1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
|
# 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 |
[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.
[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] 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.
[function]: ./syntax.md#functions
[Equality]: #equality
## Logical implication
Equivalent to `!`*b1* `||` *b2*.
[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
|
