Operators
| Name | Syntax | Associativity | Precedence |
|---|---|---|---|
| Attribute selection | attrset . attrpath [ or expr ] | none | 1 |
| Function application | func expr | left | 2 |
| Arithmetic negation | - number | none | 3 |
| Has attribute | attrset ? attrpath | none | 4 |
| List concatenation | list ++ list | right | 5 |
| Multiplication | number * number | left | 6 |
| Division | number / number | left | 6 |
| Subtraction | number - number | left | 7 |
| Addition | 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 | expr < expr | none | 10 |
| Less than or equal to | expr <= expr | none | 10 |
| Greater than | expr > expr | none | 10 |
| Greater than or equal to | 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 || bool | left | 13 |
| Logical implication | bool -> bool | right | 14 |
Attribute selection
Syntax
attrset
.attrpath [orexpr ]
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.
An attribute path is a dot-separated list of attribute names.
Syntax
attrpath = name [
.name ]...
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 of all 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:
| comparison | implementation |
|---|---|
a <= b | ! ( b < a ) |
a > b | b < 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.