nix/doc/manual/source/language/operators.md

# Operators

| Name                                   | Syntax                                     | Associativity | Precedence |
|----------------------------------------|--------------------------------------------|---------------|------------|
| [Attribute selection]                  | *attrset* `.` *attrpath* \[ `or` *expr* \] | none          | 1          |
| [Function application]                 | *func* *expr*                              | left          | 2          |
| [Arithmetic negation][arithmetic]      | `-` *number*                               | none          | 3          |
| [Has attribute]                        | *attrset* `?` *attrpath*                   | none          | 4          |
| List concatenation                     | *list* `++` *list*                         | right         | 5          |
| [Multiplication][arithmetic]           | *number* `*` *number*                      | left          | 6          |
| [Division][arithmetic]                 | *number* `/` *number*                      | left          | 6          |
| [Subtraction][arithmetic]              | *number* `-` *number*                      | left          | 7          |
| [Addition][arithmetic]                 | *number* `+` *number*                      | left          | 7          |
| [String concatenation]                 | *string* `+` *string*                      | left          | 7          |
| [Path concatenation]                   | *path* `+` *path*                          | left          | 7          |
| [Path and string concatenation]        | *path* `+` *string*                        | left          | 7          |
| [String and path concatenation]        | *string* `+` *path*                        | left          | 7          |
| Logical negation (`NOT`)               | `!` *bool*                                 | none          | 8          |
| [Update]                               | *attrset* `//` *attrset*                   | right         | 9          |
| [Less than][Comparison]                | *expr* `<` *expr*                          | none          | 10         |
| [Less than or equal to][Comparison]    | *expr* `<=` *expr*                         | none          | 10         |
| [Greater than][Comparison]             | *expr* `>` *expr*                          | none          | 10         |
| [Greater than or equal to][Comparison] | *expr* `>=` *expr*                         | none          | 10         |
| [Equality]                             | *expr* `==` *expr*                         | none          | 11         |
| Inequality                             | *expr* `!=` *expr*                         | none          | 11         |
| [Logical conjunction] (`AND`)          | *bool* `&&` *bool*                         | left          | [12](#precedence-and-disjunctive-normal-form)         |
| [Logical disjunction] (`OR`)           | *bool* <code>\|\|</code> *bool*            | left          | [13](#precedence-and-disjunctive-normal-form)         |
| [Logical implication]                  | *bool* `->` *bool*                         | right         | 14         |
| [Pipe operator] (experimental)         | *expr* `\|>` *func*                        | left          | 15         |
| [Pipe operator] (experimental)         | *func* `<\|` *expr*                        | right         | 15         |

[string]: ./types.md#type-string
[path]: ./types.md#type-path
[number]: ./types.md#type-float
[list]: ./types.md#type-list
[attribute set]: ./types.md#type-attrs

<!-- TODO(@rhendric, #10970): ^ rationalize number -> int/float -->

## Attribute selection

> **Syntax**
>
> *attrset* `.` *attrpath* \[ `or` *expr* \]

Select the attribute denoted by attribute path *attrpath* from [attribute set] *attrset*.
If the attribute doesn’t exist, return the *expr* after `or` if provided, otherwise abort evaluation.

[Attribute selection]: #attribute-selection

## Function application

> **Syntax**
>
> *func* *expr*

Apply the callable value *func* to the argument *expr*. Note the absence of any visible operator symbol.
A callable value is either:
- a [user-defined function][function]
- a [built-in][builtins] function
- an attribute set with a [`__functor` attribute](./syntax.md#attr-__functor)

> **Warning**
>
> [List][list] items are also separated by whitespace, which means that function calls in list items must be enclosed by parentheses.

## Has attribute

> **Syntax**
>
> *attrset* `?` *attrpath*

Test whether [attribute set] *attrset* contains the attribute denoted by *attrpath*.
The result is a [Boolean] value.

See also: [`builtins.hasAttr`](@docroot@/language/builtins.md#builtins-hasAttr)

[Boolean]: ./types.md#type-bool

[Has attribute]: #has-attribute

After evaluating *attrset* and *attrpath*, the computational complexity is O(log(*n*)) for *n* attributes in the *attrset*

## Arithmetic

Numbers will retain their type unless mixed with other numeric types:
Pure integer operations will always return integers, whereas any operation involving at least one floating point number returns a floating point number.

Evaluation of the following numeric operations throws an evaluation error:
- Division by zero
- Integer overflow, that is, any operation yielding a result outside of the representable range of [Nix language integers](./syntax.md#number-literal)

See also [Comparison] and [Equality].

The `+` operator is overloaded to also work on strings and paths.

[arithmetic]: #arithmetic

## String concatenation

> **Syntax**
>
> *string* `+` *string*

Concatenate two [strings][string] and merge their [string contexts](./string-context.md).

[String concatenation]: #string-concatenation

## Path concatenation

> **Syntax**
>
> *path* `+` *path*

Concatenate two [paths][path].
The result is a path.

[Path concatenation]: #path-concatenation

## Path and string concatenation

> **Syntax**
>
> *path* + *string*

Concatenate *[path]* with *[string]*.
The result is a path.

> **Note**
>
> The string must not have a [string context](./string-context.md) that refers to a [store path].

[Path and string concatenation]: #path-and-string-concatenation

## String and path concatenation

> **Syntax**
>
> *string* + *path*

Concatenate *[string]* with *[path]*.
The result is a string.

> **Important**
>
> The file or directory at *path* must exist and is copied to the [store].
> The path appears in the result as the corresponding [store path].

[store path]: @docroot@/store/store-path.md
[store]: @docroot@/glossary.md#gloss-store

[String and path concatenation]: #string-and-path-concatenation

## Update

> **Syntax**
>
> *attrset1* // *attrset2*

Update [attribute set] *attrset1* with names and values from *attrset2*.

The returned attribute set will have all of the attributes in *attrset1* and *attrset2*.
If an attribute name is present in both, the attribute value from the latter is taken.

This operator is [strict](@docroot@/language/evaluation.md#strictness) in both *attrset1* and *attrset2*.
That means that both arguments are evaluated to [weak head normal form](@docroot@/language/evaluation.md#values), so the attribute sets themselves are evaluated, but their attribute values are not evaluated.

[Update]: #update

## Comparison

Comparison is

- [arithmetic] for [numbers][number]
- lexicographic for [strings][string] and [paths][path]
- item-wise lexicographic for [lists][list]:
  elements at the same index in both lists are compared according to their type and skipped if they are equal.

All comparison operators are implemented in terms of `<`, and the following equivalencies hold:

| comparison   | implementation        |
|--------------|-----------------------|
| *a* `<=` *b* | `! (` *b* `<` *a* `)` |
| *a* `>`  *b* |       *b* `<` *a*     |
| *a* `>=` *b* | `! (` *a* `<` *b* `)` |

[Comparison]: #comparison

## Equality

- [Attribute sets][attribute set] are compared first by attribute names and then by items until a difference is found.
- [Lists][list] are compared first by length and then by items until a difference is found.
- Comparison of distinct [functions][function] returns `false`, but identical functions may be subject to [value identity optimization](#value-identity-optimization).
- Numbers are type-compatible, see [arithmetic] operators.
- Floating point numbers only differ up to a limited precision.

The `==` operator is [strict](@docroot@/language/evaluation.md#strictness) in both arguments; when comparing composite types ([attribute sets][attribute set] and [lists][list]), it is partially strict in their contained values: they are evaluated until a difference is found. <!-- this is woefully underspecified, affecting which expressions evaluate correctly; not just "ordering" or error messages. -->

### Value identity optimization

Nix performs equality comparisons of nested values by pointer equality or more abstractly, _identity_.
Nix semantics ideally do not assign a unique identity to values as they are created, but equality is an exception to this rule.
The disputable benefit of this is that it is more efficient, and it allows cyclical structures to be compared, e.g. `let x = { x = x; }; in x == x` evaluates to `true`.
However, as a consequence, it makes a function equal to itself when the comparison is made in a list or attribute set, in contradiction to a simple direct comparison.

[function]: ./syntax.md#functions

[Equality]: #equality

## Logical conjunction

> **Syntax**
>
> *bool1* `&&` *bool2*

Logical AND. Equivalent to `if` *bool1* `then` *bool2* `else false`.

This operator is [strict](@docroot@/language/evaluation.md#strictness) in *bool1*, but only evaluates *bool2* if *bool1* is `true`.

> **Example**
>
> ```nix
> true && false
> => false
>
> false && throw "never evaluated"
> => false
> ```

[Logical conjunction]: #logical-conjunction

## Logical disjunction

> **Syntax**
>
> *bool1* `||` *bool2*

Logical OR. Equivalent to `if` *bool1* `then true` `else` *bool2*.

This operator is [strict](@docroot@/language/evaluation.md#strictness) in *bool1*, but only evaluates *bool2* if *bool1* is `false`.

> **Example**
>
> ```nix
> true || false
> => true
>
> true || throw "never evaluated"
> => true
> ```

[Logical disjunction]: #logical-disjunction

### Precedence and disjunctive normal form

The precedence of `&&` and `||` aligns with disjunctive normal form.
Without parentheses, an expression describes multiple "permissible situations" (connected by `||`), where each situation consists of multiple simultaneous conditions (connected by `&&`).

For example, `A || B && C || D && E` is parsed as `A || (B && C) || (D && E)`, describing three permissible situations: A holds, or both B and C hold, or both D and E hold.

## Logical implication

> **Syntax**
>
> *bool1* `->` *bool2*

Logical implication. Equivalent to `!`*bool1* `||` *bool2* (or `if` *bool1* `then` *bool2* `else true`).

This operator is [strict](@docroot@/language/evaluation.md#strictness) in *bool1*, but only evaluates *bool2* if *bool1* is `true`.

> **Example**
>
> ```nix
> true -> false
> => false
>
> false -> throw "never evaluated"
> => true
> ```

[Logical implication]: #logical-implication

## Pipe operators

- *a* `|>` *b* is equivalent to *b* *a*
- *a* `<|` *b* is equivalent to *a* *b*

> **Example**
>
> ```
> nix-repl> 1 |> builtins.add 2 |> builtins.mul 3
> 9
>
> nix-repl> builtins.add 1 <| builtins.mul 2 <| 3
> 7
> ```

> **Warning**
>
> This syntax is part of an
> [experimental feature](@docroot@/development/experimental-features.md)
> and may change in future releases.
>
> To use this syntax, make sure the
> [`pipe-operators` experimental feature](@docroot@/development/experimental-features.md#xp-feature-pipe-operators)
> is enabled.
> For example, include the following in [`nix.conf`](@docroot@/command-ref/conf-file.md):
>
> ```
> extra-experimental-features = pipe-operators
> ```

[Pipe operator]: #pipe-operators
[builtins]: ./builtins.md
[Function application]: #function-application