Subroutines are declared using the statement:

def name(parameters) -> output_type { body }

Subroutines and their named arguments must be named according to the rules for identifiers (See Identifiers).

The subroutine will define zero or more parameters as input, consisting of both quantum and classical arguments. Quantum bits and classical values are passed to the subroutine by reference or name, while classical types are passed in by value. All arguments are declared together with their type. For example, qubit ancilla defines a quantum bit argument named ancilla.

A given qubit can be passed at most once in any subroutine call. Different qubit arguments (whether single bits or registers) cannot refer to the same underlying qubit in a call.

Subroutines return up to one value of classical type, signified by the return keyword. If there is no return value, the empty return keyword may be used to immediately exit from the subroutine, which implicitly returns the void type. Subroutines that do not return a value must be declared with no output signature:

def name(parameters) { body }

Qubit declarations are not allowed within subroutines as those declarations are global.

A subroutine is invoked with the syntax name(parameters) and may be assigned to an output as needed via an assignment operator (=, +=, etc).

Using subroutines, we can define an X-basis measurement with the program:

def xmeasure(qubit q) -> bit { h q; return measure q; }

We can also define more general classes of single-qubit measurements as:

 def pmeasure(angle[32] theta, qubit q) -> bit {
   rz(theta) q;
   h q;
   return measure q;

The type declarations are necessary if we want to mix qubit and register arguments. For example, we might define a parity check subroutine that takes qubits and registers:

def xcheck(qubit[4] d, qubit a) -> bit {
  reset a;
  for int i in [0: 3] cx d[i], a;
  return measure a;

Naturally we can also use subroutines to define purely classical operations, such as methods we can implement using low-level classical instructions, like:

const n = /* some size, known at compile time */;
def parity(bit[n] cin) -> bit {
  bit c;
  for int i in [0: n - 1] {
    c ^= cin[i];
  return c;

We can make some measurements and call this subroutine on the results as follows:

qubit q;
qubit r;
c = measure q;
c2 = measure r;
bit result;
result = parity(c ++ c2);

We require that we know the signature at compile time, as we do in this example. We could also just as easily have used an extern function for this:

const n = /* size of c + size of c2 */;
extern parity(bit[n]) -> bit;
qubit q;
qubit r;
c = measure q;
c2 = measure r;
bit result;
result = parity(c || c2);

Arrays in subroutines

Arrays may be passed as parameters to subroutines and externs. All array parameters are passed as references and must include a type modifier specifying if the parameter is readonly or mutable. The number of dimensions for all array parameters must be specified using the #dim = const expression syntax below, or specific lengths for each dimension must be provided. The unspecified-length version is provided because the lengths of the dimensions of array parameters (in the case of strided access) may not be known until runtime. Passing multiple overlapping mutable references to the same array to a subroutine is forbidden. However, the compiler will not always be able to resolve when this happens, and if it does, then no guarantees are made about the order that updates are made in.

def specified_sub(readonly array[int[8], 2, 10] arr_arg) { /* ... */ }
def arr_subroutine(readonly array[int[8], #dim = 1] arr_arg) { /* ... */ }
def mut_subroutine(mutable array[int[8], #dim = 1] arr_arg) {
  arr_arg[2] = 10; // allowed
  // ...
array[int[8], 5] aa;
array[int[8], 3, 5] bb;

arr_subroutine(bb[1, 0:3]);
mut_subroutine(aa[1:3]); // aa[3] = 10

The lifetime of the array reference is limited to within the scope of the subroutine definition, but it should be noted that since arrays are not dynamically allocated the memory associated with the array stays intact after subroutine exit. Additionally, the OpenQASM3 language is not anticipated to support explicit user-controlled creation of pointers and references outside of the specific context of passing arrays to subroutines.

The dimensions of arrays may be queried inside of subroutines using the built-in sizeof() function, which takes two parameters: the array being queried, and the zero-based dimension number requested. If the second parameter is omitted, then it defaults to 0, i.e. sizeof(arr) == sizeof(arr, 0). sizeof() returns a const uint representing the length of the requested dimension of the array argument. The array argument can be subscripted, meaning that sizeof(arr[0], 0) == sizeof(arr, 1).

def arr_subroutine(readonly array[int[8], #dim = 2] twoD_arg) {
  uint[32] firstDim  = sizeof(twoD_arg, 0);
  uint[32] secondDim = sizeof(twoD_arg, 1);
  int[32] sum = 0;
  for int ii in [0:firstDim-1] {
    for int jj in [0:secondDim-1] {
      sum += int[32](twoD_arg[ii][jj]);
  // ...