# Expressions

## Grouping

Given any expression, that same expression enclosed in parentheses
is an expression of the same type.
For instance, `(7)`

is an `Int`

expression,
`([1,2,3])`

is an expression of type array of `Int`

s,
and `((1,2))`

is an expression with type `(Int, Int)`

.

The equivalence between simple values and single-element tuples described in
the type model removes the ambiguity
between `(6)`

as a group and `(6)`

as a single-element tuple.

## Symbols

The name of a symbol bound or assigned to a value of type `'T`

is an expression of type `'T`

.
For instance, if the symbol `count`

is bound to the integer value `5`

,
then `count`

is an integer expression.

## Numeric Expressions

Numeric expressions are expressions of type `Int`

or `Double`

.
That is, they are either integer or floating-point numbers.

`Int`

literals in Q# are identical to integer literals in C#,
except that no trailing "l" or "L" is required (or allowed).
Hexadecimal integers are supported with a "0x" prefix.

`Double`

literals in Q# are identical to double literals in C#,
except that no trailing "d" or "D" is required (or allowed).

Given an array expression of any element type, an `Int`

expression
may be formed using the `Length`

built-in function, with the array
expression enclosed in parentheses, `(`

and `)`

.
For instance, if `a`

is bound to an array, then `Length(a)`

is
an integer expression.
If `b`

is an array of arrays of integers, `Int[][]`

, then
`Length(b)`

is the number of sub-arrays in `b`

, and `Length(b[1])`

is the number of integers in the second sub-array in `b`

.

Given two numeric expressions, the binary operators `+`

, `-`

, `*`

,
and `/`

may be used to form a new numeric expression.
The type of the new expression will be `Double`

if both of the
constituent expressions are `Double`

, or will be an `Int`

expression
if both are integers.

Given two integer expressions, a new integer expression may be formed
using the `%`

(modulus), `^`

(power), `&&&`

(bitwise AND), `|||`

(bitwise OR),
`^^^`

(bitwise XOR), `<<<`

(arithmetic left shift), or `>>>`

(arithmetic right shift) operations.
The second parameter to either shift operation must be greater than or
equal to zero.
The behavior for shifting negative numbers is undefined.

Integer division and integer modulus follow the same behavior for
negative numbers as C#.
That is, `a % b`

will always have the same sign as `a`

,
and `b * (a / b) + a % b`

will always equal `a`

.
For example:

`A` |
`B` |
`A / B` |
`A % B` |
---|---|---|---|

5 | 2 | 2 | 1 |

5 | -2 | -2 | 1 |

-5 | 2 | -2 | -1 |

-5 | -2 | 2 | -1 |

Given any numeric expression, a new expression may be formed using the
`-`

unary operator.
The new expression will be the same type as the constituent expression.

Given any integer expression, a new integer expression may be formed
using the `~~~`

(bitwise complement) unary operator.

## Boolean Expressions

The two `Bool`

literal values are `true`

and `false`

.

Given any two expressions of the same primitive type, the `==`

and `!=`

binary operators may be used to construct a `Bool`

expression.
The expression will be true if the two expressions are (resp. are not) equal.

ðŸ†• Values of user-defined types may not be compared. The unwrap operator must be used to compare user-defined type values for types based on primitive types. For example,

```
newtype WrappedInt = Int; // Yes, this is a contrived example
let x = WrappedInt(1);
let y = WrappedInt(2);
let z = x! == y!; // This will compile and yield z = false.
let t = x == y; // This will cause a compiler error.
```

Equality comparison for `Qubit`

values is identity equality;
that is, whether the two expressions identify the same qubit.
The state of the two qubits are not compared, accessed, measured, or modified
by this comparison.

Equality comparison for `Double`

values may be misleading
due to rounding effects.
For instance, `49.0 * (1.0/49.0) != 1.0`

.

Given any two numeric expressions, the binary operators
`>`

, `<`

, `>=`

, and `<=`

may be used to construct a new Boolean expression
that is true if the first expression is respectively greater than, less than,
greater than or equal to, or less than or equal to the second expression.

Given any two Boolean expressions, the `&&`

and `||`

binary operators
may be used to construct a new Boolean expression that is true if both of
(resp. either or both of) the two expressions are true.

Given any Boolean expression, the `not`

unary operator may be used to construct
a new Boolean expression that is true if the constituent expression is false.

## String Expressions

Q# allows strings to be used in the `fail`

statement and the `Log`

standard function.

Strings in Q# are either literals or interpolated strings.
String literals are similar to simple string literals in most languages:
a sequence of Unicode characters enclosed in double quotes, `"`

.
Inside of a string, the back-slash character `\`

may be used to escape
a double quote character, and to insert a new-line as `\n`

, a carriage
return as `\r`

, and a tab as `\t`

.
For instance:

```
"\"Hello world!\", she said.\n"
```

The Q# syntax for string interpolations is a subset of the C# 7.0 syntax;
Q# does not support verbatim (multi-line) interpolated strings.
See *Interpolated Strings*
for the C# syntax.

Expressions inside of an interpolated string follow Q# syntax, not C# syntax. Any valid Q# expression may appear in an interpolated string.

## Qubit Expressions

The only `Qubit`

expressions are symbols that are bound to `Qubit`

values
or array elements of `Qubit`

arrays.
There are no `Qubit`

literals.

## Pauli Expressions

The four `Pauli`

values, `PauliI`

, `PauliX`

, `PauliY`

, and `PauliZ`

,
are all valid `Pauli`

expressions.

Other than that, the only `Pauli`

expressions are symbols that are
bound to `Pauli`

values or array elements of `Pauli`

arrays.

## Result Expressions

The two `Result`

values, `One`

and `Zero`

, are valid `Result`

expressions.

Other than that, the only `Result`

expressions are symbols that are
bound to `Result`

values or array elements of `Result`

arrays.
In particular, note that `One`

is not the same as the integer `1`

,
and there is no direct conversion between them.
The same is true for `Zero`

and `0`

.

## Range Expressions

Given any three `Int`

expressions `start`

, `step`

, and `stop`

,
`start .. step .. stop`

is a range expression whose first element is `start`

,
second element is `start+step`

, third element is `start+step+step`

, etc.,
until `stop`

is passed.
A range may be empty if, for instance, `step`

is positive and `stop < start`

.
The last element of the range will be `stop`

if the difference between `start`

and `stop`

is an integral multiple of `step`

; that is,
the range is inclusive at both ends.

Given any two `Int`

expressions `start`

and `stop`

, `start .. stop`

is a
range expression that is equal to `start .. 1 .. stop`

.
Note that the implied `step`

is +1 even if `stop`

is less than `start`

;
in such a case, the range is empty.

Some example ranges are:

`1..3`

is the range 1, 2, 3.`2..2..5`

is the range 2, 4.`2..2..6`

is the range 2, 4, 6.`6..-2..2`

is the range 6, 4, 2.`2..1`

is the empty range.`2..6..7`

is the range 2.`2..2..1`

is the empty range.`1..-1..2`

is the empty range.

## Callable Expressions

A callable literal is the name of an operation or function defined in the
compilation scope.
For instance, `X`

is an operation literal that refers to the
standard library `X`

operation, and `Message`

is a function literal that
refers to the standard library `Message`

function.

If an operation supports the `Adjoint`

functor, then `Adjoint op`

is an operation expression.
Similarly, if the operation supports the `Controlled`

functor, then
`Controlled op`

is an operation expression.
The types of these expressions are specified in
Functors.

ðŸ†• Functor modifiers (`Adjoint`

and `Controlled`

) bind more closely than
all other operators, except for the unwrap operator `!`

and array indexing
with `[]`

.
Thus, the following are all legal, assuming that the operations support the
functors used:

```
Adjoint Op(qs)
Controlled Op(controls, targets)
Controlled Adjoint Op(controls, targets)
Adjoint WrappedOp!(qs)
```

ðŸ†• A callable literal may be used as a value, say to assign to a variable or to pass to another callable. In this case, if the callable has type parameters, they must be provided as part of the callable value. A callable value cannot have any unspecified type parameters.

For instance, if `Fun`

is a function with signature `'T1->Unit`

:

```
let f = Fun<Int>; // f is Int->Unit.
SomeOtherFun(Fun<Double>); // A Double->Unit is passed to SomOtherFun.
let g = Fun; // This causes a compilation error.
SomeOtherFun(Fun); // This also causes a compilation error.
```

## Callable Invocation Expressions

Given a callable (operation or function) expression and a tuple expression of the input type of the callable's signature, an invocation expression may be formed by appending the tuple expression to the callable expression. The type of the invocation expression is the output type of the callable's signature.

For example, if `Op`

is an operation with signature
`((Int, Qubit) => Double)`

, `Op(3, qubit1)`

is an expression of type `Double`

.
Similarly, if `Sin`

is a function with signature `(Double -> Double)`

,
`Sin(0.1)`

is an expression of type `Double`

.
Finally, if `Builder`

is a function with signature
`(Int -> (Int -> Int))`

, then `Builder(3)`

is a function from Into to Int.

Invoking the result of a callable-valued expression requires an extra pair
of parentheses around the callable expression.
Thus, to invoke the result of calling `Builder`

from the previous paragraph,
the correct syntax is:

```
(Builder(3))(2)
```

ðŸ†• When invoking a type-parameterized callable, the actual type parameters
may be specified within angle brackets `<`

and `>`

after the callable
expression.
This is usually unnecessary as the Q# compiler will infer the actual types.
It is required for partial application (see below) if a type-parameterized
argument is left unspecified.
It is also sometimes useful when passing operations with different functor
supports to a callable.

For instance, if `Func`

has signature `('T1, 'T2, 'T1) -> 'T2`

, `Op1`

and
`Op2`

have signature `(Qubit[] => Unit : Adjoint)`

, and `Op3`

has signature
`(Qubit[] => Unit)`

, to invoke `Func`

with `Op1`

as the first argument, `Op2`

as the second, and `Op3`

as the third:

```
let combinedOp = Func<(Qubit[] => Unit), (Qubit[] => Unit : Adjoint)>(Op1, Op2, Op3);
```

The type specification is required because `Op3`

and `Op1`

have different
types, so the compiler will treat this as ambiguous without the specification.

### Partial Application

Given a callable expression, a new callable may be created by providing a
subset of the arguments to the callable.
This is called *partial application*.

In Q#, a partially applied callable is expressed by writing a normal
invocation expression, but using an underscore, `_`

, for the unspecified
arguments.
The resulting callable has the same result type as the base callable,
and the same specializations for operations.
The input type of the partial application is simply the original type
with the specified arguments removed.

If a mutable variable is passed as a specified argument when creating a partial application, the current value of the variable is used. Changing the value of the variable afterward will not impact the partial application.

For example, if `Op`

has type
`((Int, ((Qubit, Qubit), Double)) => Unit : Adjoint)`

:

`Op(5,(_,_))`

has type`(((Qubit,Qubit), Double) => Unit:Adjoint)`

, and so has`Op(5,_)`

.`Op(_,(_,1.0))`

has type`((Int, (Qubit,Qubit)) => Unit:Adjoint)`

.`Op(_,((q1,q2),_))`

has type`((Int,Double) => Unit:Adjoint)`

. Note that we have applied singleton tuple equivalence here.

ðŸ†• If the partially-applied callable has type parameters that cannot be inferred by the compiler, they must be provided at the invocation site. The partial application cannot have any unspecified type parameters.

For example, if `Op`

has type
`(('T1, Qubit, 'T1) => Unit : Adjoint)`

:

```
let f1 = Op<Int>(_, qb, _); // f1 is Int=>Unit:Adjoint
let f2 = Op(5, qb, _); // f2 is Int=>Unit:Adjoint
let f3 = Op(_,qb, _); // f3 generates a compilation error
```

### Recursion

Q# callables are allowed to be directly or indirectly recursive. That is, an operation or function may call itself, or it may call another callable that directly or indirectly calls the callable operation.

There are two important comments about the use of recursion, however:

- The use of recursion in operations is likely to interfere with certain optimizations. This may have a substantial impact on the execution time of the algorithm.
- When executing on an actual quantum device, stack space may be limited, and so deep recursion may lead to a runtime error. In particular, the Q# compiler and runtime do not identify and optimize tail recursion.

## Tuple Expressions

A tuple literal is a sequence of element expressions of the appropriate type,
separated by commas, enclosed in `(`

and `)`

.
For instance, `(1, One)`

is an `(Int, Result)`

expression.

Other than literals, the only tuple expressions are symbols that are bound to tuple values, array elements of tuple arrays, and callable invocations that return tuples.

## User-Defined Type Expressions

A literal of a user-defined type consists of the type name followed by a
tuple literal of the typeâ€™s base tuple type.
For instance, if `IntPair`

is a user-defined type based on `(Int, Int)`

,
then `IntPair(2,3)`

would be a valid literal of that type.

Other than literals, the only expressions of a user-defined type are symbols that are bound to values of that type, array elements of arrays of that type, and callable invocations that return that type.

## ðŸ†• Unwrap Expressions

Important

Q# 0.3 introduces a significant difference in the behavior of user-defined types.

To access the base value of a user-defined type value, the value must be
unwrapped.
In Q#, the unwrap operator is a trailing exclamation mark `!`

.
For instance, if `IntPair`

is a user-defined type based on `(Int, Int)`

,
and `s`

was a variable with value `IntPair(2,3)`

, then `s!`

would be `(2,3)`

.

For user-defined types defined in terms of other user-defined types. the
unwrap operator may be repeated; for instance, `s!!`

indicates the
doubly-unwrapped value of `s`

.
Thus, if `WrappedPair`

is a user-defined type based on `IntPair`

, and
`t`

is a variable with value `WrappedPair(IntPair(1,2))`

, then `t!!`

would
be `(1,2)`

.

The `!`

operator has higher precedence than all other operators other than
`[]`

for array indexing and slicing.
`!`

and `[]`

bind positionally; that is, `a[i]![3]`

should be read as
`((a[i])!)[3]`

: take the `i`

'th element of `a`

, unwrap it, and then
get the 3rd element of the unwrapped value (which must be an array).

The precedence of the `!`

operator has one impact that might not be obvious.
If a function or operation returns a value that then gets unwrapped, the
function or operation call must be enclosed in parentheses so that the
argument tuple binds to the call rather than to the unwrap.
For example:

```
let f = (Foo(arg))!; // Calls Foo(arg), then unwraps the result
let g = Foo(arg)!; // Syntax error
```

## Array Expressions

An array literal is a sequence of one or more element expressions,
separated by ðŸ†• commas ~~semi-colons~~, enclosed in `[`

and `]`

.
All elements must be compatible with the same type.

If the common element type is an operation or function type, all of the
elements must have the same input and output types.
The element type of the array will support any functors that are supported
by all of the elements.
For example, if `Op1`

, `Op2`

, and `Op3`

all are `Qubit[] => Unit`

, but `Op1`

supports `Adjoint`

, `Op2`

supports `Controlled`

, and `Op3`

supports both:

`[Op1, Op2]`

is an array of`(Qubit[] => Unit)`

operations.`[Op1, Op3]`

is an array of`(Qubit[] => Unit) : Adjoint`

operations.`[Op2, Op3]`

is an array of`(Qubit[] => Unit) : Controlled)`

operations.

Empty array literals, `[]`

, are not allowed.
Instead using `new â˜…[0]`

,
where `â˜…`

is as placeholder for a suitable type, allows to create the
desired array of length zero.

Given two arrays of the same type, the binary `+`

operator may be used to
form a new array that is the concatenation of the two arrays.
For instance, `[1,2,3] + [4,5,6]`

is `[1,2,3,4,5,6]`

.

### Array Creation

Given a type and an `Int`

expression, the `new`

operator may be used
to allocate a new array of the given size.
For instance, `new Int[i+1]`

would allocate a new `Int`

array with
`i+1`

elements.

The elements of a new array are initialized to a type-dependent default value. In most cases this is some variation of zero.

For qubits and callables, which are references to entities, there is no
reasonable default value.
Thus, for these types, the default is an invalid
reference that cannot be used without causing a runtime error.
This is similar to a null reference in languages such as C# or Java.
Arrays containing qubits or callables must be filled in using
`set`

statements before their elements may be safely used.
Array elements can only be set if the array is declared as mutable, e.g.
`mutable array = new Int[5]`

.
Arrays passed as arguments are immutable.

The default values for each type are:

Type | Default |
---|---|

`Int` |
`0` |

`Double` |
`0.0` |

`Bool` |
`false` |

`String` |
`""` |

`Qubit` |
invalid qubit |

`Pauli` |
`PauliI` |

`Result` |
`Zero` |

`Range` |
The empty range, `1..1..0` |

`Callable` |
invalid callable |

`Array['T]` |
`'T[0]` |

Tuple types are initialized element-by-element.

Array creation is primarily of use initializing mutable arrays, on the
right-hand side of a
`mutable`

statement.

### Array Slices

Given an array expression and a `Range`

expression, a new expression
may be formed using the `[`

and `]`

array slice operator.
The new expression will be the same type as the array and will contain
the array items indexed by the elements of the `Range`

,
in the order defined by the `Range`

.
For instance, if `a`

is bound to an array of `Double`

s,
then `a[3..-1..0]`

is a `Double[]`

expression that contains the first four
elements of `a`

but in the reverse order as they appear in `a`

.

If the `Range`

is empty, then the resulting array slice will be zero length.

If the array expression is not a simple identifier, it must be enclosed
it parentheses in order to slice.
For instance, if `a`

and `b`

are both arrays of `Int`

s, a slice from the
concatenation would be expressed as:

```
(a+b)[1..2..7]
```

All arrays in Q# are zero-based.
That is, the first element of an array `a`

is always `a[0]`

.

## Array Element Expressions

Given an array expression and an `Int`

expression, a new expression
may be formed using the `[`

and `]`

array element operator.
The new expression will be the same type as the element type of the array.
For instance, if `a`

is bound to an array of `Double`

s,
then `a[4]`

is a `Double`

expression.

If the array expression is not a simple identifier, it must be enclosed
it parentheses in order to select an element.
For instance, if `a`

and `b`

are both arrays of `Int`

s, an element from the
concatenation would be expressed as:

```
(a+b)[13]
```

All arrays in Q# are zero-based.
That is, the first element of an array `a`

is always `a[0]`

.

## ðŸ†• Conditional Expressions

Given two other expressions of the same type and a Boolean expression,
the conditional expression may be formed using the question mark `?`

and
the vertical bar `|`

.
For instance, `a==b ? c | d`

.
In this example, the value of the conditional expression will be `c`

if
`a==b`

is true and `d`

if it is false.

The two expressions may evaluate to operations that have the same inputs
and outputs but support different functors.
In this case, the type of the conditional expression is an operation with
those inputs and outputs that supports any functors supported by both
expressions.
For example, if `Op1`

, `Op2`

, and `Op3`

all are `Qubit[]=>Unit`

, but `Op1`

supports `Adjoint`

, `Op2`

supports `Controlled`

, and `Op3`

supports both:

`flag ? Op1 | Op2`

is a`(Qubit[]=>Unit)`

operation.`flag ? Op1 | Op3`

is a`(Qubit[]=>Unit):Adjoint`

operation.`flag ? Op2 | Op3`

is a`(Qubit[]=>Unit:Controlled)`

operation.

If either of the two possible result expressions include a function or
operation call, that call will only take place if that result is the one
that will be the value of the call.
For instance, in the case `a==b ? C(qs) | D(qs)`

, if `a==b`

is true then
the `C`

operation will be invoked, and if it is false then only `D`

will
be invoked.
This is similar to short-circuiting in other languages.

## Operator Precedence

All binary operators are right-associative, except for `^`

.

Brackets, `[`

and `]`

, for array slicing and indexing,
bind before any operator.

The functors `Adjoint`

and `Controlled`

bind after array indexing
but before all other operators.

Parentheses for operation and function invocation also bind before any operator but after array indexing and functors.

Operators in order of precedence, from highest to lowest:

Operator | Arity | Description | Operand Types |
---|---|---|---|

trailing `!` |
Unary | Unwrap | Any user-defined type |

`-` , `~~~` ,`not` |
Unary | Numeric negative, bitwise complement, logical negation | `Int` or `Double` for `-` , `Int` for `~~~` , `Bool` for `not` |

`^` |
Binary | Integer power | `Int` |

`/` , `*` , `%` |
Binary | Division, multiplication, integer modulus | `Int` or `Double` for `/` and `*` , `Int` for `%` |

`+` , `-` |
Binary | Addition or string and array concatenation, subtraction | `Int` or `Double` , additionally `String` or any array type for `+` |

`<<<` , `>>>` |
Binary | Left shift, right shift | `Int` |

`<` , `<=` , `>` , `>=` |
Binary | Less-than, less-than-or-equal, greater-than, greater-than-or-equal comparisons | `Int` or `Double` |

`==` , `!=` |
Binary | equal, not-equal comparisons | any primitive type |

`&&&` |
Binary | Bitwise AND | `Int` |

`^^^` |
Binary | Bitwise XOR | `Int` |

` | ` | ||

`&&` |
Binary | Logical AND | `Bool` |

` | ` | Binary |