# Statements and Other Constructs

Comments begin with two forward slashes, //, and continue until the end of line. While expressions can be split across multiple lines, end of line comments in the middle of an expression are not supported. A comment may appear anywhere in a Q# source file, including where statements are not valid.

Comments that begin with three forward slashes, ///, are treated specially by the compiler when they appear immediately before an operation, function, or type definition. In that case, their contents are taken as documentation for the defined callable or user-defined type, as for other .NET languages.

Within /// comments, text to appear as a part of API documentation is formatted as Markdown, with different parts of the documentation being indicated by specially named headers. As an extension to Markdown, cross references to operations, functions and user-defined types in Q# can be included using the @"<ref target>", where <ref target> is replaced by the fully qualified name of the code object being referenced. Optionally, a documentation engine may also support additional Markdown extensions.

For example:

/// # Summary
/// Given an operation and a target for that operation,
/// applies the given operation twice.
///
/// # Input
/// ## op
/// The operation to be applied.
/// ## target
/// The target to which the operation is to be applied.
///
/// # Type Parameters
/// ## 'T
/// The type expected by the given operation as its input.
///
/// # Example
/// Q#
/// // Should be equivalent to the identity.
/// ApplyTwice(H, qubit);
/// 
///
/// - Microsoft.Quantum.Primitive.H
operation ApplyTwice<'T>(op : ('T => ()), target : 'T) : () {
body {
op(target);
op(target);
}
}


The following names are recognized as documentation comment headers.

• Summary: A short summary of the behavior of a function or operation, or of the purpose of a type.
• Input: A description of the input tuple for an operation or function. May contain additional Markdown subsections indicating each individual element of the input tuple.
• Output: A description of the tuple returned by an operation or function.
• Type Parameters: An empty section which contains one additional subsection for each generic type parameter.
• Example: A short example of the operation, function or type in use.
• Remarks: Miscellaneous prose describing some aspect of the operation, function, or type.
• See Also: A list of fully qualified names indicating related functions, operations, or user-defined types.
• References: A list of references and citations for the item being documented.

## Namespaces

Every Q# operation, function, and user-defined type is defined within a namespace. Q# follows the same rules for naming as other .NET languages. However, Q# does not support nested namespaces. In particular, for two defined namespaces NS.Name1 and NS.Name2, only the fully qualified namespace can be opened - a directive open NS; is invalid.

Every Q# file must include at least one namespace directive. This consists of the namespace keyword, followed by the namespace name, an opening {, the construct definitions, and a closing }. All user-defined types, functions, and operations must appear inside of a namespace block; only comments may appear outside of a namespace block.

Within a namespace block, the open directive may be used to allow abbreviated reference to constructs from another namespace. This consists of the open keyword, followed by the namespace to be opened and a terminating semicolon. open directives must appear before any function, operation, or newtype directives in the namespace block. The open directive applies to the entire namespace block.

A reference to a construct defined in another namespace that is not opened in the current namespace must be by its fully-qualified name. For example, an operation named Op in the X.Y namespace must be referenced by its fully-qualified name X.Y.Op, unless the X.Y namespace has been opened earlier in the current block.

Generally it is preferable to include a namespace with an open directive. Using a fully-qualified name is required if two namespace define constructs with the same name, and the current source uses constructs from both.

## Formatting

Most Q# statements and directives end with a terminating semicolon, ;. Statements and directives such as for and operation that end with a statement block do not require a terminating semicolon. Each statement description notes whether the terminating semicolon is required.

Statements may be broken out across multiple lines. There must be a line-end between statements, so it is not possible to have multiple statements on a single line. Directives and declarations, such as operation declarations, may also span multiple lines.

## Statement Blocks

Q# statements are grouped into statement blocks. A statement block starts with an opening { and ends with a closing }.

A statement block that is lexically enclosed within another block is considered to be a sub-block of the containing block; containing and sub-blocks are also called outer and inner blocks.

## Symbol Binding and Assignment

Q# distinguishes between mutable and immutable symbols. In general, the use of immutable symbols is encouraged because it allows the compiler to perform more optimizations.

For arrays, mutability or immutability applies both to the array as a whole and to array elements. That is, the elements of an immutable array may not be changed.

The contents of a tuple instance or of a user-defined type based on a tuple may not be changed regardless of the mutability of the symbol bound to the instance. If a mutable symbol is bound to a tuple, the symbol may be bound to a new tuple, but the existing tuple may not be modified in place.

The arguments to an operation or a function are treated as immutable. This means that Q# does not support "out" variables. Also, the elements of an array passed to an operation or function may not be changed.

### Immutable Symbols

Immutable symbols are bound using the let statement. This is roughly equivalent to variable declaration and initialization in languages such as C#, except that Q# symbols, once bound, may not be changed; let bindings are immutable.

A simple binding statement consists of the keyword let, followed by an identifier, an equals sign =, an expression to bind the identifier to, and a terminating semicolon. The type of the identifier is defined to be the same as the type of the expression it is bound to.

For example, the statement

let i = 5;


binds the symbol i as an Int with the value 5.

If the right-hand side of the binding is a tuple or a user-defined type, then an extended syntax may be used to deconstruct the tuple:

let (i, f) = (5, 0.1);


This statement will bind 5 to i and 0.1 to f.

Deconstructing binds may also be used for more complex tuples, such as:

let (a, (b, c)) = (1, (2, 3))
let (x, y) = (1, (2, 3))


In these examples, a and x both get bound to 1, b gets bound to 2, c to 3, and y to (2, 3).

Tuple deconstruction can also be used when the right-hand side of the = is a tuple-valued expression:

let (r1, r2) = MeasureTwice(q1, PauliX, q2, PauliY);


### Mutable Symbols

Mutable symbols are defined and initialized using the mutable statement. This statement defines a new symbol binding and specifies that the bound value may be changed later in the code.

A mutable binding statement consists of the keyword mutable, followed by an identifier, an equals sign =, an expression to bind the identifier to, and a terminating semicolon. The type of the identifier is defined to be the same as the type of the expression it is bound to.

For example, the statement

mutable counter = 0;


defines the symbol counter as a mutable Int with initial value 0.

The mutable statement does not support tuple deconstruction.

If a mutable symbol is bound to an immutable array, a copy of the array is created and bound to the symbol. Modifying the elements of the mutable array will not change the contents of the original immutable array.

### Updating Mutable Symbols

The value bound to a mutable symbol may be changed by binding the symbol to a new value using the set statement.

A mutable rebinding statement consists of the keyword set, followed by an identifier, an equals sign =, an expression to rebind the identifier to, and a terminating semicolon. The new value must be compatible with the original type, and will be promoted to the original type.

For example, the statement

set counter = counter + 1;


increments the counter symbol from the mutable example.

The set statement is also used to set the value of an item in a mutable array:

set result[1] = One;


sets the second element of the result array to One. Note that the result array must have been defined in a mutable statement for this to be valid.

### Binding Scopes

In general, symbol bindings go out of scope and become inoperative at the end of the statement block they occur in. There are two exceptions to this rule:

• The binding of the loop variable of a for loop is in scope for the body of the for loop, but not after the end of the loop.
• All three portions of a repeat/until loop (the body, the test, and the fixup) are treated as a single scope, so symbols that are bound in the body are available in the test and in the fixup. For both types of loops, each pass through the loop executes in its own scope, so bindings from an earlier pass are not available in a later pass.

Symbol bindings from outer blocks are inherited by inner blocks. A symbol may only be bound once per block; it is illegal to bind a symbol that is already bound. Thus, the following sequences would be legal:

if a == b {
...
let n = 5;
...             // n is 5
}
let n = 8;
...                 // n is 8


and

if a == b {
...
let n = 5;
...             // n is 5
} else {
...
let n = 8;
...             // n is 8
}


But this would be illegal:

let n = 5;
...                 // n is 5
let n = 8;          // Error!!
...


as would:

let n = 8;
if a == b {
...             // n is 8
let n = 5;      // Error!
...
}
...


## Control Flow

### For-Loop

The for statement supports iteration through a simple integer range. The statement consists of the keyword for, followed by an identifier, the keyword in, a Range expression, and a statement block. The statement block (the body of the loop) is executed repeatedly, with the identifier (the loop variable) bound to each value in the range expression. Note that if the range expression evaluates to an empty range, the body will not be executed at all.

The range is fully evaluated before entering the loop, and will not change while the loop is executing.

For example,

for (index in 0 .. n-2) {
set results[index] = Measure([PauliX], [qubits[index]]);
}


The loop variable is bound at each entrance to the loop body, and unbound at the end of the body. In particular, the loop variable is not bound after the for loop is completed.

### Repeat-Until-Success Loop

The repeat statement supports the quantum “repeat until success” pattern. It consists of the keyword repeat, followed by a statement block (the loop body), the keyword until, a Boolean expression, the keyword fixup, and another statement block (the fixup). The loop body, condition, and fixup are all considered to be a single scope, so symbols bound in the body are available in the condition and fixup.

Note that the fixup block is required, even if there is no fixup to be done. In this case, the fixup should be a single expression statement, ().

The loop body is executed, and then the condition is evaluated. If the condition is true, then the statement is completed; otherwise, the fixup is executed, and the statement is re-executed starting with the loop body. Note that completing the execution of the fixup ends the scope for the statement, so that symbol bindings made during the body or fixup are not available in subsequent repetitions.

For example, the following code is a probabilistic circuit that implements an important rotation gate $V_3 = (\boldone + 2 i Z) / \sqrt{5}$ using the Hadamard and T gates. The loop terminates in 8/5 repetitions on average. See Repeat-Until-Success: Non-deterministic decomposition of single-qubit unitaries (Paetznick and Svore, 2014) for details.

using ancilla = Qubit[1] {
repeat {
let anc = ancilla[0];
H(anc);
T(anc);
CNOT(target,anc);
H(anc);
H(anc);
T(anc);
H(anc);
CNOT(target,anc);
T(anc);
Z(target);
H(anc);
let result = M([anc],[PauliZ]);
} until result == Zero
fixup {
();
}
}


### Conditional Statement

The if statement supports conditional execution. It consists of the keyword if, followed by a Boolean expression and a statement block (the then block). This may be followed by any number of else-if clauses, each of which consists of the keyword elif, followed by a Boolean expression and a statement block (the else-if block). Finally, the statement may optionally finish with an else clause, which consists of the keyword else followed by another statement block (the else block). The condition is evaluated, and if it is true, the then block is executed. If the condition is false, then the first else-if condition is evaluated; if it is true, that else-if block is executed. Otherwise, the second else-if block is tested, and then the third, and so on until either a clause with a true condition is encountered or there are no more else-if clauses. If the original if condition and all else-if clauses evaluate to false, the else block is executed if one was provided.

Note that whichever block is executed is executed in its own scope. Bindings made inside of a then, else-if, or else block are not visible after the end of the if statement.

For example,

if (result == One) {
X(target);
} else {
Z(target);
}


or

if (i == 1) {
X(target);
} elif (i == 2) {
Y(target);
} else {
Z(target);
}


### Return

The return statement ends execution of an operation or function and returns a value to the caller. It consists of the keyword return, followed by an expression of the appropriate type, and a terminating semicolon.

A callable that returns an empty tuple, (), does not require a return statement. If an early exit is desired, return () may be used in this case. Callables that return any other type require a final return statement.

There is no maximum number of return statements within an operation. The compiler may emit a warning if statements follow a return statement within a block.

For example,

return 1;


or

return ();


or

return (results, qubits);


### Fail

The fail statement ends execution of an operation and returns an error value to the caller. It consists of the keyword fail, followed by a string and a terminating semicolon. The string is returned to the classical driver as the error message.

There is no restriction on the number of fail statements within an operation. The compiler may emit a warning if statements follow a fail statement within a block.

For example,

fail $"Impossible state reached";  or fail$"Syndrome {syn} is incorrect";


## Qubit Management

Note that none of these statements are allowed within the body of a function.

### Clean Qubits

The using statement is used to acquire new qubits for use during a statement block. The qubits are guaranteed to be initialized to the computational Zero state. The qubits should be in the computational Zero state at the end of the statement block; simulators are encouraged to enforce this.

The statement consists of the keyword using, followed by the symbol that should be bound to the resulting array of qubits, =, the type to acquire (Qubit), [, an Int expression, ], and the statement block within which the qubits will be available.

For example,

using (qubits = Qubit[bits * 2 + 3]) {
...
}


### Dirty Qubits

The borrowing statement is used to allocate qubits for temporary use during a statement block. The borrower commits to leaving the qubits in the same state they were in when they were borrowed. Such qubits are often known as “dirty ancilla”. See Factoring using 2n+2 qubits with Toffoli based modular multiplication (Haner, Roetteler, and Svore 2017) for an example of dirty ancilla use.

When borrowing qubits, the system will first try to fill the request from qubits that are in use but that are not accessed during the body of the borrowing statement. If there aren't enough such qubits, then it will allocate new qubits to complete the request.

The statement consists of the keyword borrowing, followed by the symbol that should be bound to the resulting array of qubits, =, the type to acquire (Qubit), [, an Int expression, ], and the statement block within which the qubits will be available.

For example,

borrowing qubits = Qubit[bits * 2 + 3] {
...
}


## Expression Evaluation Statements

Any valid Q# expression of type () may be evaluated as a statement. This is primarily of use when calling operations on qubits that return () because the purpose of the statement is to modify the implicit quantum state. Expression evaluation statements require a terminating semicolon.

For example,

X(q);


or

CNOT(control, target);


or

(Adjoint T)(q1);