pyrox.dev
1# Perl {#sec-language-perl}
2
3## Running perl programs on the shell {#ssec-perl-running}
4
5When executing a Perl script, it is possible you get an error such as `./myscript.pl: bad interpreter: /usr/bin/perl: no such file or directory`. This happens when the script expects Perl to be installed at `/usr/bin/perl`, which is not the case when using Perl from nixpkgs. You can fix the script by changing the first line to:
6
7```perl
8#!/usr/bin/env perl
9```
10
11to take the Perl installation from the `PATH` environment variable, or invoke Perl directly with:
12
13```ShellSession
14$ perl ./myscript.pl
15```
16
17When the script is using a Perl library that is not installed globally, you might get an error such as `Can't locate DB_File.pm in @INC (you may need to install the DB_File module)`. In that case, you can use `nix-shell` to start an ad-hoc shell with that library installed, for instance:
18
19```ShellSession
20$ nix-shell -p perl perlPackages.DBFile --run ./myscript.pl
21```
22
23If you are always using the script in places where `nix-shell` is available, you can embed the `nix-shell` invocation in the shebang like this:
24
25```perl
26#!/usr/bin/env nix-shell
27#! nix-shell -i perl -p perl perlPackages.DBFile
28```
29
30## Packaging Perl programs {#ssec-perl-packaging}
31
32Nixpkgs provides a function `buildPerlPackage`, a generic package builder function for any Perl package that has a standard `Makefile.PL`. It’s implemented in [pkgs/development/perl-modules/generic](https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/perl-modules/generic).
33
34Perl packages from CPAN are defined in [pkgs/top-level/perl-packages.nix](https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/perl-packages.nix) rather than `pkgs/all-packages.nix`. Most Perl packages are so straight-forward to build that they are defined here directly, rather than having a separate function for each package called from `perl-packages.nix`. However, more complicated packages should be put in a separate file, typically in `pkgs/development/perl-modules`. Here is an example of the former:
35
36```nix
37ClassC3 = buildPerlPackage rec {
38 name = "Class-C3-0.21";
39 src = fetchurl {
40 url = "mirror://cpan/authors/id/F/FL/FLORA/${name}.tar.gz";
41 sha256 = "1bl8z095y4js66pwxnm7s853pi9czala4sqc743fdlnk27kq94gz";
42 };
43};
44```
45
46Note the use of `mirror://cpan/`, and the `${name}` in the URL definition to ensure that the name attribute is consistent with the source that we’re actually downloading. Perl packages are made available in `all-packages.nix` through the variable `perlPackages`. For instance, if you have a package that needs `ClassC3`, you would typically write
47
48```nix
49foo = import ../path/to/foo.nix {
50 inherit stdenv fetchurl ...;
51 inherit (perlPackages) ClassC3;
52};
53```
54
55in `all-packages.nix`. You can test building a Perl package as follows:
56
57```ShellSession
58$ nix-build -A perlPackages.ClassC3
59```
60
61`buildPerlPackage` adds `perl-` to the start of the name attribute, so the package above is actually called `perl-Class-C3-0.21`. So to install it, you can say:
62
63```ShellSession
64$ nix-env -i perl-Class-C3
65```
66
67(Of course you can also install using the attribute name: `nix-env -i -A perlPackages.ClassC3`.)
68
69So what does `buildPerlPackage` do? It does the following:
70
711. In the configure phase, it calls `perl Makefile.PL` to generate a Makefile. You can set the variable `makeMakerFlags` to pass flags to `Makefile.PL`
722. It adds the contents of the `PERL5LIB` environment variable to `#! .../bin/perl` line of Perl scripts as `-Idir` flags. This ensures that a script can find its dependencies. (This can cause this shebang line to become too long for Darwin to handle; see the note below.)
733. In the fixup phase, it writes the propagated build inputs (`propagatedBuildInputs`) to the file `$out/nix-support/propagated-user-env-packages`. `nix-env` recursively installs all packages listed in this file when you install a package that has it. This ensures that a Perl package can find its dependencies.
74
75`buildPerlPackage` is built on top of `stdenv`, so everything can be customised in the usual way. For instance, the `BerkeleyDB` module has a `preConfigure` hook to generate a configuration file used by `Makefile.PL`:
76
77```nix
78{ buildPerlPackage, fetchurl, db }:
79
80buildPerlPackage rec {
81 name = "BerkeleyDB-0.36";
82
83 src = fetchurl {
84 url = "mirror://cpan/authors/id/P/PM/PMQS/${name}.tar.gz";
85 sha256 = "07xf50riarb60l1h6m2dqmql8q5dij619712fsgw7ach04d8g3z1";
86 };
87
88 preConfigure = ''
89 echo "LIB = ${db.out}/lib" > config.in
90 echo "INCLUDE = ${db.dev}/include" >> config.in
91 '';
92}
93```
94
95Dependencies on other Perl packages can be specified in the `buildInputs` and `propagatedBuildInputs` attributes. If something is exclusively a build-time dependency, use `buildInputs`; if it’s (also) a runtime dependency, use `propagatedBuildInputs`. For instance, this builds a Perl module that has runtime dependencies on a bunch of other modules:
96
97```nix
98ClassC3Componentised = buildPerlPackage rec {
99 name = "Class-C3-Componentised-1.0004";
100 src = fetchurl {
101 url = "mirror://cpan/authors/id/A/AS/ASH/${name}.tar.gz";
102 sha256 = "0xql73jkcdbq4q9m0b0rnca6nrlvf5hyzy8is0crdk65bynvs8q1";
103 };
104 propagatedBuildInputs = [
105 ClassC3 ClassInspector TestException MROCompat
106 ];
107};
108```
109
110On Darwin, if a script has too many `-Idir` flags in its first line (its “shebang line”), it will not run. This can be worked around by calling the `shortenPerlShebang` function from the `postInstall` phase:
111
112```nix
113{ lib, stdenv, buildPerlPackage, fetchurl, shortenPerlShebang }:
114
115ImageExifTool = buildPerlPackage {
116 pname = "Image-ExifTool";
117 version = "11.50";
118
119 src = fetchurl {
120 url = "https://www.sno.phy.queensu.ca/~phil/exiftool/Image-ExifTool-11.50.tar.gz";
121 sha256 = "0d8v48y94z8maxkmw1rv7v9m0jg2dc8xbp581njb6yhr7abwqdv3";
122 };
123
124 buildInputs = lib.optional stdenv.isDarwin shortenPerlShebang;
125 postInstall = lib.optionalString stdenv.isDarwin ''
126 shortenPerlShebang $out/bin/exiftool
127 '';
128};
129```
130
131This will remove the `-I` flags from the shebang line, rewrite them in the `use lib` form, and put them on the next line instead. This function can be given any number of Perl scripts as arguments; it will modify them in-place.
132
133### Generation from CPAN {#ssec-generation-from-CPAN}
134
135Nix expressions for Perl packages can be generated (almost) automatically from CPAN. This is done by the program `nix-generate-from-cpan`, which can be installed as follows:
136
137```ShellSession
138$ nix-env -i nix-generate-from-cpan
139```
140
141This program takes a Perl module name, looks it up on CPAN, fetches and unpacks the corresponding package, and prints a Nix expression on standard output. For example:
142
143```ShellSession
144$ nix-generate-from-cpan XML::Simple
145 XMLSimple = buildPerlPackage rec {
146 name = "XML-Simple-2.22";
147 src = fetchurl {
148 url = "mirror://cpan/authors/id/G/GR/GRANTM/${name}.tar.gz";
149 sha256 = "b9450ef22ea9644ae5d6ada086dc4300fa105be050a2030ebd4efd28c198eb49";
150 };
151 propagatedBuildInputs = [ XMLNamespaceSupport XMLSAX XMLSAXExpat ];
152 meta = {
153 description = "An API for simple XML files";
154 license = with lib.licenses; [ artistic1 gpl1Plus ];
155 };
156 };
157```
158
159The output can be pasted into `pkgs/top-level/perl-packages.nix` or wherever else you need it.
160
161### Cross-compiling modules {#ssec-perl-cross-compilation}
162
163Nixpkgs has experimental support for cross-compiling Perl modules. In many cases, it will just work out of the box, even for modules with native extensions. Sometimes, however, the Makefile.PL for a module may (indirectly) import a native module. In that case, you will need to make a stub for that module that will satisfy the Makefile.PL and install it into `lib/perl5/site_perl/cross_perl/${perl.version}`. See the `postInstall` for `DBI` for an example.