# 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 |
| Logical disjunction (`OR`) | *bool* <code>\|\|</code> *bool* | left | 13 |
| [Logical implication] | *bool* `->` *bool* | right | 14 |
| [Pipe operator] (experimental) | *expr* `\|>` *func* | left | 15 |
| [Pipe operator] (experimental) | *func* `<\|` *expr* | right | 15 |
<!-- 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.
## 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)
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.
## String concatenation
> **Syntax**
>
> *string* `+` *string*
Concatenate two [strings][string] and merge their [string contexts](./string-context.md).
## Path concatenation
> **Syntax**
>
> *path* `+` *path*
Concatenate two [paths][path].
The result is a path.
## 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].
## 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].
## 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.
## 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* `)` |
## Equality
- [Attribute sets][attribute set] and [lists][list] are compared recursively, and therefore are fully evaluated.
- Comparison of [functions][function] always returns `false`.
- Numbers are type-compatible, see [arithmetic] operators.
- Floating point numbers only differ up to a limited precision.
## Logical implication
Equivalent to `!`*b1* `||` *b2* (or `if` *b1* `then` *b2* `else true`)
## 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
> ```