The Type Model

This section lays out the Q# type model and describes the syntax for specifying and working with types.

Primitive Types

Q# provides several primitive types, out of which all other types are constructed:

  • The Int type represents a 64- bit signed (two's complement) integer.
  • The Double type represents a double-precision floating-point number.
  • The Bool type represents a Boolean value, either true or false.
  • The Qubit type represents a quantum bit or qubit. Qubits are opaque to the user; the only operation possible with them, other than passing them to another operation, is to test for identity (equality). Ultimately, actions on Qubits are implemented by calling operations in the Q# standard library.
  • The Pauli type represents an element of the single-qubit Pauli group. This type is used to denote the base operation for rotations and to specify the basis of a measurement. This type is a discriminated union with four possible values: PauliI, PauliX, PauliY, and PauliZ.
  • The Result type represents the result of a measurement. This type is a discriminated union with two possible values: One and Zero. Zero indicates that the +1 eigenvalue was measured; One indicates the -1 eigenvalue.
  • The Range type represents a sequence of integers.
  • The String type is a sequence of Unicode characters that is opaque to the user once created. This type is used to report messages to a classical host.

Note that this implies that true, false, PauliI, PauliX, PauliY, PauliZ, One, and Zero are all reserved symbols in Q#.

Array Types

Given any valid Q# type 'T, there is a type that represents an array of values of type 'T. This array type is represented as 'T[]; for example, Qubit[] or Int[][].

In the second example, note that this represents a potentially jagged array of arrays, and not a rectangular two-dimensional array. Q# does not include support for rectangular multi-dimensional arrays.

Tuple Types

Given any valid Q# types 'T1, 'T2, 'T3, etc., there is a type that represents a tuple of values of types 'T1, 'T2, 'T3, etc., respectively. This tuple type is represented as ('T1, 'T2, 'T3, …). Any number of types may be tupled together. The empty tuple, (), is equivalent to unit in F#.

It is possible to create arrays of tuples, tuples of arrays, tuples of sub-tuples, etc.

Tuple instances are immutable. Q# does not provide a mechanism to change the contents of a tuple once created.

Singleton Tuple Equivalence

It is possible to create a singleton (single-element) tuple, ('T1), such as (5) or ([1;2;3]). However, Q# treats a singleton tuple as completely equivalent to a value of the enclosed type. That is, there is no difference between 5 and (5), or between 5 and (((5))), or between (5, (6)) and (5, 6).

This equivalence applies for all purposes, including assignment and expressions. It is just as valid to write (5)+3 as to write 5+3, and both expressions will evaluate to 8.

We refer to this property as singleton tuple equivalence.

User-Defined Types

A Q# file may define a new named type based on a standard type. Any legal type may be used as the base for a user-defined type.

User-defined types may be used anywhere any other type may be used. In particular, it is possible to define an array of a user-defined type and to include a user-defined type as an element of a tuple type.

It is not possible to create recursive type structures. That is, the type that defines a user-defined type may not be a tuple type that includes an element of the user-defined type. More generally, user-defined types may not have cyclic dependencies on each other, so the following set of type definitions would be illegal:

newtype TypeA = (Int, TypeB);
newtype TypeB = (Double, TypeC);
newtype TypeC = (TypeA, Range);

The mutability of instances of user-defined types is the same as the mutability of instances of the base type of the user-defined type. Specifically, instances of user-defined types based on tuples are immutable; instances of user-defined types based on arrays are potentially mutable.

Type Compatibility

Effectively, a user-defined type is a subtype of the base type. Thus, a value of a user-defined type may be used anywhere a value of the base type is expected. This is applied recursively.

For example, suppose type IntPair is a user-defined type with base type (Int, Int), and type IntPair2 is a user-defined type with base type IntPair. A value of type IntPair2 may be used anywhere a value of type IntPair2, IntPair, or (Int, Int) is expected. A value of type IntPair may be used anywhere a value of type IntPair or (Int, Int) is expected.

Different user-defined types based on the same base type are treated as distinct and unrelated types. In the previous example, if IntPair3 is also a user-defined type with base type (Int, Int), then IntPair and IntPair3 are unrelated and a value of one may not be used where a value of the other is expected.

Operation and Function Types

A Q# operation is a quantum subroutine. That is, it is a callable routine that contains quantum operations.

A Q# function is a classical subroutine used within a quantum algorithm. It may contain classical code but no quantum operations. Functions may not allocate or borrow qubits, nor may they call operations. It is possible, however, to pass them operations or qubits for processing.

Together, operations and functions are known as callables.

All Q# callables are considered to take a single value as input and return a single value as output. Both the input and output values may be tuples. Callables that have no result return the empty tuple, (); callables that have no input take the empty tuple as input.

The basic signature for any callable is written as ('Tinput => 'Tresult) or ('Tinput -> 'Tresult), where both 'Tinput and 'Tresult are type specifiers. The first form, with =>, is used for operations; the second form, with ->, for functions. For example, ((Qubit, Pauli) => Result) represents the signature for a possible single-qubit measurement operation.

Function types are completely specified by their signature. For example, a function that computes the sine of an angle would have type (Double -> Double).

Operations -- but not functions -- may allow the application of one or more functors. Functors are meta-operations that generate a variant of a base operation; see Functors, below.

Operation types are specified by the their signature and the list of functors they support. For example, the Pauli X operation has type (Qubit => () : Adjoint, Controlled). An operation type that does not support any functors is specified by its signature alone, with no trailing :.

Type-Parameterized Functions and Operations

Callable signatures may contain type parameters. Type parameters are indicated by a symbol prefixed by a single quote; for example, 'A is a legal type parameter. Type-parameterized functions and operations are similar to generic functions in many programming languages, but Q# does not provide a full generic type/function capability.

A type parameter may appear more than once in a single signature. For example, a function that applies another function to each element of an array and returns the collected results would have signature (('A[], 'A->'A) -> 'A[]). Similarly, a function that returns the composition of two operations might have signature ((('A=>'B), ('B=>'C)) -> ('A=>'C)).

When invoking a type-parameterized callable, all arguments that have the same type parameter must be of the same type, or be compatible with the same type; that is.

Q# does not provide a mechanism for constraining the possible types that might be substituted for a type parameter. Thus, type parameters are primarily useful for functions on arrays and for composing callables.

Type Compatibility

An operation with additional functors supported may be used anywhere an operation with fewer functors but the same signature is expected. For instance, an operation of type (Qubit=>():Adjoint) may be used anywhere an operation of type (Qubit=>()) is expected.

Q# is covariant with respect to callable return types: a callable that returns a type 'A is compatible with a callable with the same input type and a result type that 'A is compatible with.

Q# is contravariant with respect to input types: a callable that takes a type 'A as input is compatible with a callable with the same result type and an input type that is compatible with 'A.

That is, given the following type definitions:

newtype IntPair : (Int, Int) 
newtype IntPairTransform : ((Int, Int) -> (Int, Int))
newType IntPairTransform2 : ((Int, Int) -> IntPair)   
newType IntPairTransform3 : (IntPair -> (Int, Int))   

the following are true:

  • A value of type IntPairTransform may be invoked with a single argument of type IntPair.
  • The result of invoking a function of type IntPairTransform may not be used where a value of type IntPair is expected.
  • A value of type IntPairTransform2 may be used when an IntPairTransform is expected, but not vice versa.
  • A value of type IntPairTransform may be used when an IntPairTransform3 is expected, but not vice versa.

Functors

A functor in Q# is a factory that defines a new operation from another operation. Functors have access to the implementation of the base operation when defining the implementation of the new operation. Thus, functors can perform more complex functions than traditional higher-level functions.

A functor is used by applying it to an operation, returning a new operation. For example, the operation that results from applying the Adjoint functor to the Y operation is written as (Adjoint Y). The new operation may then be invoked like any other operation. Thus, (Adjoint Y)(q1) applies the adjoint functor to the Y operation to generate a new operation, and applies that new operation to q1.

Similarly, (Controlled X)(controls, target)

The two standard functors in Q# are Adjoint and Controlled.

Adjoint

In quantum computing, the adjoint of an operation is the complex conjugate transpose of the operation. For operations that implement a unitary operator, the adjoint is the inverse of the operation. For a simple operation that just invokes a sequence of other unitary operations on a set of qubits, the adjoint may be computed by applying the adjoints of the sub-operations on the same qubits, in the reverse sequence.

Given an operation expression, a new operation expression may be formed using the Adjoint functor, with the base operation expression enclosed in parentheses, ( and ). The new operation has the same signature and type as the base operation. In particular, the new operation also allows Adjoint, and will allow Controlled if and only if the base operation did.

For instance, (Adjoint QFT) designates the adjoint of the QFT operation.

Controlled

The controlled version of an operation is a new operation that effectively applies the base operation only if all of the control qubits are in a specified state. If the control qubits are in superposition, then the base operation is applied coherently to the appropriate part of the superposition. Thus, controlled operations are often used to generate entanglement.

In Q#, controlled versions always take an array of control qubits, and the specified state is always for all of the control qubits to be in the computational (PauliZ) One state, $\ket{1}$. Controlling based on other states may be achieved by applying the appropriate Clifford operations to the control qubits before the controlled operation, and then applying the inverses of the Cliffords after the controlled operation. For example, applying an X operation to a control qubit before and after a controlled operation will cause the operation to control on the Zero state ($\ket{0}$) for that qubit; applying an H operation will control on the PauliX Zero state $\ket{+} \mathrel{:=} (\ket{0} + \ket{1}) / \sqrt{2}$ rather than the PauliZ Zero state.

Given an operation expression, a new operation expression may be formed using the Controlled functor, with the base operation expression enclosed in parentheses, ( and ). The signature of the new operation is based on the signature of the base operation. The result type is the same, but the input type is a two-tuple with a qubit array that holds the control qubit(s) as the first element and the arguments of the base operation as the second element. If the base operation took no arguments, (), then the input type of the controlled version is just the array of control qubits. The new operation allows Controlled, and will allow Adjoint if and only if the base operation did.

If the base function took only a single argument, then singleton tuple equivalence will come into play here. For instance, Controlled(X) is the controlled version of the X operation. X has type (Qubit => () : Adjoint, Controlled), so Controlled(X) has type ((Qubit[], (Qubit)) => () : Adjoint, Controlled); because of singleton tuple equivalence, this is the same as ((Qubit[], Qubit) => () : Adjoint, Controlled).

Similarly, Controlled(Rz) is the controlled version of the Rz operation. Rz has type ((Double, Qubit) => () : Adjoint, Controlled), so Controlled(Rz) has type ((Qubit[], (Double, Qubit)) => () : Adjoint, Controlled). For example, ((Controlled(Rz))(controls, (0.1, target)) would be a valid invocation of Controlled(Rz).

As another example, CNOT(control, target) can be implemented as (Controlled(X))([control], target). If a target should be controlled by 2 control qubits (CCNOT), we can use (Controlled(X))([control1;control2], target) statement.