeed no Array type in Q#; in this case the second argument of your operation is Double[].

Combining these two, the signature of your operation should be

operation subCircuit(q: Qubit[], x: Double[], n: Int) : Unit {

At this point your code will compile. (It will run into a runtime error because within the operation the for loop goes from 0 to n, inclusive, so it will try to access array elements out of array bounds; the iteration range should be 0 .. n - 1.)