about summary refs log tree commit diff
path: root/doc/manual/expressions/expression-syntax.xml
blob: b674b0d60001423335d8ec70ce8afd1eb619b70e (plain) (blame)
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
<section xmlns="http://docbook.org/ns/docbook"
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:xi="http://www.w3.org/2001/XInclude"
      version="5.0"
      xml:id='sec-expression-syntax'>

<title>Expression Syntax</title>

<example xml:id='ex-hello-nix'><title>Nix expression for GNU Hello
(<filename>default.nix</filename>)</title>
<programlisting>
{ stdenv, fetchurl, perl }: <co xml:id='ex-hello-nix-co-1' />

stdenv.mkDerivation { <co xml:id='ex-hello-nix-co-2' />
  name = "hello-2.1.1"; <co xml:id='ex-hello-nix-co-3' />
  builder = ./builder.sh; <co xml:id='ex-hello-nix-co-4' />
  src = fetchurl { <co xml:id='ex-hello-nix-co-5' />
    url = ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz;
    sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
  };
  inherit perl; <co xml:id='ex-hello-nix-co-6' />
}</programlisting>
</example>

<para><xref linkend='ex-hello-nix' /> shows a Nix expression for GNU
Hello.  It's actually already in the Nix Packages collection in
<filename>pkgs/applications/misc/hello/ex-1/default.nix</filename>.
It is customary to place each package in a separate directory and call
the single Nix expression in that directory
<filename>default.nix</filename>.  The file has the following elements
(referenced from the figure by number):

<calloutlist>

  <callout arearefs='ex-hello-nix-co-1'>

    <para>This states that the expression is a
    <emphasis>function</emphasis> that expects to be called with three
    arguments: <varname>stdenv</varname>, <varname>fetchurl</varname>,
    and <varname>perl</varname>.  They are needed to build Hello, but
    we don't know how to build them here; that's why they are function
    arguments.  <varname>stdenv</varname> is a package that is used
    by almost all Nix Packages packages; it provides a
    <quote>standard</quote> environment consisting of the things you
    would expect in a basic Unix environment: a C/C++ compiler (GCC,
    to be precise), the Bash shell, fundamental Unix tools such as
    <command>cp</command>, <command>grep</command>,
    <command>tar</command>, etc.  <varname>fetchurl</varname> is a
    function that downloads files.  <varname>perl</varname> is the
    Perl interpreter.</para>

    <para>Nix functions generally have the form <literal>{ x, y, ...,
    z }: e</literal> where <varname>x</varname>, <varname>y</varname>,
    etc. are the names of the expected arguments, and where
    <replaceable>e</replaceable> is the body of the function.  So
    here, the entire remainder of the file is the body of the
    function; when given the required arguments, the body should
    describe how to build an instance of the Hello package.</para>

  </callout>

  <callout arearefs='ex-hello-nix-co-2'>

    <para>So we have to build a package.  Building something from
    other stuff is called a <emphasis>derivation</emphasis> in Nix (as
    opposed to sources, which are built by humans instead of
    computers).  We perform a derivation by calling
    <varname>stdenv.mkDerivation</varname>.
    <varname>mkDerivation</varname> is a function provided by
    <varname>stdenv</varname> that builds a package from a set of
    <emphasis>attributes</emphasis>.  A set is just a list of
    key/value pairs where each key is a string and each value is an
    arbitrary Nix expression.  They take the general form <literal>{
    <replaceable>name1</replaceable> =
    <replaceable>expr1</replaceable>; <replaceable>...</replaceable>
    <replaceable>nameN</replaceable> =
    <replaceable>exprN</replaceable>; }</literal>.</para>

  </callout>

  <callout arearefs='ex-hello-nix-co-3'>

    <para>The attribute <varname>name</varname> specifies the symbolic
    name and version of the package.  Nix doesn't really care about
    these things, but they are used by for instance <command>nix-env
    -q</command> to show a <quote>human-readable</quote> name for
    packages.  This attribute is required by
    <varname>mkDerivation</varname>.</para>

  </callout>

  <callout arearefs='ex-hello-nix-co-4'>

    <para>The attribute <varname>builder</varname> specifies the
    builder.  This attribute can sometimes be omitted, in which case
    <varname>mkDerivation</varname> will fill in a default builder
    (which does a <literal>configure; make; make install</literal>, in
    essence).  Hello is sufficiently simple that the default builder
    would suffice, but in this case, we will show an actual builder
    for educational purposes.  The value
    <command>./builder.sh</command> refers to the shell script shown
    in <xref linkend='ex-hello-builder' />, discussed below.</para>

  </callout>

  <callout arearefs='ex-hello-nix-co-5'>

    <para>The builder has to know what the sources of the package
    are.  Here, the attribute <varname>src</varname> is bound to the
    result of a call to the <command>fetchurl</command> function.
    Given a URL and an MD5 hash of the expected contents of the file
    at that URL, this function builds a derivation that downloads the
    file and checks its hash.  So the sources are a dependency that
    like all other dependencies is built before Hello itself is
    built.</para>

    <para>Instead of <varname>src</varname> any other name could have
    been used, and in fact there can be any number of sources (bound
    to different attributes).  However, <varname>src</varname> is
    customary, and it's also expected by the default builder (which we
    don't use in this example).</para>

  </callout>

  <callout arearefs='ex-hello-nix-co-6'>

    <para>Since the derivation requires Perl, we have to pass the
    value of the <varname>perl</varname> function argument to the
    builder.  All attributes in the set are actually passed as
    environment variables to the builder, so declaring an attribute

    <programlisting>
perl = perl;</programlisting>

    will do the trick: it binds an attribute <varname>perl</varname>
    to the function argument which also happens to be called
    <varname>perl</varname>.  However, it looks a bit silly, so there
    is a shorter syntax.  The <literal>inherit</literal> keyword
    causes the specified attributes to be bound to whatever variables
    with the same name happen to be in scope.</para>

  </callout>

</calloutlist>

</para>

</section>