pyrox.dev
1# Swift {#swift}
2
3The Swift compiler is provided by the `swift` package:
4
5```sh
6# Compile and link a simple executable.
7nix-shell -p swift --run 'swiftc -' <<< 'print("Hello world!")'
8# Run it!
9./main
10```
11
12The `swift` package also provides the `swift` command, with some caveats:
13
14- Swift Package Manager (SwiftPM) is packaged separately as `swiftpm`. If you
15 need functionality like `swift build`, `swift run`, `swift test`, you must
16 also add the `swiftpm` package to your closure.
17- On Darwin, the `swift repl` command requires an Xcode installation. This is
18 because it uses the system LLDB debugserver, which has special entitlements.
19
20## Module search paths {#ssec-swift-module-search-paths}
21
22Like other toolchains in Nixpkgs, the Swift compiler executables are wrapped
23to help Swift find your application's dependencies in the Nix store. These
24wrappers scan the `buildInputs` of your package derivation for specific
25directories where Swift modules are placed by convention, and automatically
26add those directories to the Swift compiler search paths.
27
28Swift follows different conventions depending on the platform. The wrappers
29look for the following directories:
30
31- On Darwin platforms: `lib/swift/macosx`
32 (If not targeting macOS, replace `macosx` with the Xcode platform name.)
33- On other platforms: `lib/swift/linux/x86_64`
34 (Where `linux` and `x86_64` are from lowercase `uname -sm`.)
35- For convenience, Nixpkgs also adds `lib/swift` to the search path.
36 This can save a bit of work packaging Swift modules, because many Nix builds
37 will produce output for just one target any way.
38
39## Core libraries {#ssec-swift-core-libraries}
40
41In addition to the standard library, the Swift toolchain contains some
42additional 'core libraries' that, on Apple platforms, are normally distributed
43as part of the OS or Xcode. These are packaged separately in Nixpkgs, and can
44be found (for use in `buildInputs`) as:
45
46- `swiftPackages.Dispatch`
47- `swiftPackages.Foundation`
48- `swiftPackages.XCTest`
49
50## Packaging with SwiftPM {#ssec-swift-packaging-with-swiftpm}
51
52Nixpkgs includes a small helper `swiftpm2nix` that can fetch your SwiftPM
53dependencies for you, when you need to write a Nix expression to package your
54application.
55
56The first step is to run the generator:
57
58```sh
59cd /path/to/my/project
60# Enter a Nix shell with the required tools.
61nix-shell -p swift swiftpm swiftpm2nix
62# First, make sure the workspace is up-to-date.
63swift package resolve
64# Now generate the Nix code.
65swiftpm2nix
66```
67
68This produces some files in a directory `nix`, which will be part of your Nix
69expression. The next step is to write that expression:
70
71```nix
72{ stdenv, swift, swiftpm, swiftpm2nix, fetchFromGitHub }:
73
74let
75 # Pass the generated files to the helper.
76 generated = swiftpm2nix.helpers ./nix;
77in
78
79stdenv.mkDerivation rec {
80 pname = "myproject";
81 version = "0.0.0";
82
83 src = fetchFromGitHub {
84 owner = "nixos";
85 repo = pname;
86 rev = version;
87 hash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
88 };
89
90 # Including SwiftPM as a nativeBuildInput provides a buildPhase for you.
91 # This by default performs a release build using SwiftPM, essentially:
92 # swift build -c release
93 nativeBuildInputs = [ swift swiftpm ];
94
95 # The helper provides a configure snippet that will prepare all dependencies
96 # in the correct place, where SwiftPM expects them.
97 configurePhase = generated.configure;
98
99 installPhase = ''
100 # This is a special function that invokes swiftpm to find the location
101 # of the binaries it produced.
102 binPath="$(swiftpmBinPath)"
103 # Now perform any installation steps.
104 mkdir -p $out/bin
105 cp $binPath/myproject $out/bin/
106 '';
107}
108```
109
110### Custom build flags {#ssec-swiftpm-custom-build-flags}
111
112If you'd like to build a different configuration than `release`:
113
114```nix
115{
116 swiftpmBuildConfig = "debug";
117}
118```
119
120It is also possible to provide additional flags to `swift build`:
121
122```nix
123{
124 swiftpmFlags = [ "--disable-dead-strip" ];
125}
126```
127
128The default `buildPhase` already passes `-j` for parallel building.
129
130If these two customization options are insufficient, provide your own
131`buildPhase` that invokes `swift build`.
132
133### Running tests {#ssec-swiftpm-running-tests}
134
135Including `swiftpm` in your `nativeBuildInputs` also provides a default
136`checkPhase`, but it must be enabled with:
137
138```nix
139{
140 doCheck = true;
141}
142```
143
144This essentially runs: `swift test -c release`
145
146### Patching dependencies {#ssec-swiftpm-patching-dependencies}
147
148In some cases, it may be necessary to patch a SwiftPM dependency. SwiftPM
149dependencies are located in `.build/checkouts`, but the `swiftpm2nix` helper
150provides these as symlinks to read-only `/nix/store` paths. In order to patch
151them, we need to make them writable.
152
153A special function `swiftpmMakeMutable` is available to replace the symlink
154with a writable copy:
155
156```nix
157{
158 configurePhase = generated.configure ++ ''
159 # Replace the dependency symlink with a writable copy.
160 swiftpmMakeMutable swift-crypto
161 # Now apply a patch.
162 patch -p1 -d .build/checkouts/swift-crypto -i ${./some-fix.patch}
163 '';
164}
165```
166
167## Considerations for custom build tools {#ssec-swift-considerations-for-custom-build-tools}
168
169### Linking the standard library {#ssec-swift-linking-the-standard-library}
170
171The `swift` package has a separate `lib` output containing just the Swift
172standard library, to prevent Swift applications needing a dependency on the
173full Swift compiler at run-time. Linking with the Nixpkgs Swift toolchain
174already ensures binaries correctly reference the `lib` output.
175
176Sometimes, Swift is used only to compile part of a mixed codebase, and the
177link step is manual. Custom build tools often locate the standard library
178relative to the `swift` compiler executable, and while the result will work,
179when this path ends up in the binary, it will have the Swift compiler as an
180unintended dependency.
181
182In this case, you should investigate how your build process discovers the
183standard library, and override the path. The correct path will be something
184like: `"${swift.swift.lib}/${swift.swiftModuleSubdir}"`