Operators

NameSyntaxAssociativityPrecedence
Attribute selectionattrset . attrpath [ or expr ]none1
Function applicationfunc exprleft2
Arithmetic negation- numbernone3
Has attributeattrset ? attrpathnone4
List concatenationlist ++ listright5
Multiplicationnumber * numberleft6
Divisionnumber / numberleft6
Subtractionnumber - numberleft7
Additionnumber + numberleft7
String concatenationstring + stringleft7
Path concatenationpath + pathleft7
Path and string concatenationpath + stringleft7
String and path concatenationstring + pathleft7
Logical negation (NOT)! boolnone8
Updateattrset // attrsetright9
Less thanexpr < exprnone10
Less than or equal toexpr <= exprnone10
Greater thanexpr > exprnone10
Greater than or equal toexpr >= exprnone10
Equalityexpr == exprnone11
Inequalityexpr != exprnone11
Logical conjunction (AND)bool && boolleft12
Logical disjunction (OR)bool || boolleft13
Logical implicationbool -> boolright14
Pipe operator (experimental)expr |> funcleft15
Pipe operator (experimental)func <| exprright15

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.

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

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

Arithmetic

Numbers are type-compatible: Pure integer operations will always return integers, whereas any operation involving at least one floating point number return a floating point number.

See also Comparison and Equality.

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

String concatenation

Syntax

string + string

Concatenate two strings and merge their string contexts.

Path concatenation

Syntax

path + path

Concatenate two paths. 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 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
  • lexicographic for strings and paths
  • item-wise lexicographic for lists: 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:

comparisonimplementation
a <= b! ( b < a )
a > bb < a
a >= b! ( a < b )

Equality

  • Attribute sets and lists are compared recursively, and therefore are fully evaluated.
  • Comparison of functions 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.

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 and may change in future releases.

To use this syntax, make sure the pipe-operators experimental feature is enabled. For example, include the following in nix.conf:

extra-experimental-features = pipe-operators