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1<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-nix-syntax-summary">
2 <title>Syntax Summary</title>
3 <para>
4 Below is a summary of the most important syntactic constructs in the
5 Nix expression language. It’s not complete. In particular, there are
6 many other built-in functions. See the
7 <link xlink:href="https://nixos.org/nix/manual/#chap-writing-nix-expressions">Nix
8 manual</link> for the rest.
9 </para>
10 <informaltable>
11 <tgroup cols="2">
12 <colspec align="left" />
13 <colspec align="left" />
14 <thead>
15 <row>
16 <entry>
17 Example
18 </entry>
19 <entry>
20 Description
21 </entry>
22 </row>
23 </thead>
24 <tbody>
25 <row>
26 <entry>
27 <emphasis>Basic values</emphasis>
28 </entry>
29 <entry>
30 </entry>
31 </row>
32 <row>
33 <entry>
34 <literal>"Hello world"</literal>
35 </entry>
36 <entry>
37 A string
38 </entry>
39 </row>
40 <row>
41 <entry>
42 <literal>"${pkgs.bash}/bin/sh"</literal>
43 </entry>
44 <entry>
45 A string containing an expression (expands to
46 <literal>"/nix/store/hash-bash-version/bin/sh"</literal>)
47 </entry>
48 </row>
49 <row>
50 <entry>
51 <literal>true</literal>, <literal>false</literal>
52 </entry>
53 <entry>
54 Booleans
55 </entry>
56 </row>
57 <row>
58 <entry>
59 <literal>123</literal>
60 </entry>
61 <entry>
62 An integer
63 </entry>
64 </row>
65 <row>
66 <entry>
67 <literal>./foo.png</literal>
68 </entry>
69 <entry>
70 A path (relative to the containing Nix expression)
71 </entry>
72 </row>
73 <row>
74 <entry>
75 <emphasis>Compound values</emphasis>
76 </entry>
77 <entry>
78 </entry>
79 </row>
80 <row>
81 <entry>
82 <literal>{ x = 1; y = 2; }</literal>
83 </entry>
84 <entry>
85 A set with attributes named <literal>x</literal> and
86 <literal>y</literal>
87 </entry>
88 </row>
89 <row>
90 <entry>
91 <literal>{ foo.bar = 1; }</literal>
92 </entry>
93 <entry>
94 A nested set, equivalent to
95 <literal>{ foo = { bar = 1; }; }</literal>
96 </entry>
97 </row>
98 <row>
99 <entry>
100 <literal>rec { x = "foo"; y = x + "bar"; }</literal>
101 </entry>
102 <entry>
103 A recursive set, equivalent to
104 <literal>{ x = "foo"; y = "foobar"; }</literal>
105 </entry>
106 </row>
107 <row>
108 <entry>
109 <literal>[ "foo" "bar" ]</literal>
110 </entry>
111 <entry>
112 A list with two elements
113 </entry>
114 </row>
115 <row>
116 <entry>
117 <emphasis>Operators</emphasis>
118 </entry>
119 <entry>
120 </entry>
121 </row>
122 <row>
123 <entry>
124 <literal>"foo" + "bar"</literal>
125 </entry>
126 <entry>
127 String concatenation
128 </entry>
129 </row>
130 <row>
131 <entry>
132 <literal>1 + 2</literal>
133 </entry>
134 <entry>
135 Integer addition
136 </entry>
137 </row>
138 <row>
139 <entry>
140 <literal>"foo" == "f" + "oo"</literal>
141 </entry>
142 <entry>
143 Equality test (evaluates to <literal>true</literal>)
144 </entry>
145 </row>
146 <row>
147 <entry>
148 <literal>"foo" != "bar"</literal>
149 </entry>
150 <entry>
151 Inequality test (evaluates to <literal>true</literal>)
152 </entry>
153 </row>
154 <row>
155 <entry>
156 <literal>!true</literal>
157 </entry>
158 <entry>
159 Boolean negation
160 </entry>
161 </row>
162 <row>
163 <entry>
164 <literal>{ x = 1; y = 2; }.x</literal>
165 </entry>
166 <entry>
167 Attribute selection (evaluates to <literal>1</literal>)
168 </entry>
169 </row>
170 <row>
171 <entry>
172 <literal>{ x = 1; y = 2; }.z or 3</literal>
173 </entry>
174 <entry>
175 Attribute selection with default (evaluates to
176 <literal>3</literal>)
177 </entry>
178 </row>
179 <row>
180 <entry>
181 <literal>{ x = 1; y = 2; } // { z = 3; }</literal>
182 </entry>
183 <entry>
184 Merge two sets (attributes in the right-hand set taking
185 precedence)
186 </entry>
187 </row>
188 <row>
189 <entry>
190 <emphasis>Control structures</emphasis>
191 </entry>
192 <entry>
193 </entry>
194 </row>
195 <row>
196 <entry>
197 <literal>if 1 + 1 == 2 then "yes!" else "no!"</literal>
198 </entry>
199 <entry>
200 Conditional expression
201 </entry>
202 </row>
203 <row>
204 <entry>
205 <literal>assert 1 + 1 == 2; "yes!"</literal>
206 </entry>
207 <entry>
208 Assertion check (evaluates to
209 <literal>"yes!"</literal>). See
210 <xref linkend="sec-assertions" /> for using assertions in
211 modules
212 </entry>
213 </row>
214 <row>
215 <entry>
216 <literal>let x = "foo"; y = "bar"; in x + y</literal>
217 </entry>
218 <entry>
219 Variable definition
220 </entry>
221 </row>
222 <row>
223 <entry>
224 <literal>with pkgs.lib; head [ 1 2 3 ]</literal>
225 </entry>
226 <entry>
227 Add all attributes from the given set to the scope
228 (evaluates to <literal>1</literal>)
229 </entry>
230 </row>
231 <row>
232 <entry>
233 <emphasis>Functions (lambdas)</emphasis>
234 </entry>
235 <entry>
236 </entry>
237 </row>
238 <row>
239 <entry>
240 <literal>x: x + 1</literal>
241 </entry>
242 <entry>
243 A function that expects an integer and returns it increased
244 by 1
245 </entry>
246 </row>
247 <row>
248 <entry>
249 <literal>(x: x + 1) 100</literal>
250 </entry>
251 <entry>
252 A function call (evaluates to 101)
253 </entry>
254 </row>
255 <row>
256 <entry>
257 <literal>let inc = x: x + 1; in inc (inc (inc 100))</literal>
258 </entry>
259 <entry>
260 A function bound to a variable and subsequently called by
261 name (evaluates to 103)
262 </entry>
263 </row>
264 <row>
265 <entry>
266 <literal>{ x, y }: x + y</literal>
267 </entry>
268 <entry>
269 A function that expects a set with required attributes
270 <literal>x</literal> and <literal>y</literal> and
271 concatenates them
272 </entry>
273 </row>
274 <row>
275 <entry>
276 <literal>{ x, y ? "bar" }: x + y</literal>
277 </entry>
278 <entry>
279 A function that expects a set with required attribute
280 <literal>x</literal> and optional <literal>y</literal>,
281 using <literal>"bar"</literal> as default value
282 for <literal>y</literal>
283 </entry>
284 </row>
285 <row>
286 <entry>
287 <literal>{ x, y, ... }: x + y</literal>
288 </entry>
289 <entry>
290 A function that expects a set with required attributes
291 <literal>x</literal> and <literal>y</literal> and ignores
292 any other attributes
293 </entry>
294 </row>
295 <row>
296 <entry>
297 <literal>{ x, y } @ args: x + y</literal>
298 </entry>
299 <entry>
300 A function that expects a set with required attributes
301 <literal>x</literal> and <literal>y</literal>, and binds the
302 whole set to <literal>args</literal>
303 </entry>
304 </row>
305 <row>
306 <entry>
307 <emphasis>Built-in functions</emphasis>
308 </entry>
309 <entry>
310 </entry>
311 </row>
312 <row>
313 <entry>
314 <literal>import ./foo.nix</literal>
315 </entry>
316 <entry>
317 Load and return Nix expression in given file
318 </entry>
319 </row>
320 <row>
321 <entry>
322 <literal>map (x: x + x) [ 1 2 3 ]</literal>
323 </entry>
324 <entry>
325 Apply a function to every element of a list (evaluates to
326 <literal>[ 2 4 6 ]</literal>)
327 </entry>
328 </row>
329 </tbody>
330 </tgroup>
331 </informaltable>
332</section>