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
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
|
# Nix Language
The Nix language is a pure, lazy, functional language. Purity
means that operations in the language don't have side-effects (for
instance, there is no variable assignment). Laziness means that
arguments to functions are evaluated only when they are needed.
Functional means that functions are “normal” values that can be passed
around and manipulated in interesting ways. The language is not a
full-featured, general purpose language. Its main job is to describe
packages, compositions of packages, and the variability within packages.
This section presents the various features of the language.
# Syntax Summary
Below is a summary of the most important syntactic constructs in the Nix
expression language. It's not complete. In particular, there are many
other built-in functions. See the [Nix
manual](https://nixos.org/nix/manual/#chap-writing-nix-expressions) for
the rest.
<table>
<tr>
<th>
Example
</th>
<th>
Description
</th>
</tr>
<tr>
<td>
*Basic values*
</td>
<td>
</td>
</tr>
<tr>
<td>
`"Hello world"`
</td>
<td>
A string
</td>
</tr>
<tr>
<td>
`"${pkgs.bash}/bin/sh"`
</td>
<td>
A string containing an expression (expands to `"/nix/store/hash-bash-version/bin/sh"`)
</td>
</tr>
<tr>
<td>
`true`, `false`
</td>
<td>
Booleans
</td>
</tr>
<tr>
<td>
`123`
</td>
<td>
An integer
</td>
</tr>
<tr>
<td>
`./foo.png`
</td>
<td>
A path (relative to the containing Nix expression)
</td>
</tr>
<tr>
<td>
*Compound values*
</td>
<td>
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; }`
</td>
<td>
A set with attributes named `x` and `y`
</td>
</tr>
<tr>
<td>
`{ foo.bar = 1; }`
</td>
<td>
A nested set, equivalent to `{ foo = { bar = 1; }; }`
</td>
</tr>
<tr>
<td>
`rec { x = "foo"; y = x + "bar"; }`
</td>
<td>
A recursive set, equivalent to `{ x = "foo"; y = "foobar"; }`
</td>
</tr>
<tr>
<td>
`[ "foo" "bar" ]`
</td>
<td>
A list with two elements
</td>
</tr>
<tr>
<td>
*Operators*
</td>
<td>
</td>
</tr>
<tr>
<td>
`"foo" + "bar"`
</td>
<td>
String concatenation
</td>
</tr>
<tr>
<td>
`1 + 2`
</td>
<td>
Integer addition
</td>
</tr>
<tr>
<td>
`"foo" == "f" + "oo"`
</td>
<td>
Equality test (evaluates to `true`)
</td>
</tr>
<tr>
<td>
`"foo" != "bar"`
</td>
<td>
Inequality test (evaluates to `true`)
</td>
</tr>
<tr>
<td>
`!true`
</td>
<td>
Boolean negation
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; }.x`
</td>
<td>
Attribute selection (evaluates to `1`)
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; }.z or 3`
</td>
<td>
Attribute selection with default (evaluates to `3`)
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; } // { z = 3; }`
</td>
<td>
Merge two sets (attributes in the right-hand set taking precedence)
</td>
</tr>
<tr>
<td>
*Control structures*
</td>
<td>
</td>
</tr>
<tr>
<td>
`if 1 + 1 == 2 then "yes!" else "no!"`
</td>
<td>
Conditional expression
</td>
</tr>
<tr>
<td>
`assert 1 + 1 == 2; "yes!"`
</td>
<td>
Assertion check (evaluates to `"yes!"`).
</td>
</tr>
<tr>
<td>
`let x = "foo"; y = "bar"; in x + y`
</td>
<td>
Variable definition
</td>
</tr>
<tr>
<td>
`with pkgs.lib; head [ 1 2 3 ]`
</td>
<td>
Add all attributes from the given set to the scope (evaluates to `1`)
</td>
</tr>
<tr>
<td>
*Functions (lambdas)*
</td>
<td>
</td>
</tr>
<tr>
<td>
`x: x + 1`
</td>
<td>
A function that expects an integer and returns it increased by 1
</td>
</tr>
<tr>
<td>
`(x: x + 1) 100`
</td>
<td>
A function call (evaluates to 101)
</td>
</tr>
<tr>
<td>
`let inc = x: x + 1; in inc (inc (inc 100))`
</td>
<td>
A function bound to a variable and subsequently called by name (evaluates to 103)
</td>
</tr>
<tr>
<td>
`{ x, y }: x + y`
</td>
<td>
A function that expects a set with required attributes `x` and `y` and concatenates them
</td>
</tr>
<tr>
<td>
`{ x, y ? "bar" }: x + y`
</td>
<td>
A function that expects a set with required attribute `x` and optional `y`, using `"bar"` as default value for `y`
</td>
</tr>
<tr>
<td>
`{ x, y, ... }: x + y`
</td>
<td>
A function that expects a set with required attributes `x` and `y` and ignores any other attributes
</td>
</tr>
<tr>
<td>
`{ x, y } @ args: x + y`
</td>
<td>
A function that expects a set with required attributes `x` and `y`, and binds the whole set to `args`
</td>
</tr>
<tr>
<td>
*Built-in functions*
</td>
<td>
</td>
</tr>
<tr>
<td>
`import ./foo.nix`
</td>
<td>
Load and return Nix expression in given file
</td>
</tr>
<tr>
<td>
`map (x: x + x) [ 1 2 3 ]`
</td>
<td>
Apply a function to every element of a list (evaluates to `[ 2 4 6 ]`)
</td>
</tr>
</table>
|
