8.5 Build Phases

Almost all package build systems implement a notion build phases: a sequence of actions that the build system executes, when you build the package, leading to the installed byproducts in the store. A notable exception is the “bare-bones” trivial-build-system (see Build Systems).

As discussed in the previous section, those build systems provide a standard list of phases. For gnu-build-system, the standard phases include an unpack phase to unpack the source code tarball, a configure phase to run ./configure, a build phase to run make, and (among others) an install phase to run make install; see Build Systems, for a more detailed view of these phases. Likewise, cmake-build-system inherits these phases, but its configure phase runs cmake instead of ./configure. Other build systems, such as python-build-system, have a wholly different list of standard phases. All this code runs on the build side: it is evaluated when you actually build the package, in a dedicated build process spawned by the build daemon (see Invoking guix-daemon).

Build phases are represented as association lists or “alists” (see Association Lists in GNU Guile Reference Manual) where each key is a symbol for the name of the phase and the associated value is a procedure that accepts an arbitrary number of arguments. By convention, those procedures receive information about the build in the form of keyword parameters, which they can use or ignore.

For example, here is how (guix build gnu-build-system) defines %standard-phases, the variable holding its alist of build phases¹⁷:

;; The build phases of 'gnu-build-system'.

(define* (unpack #:key source #:allow-other-keys)
  ;; Extract the source tarball.
  (invoke "tar" "xvf" source))

(define* (configure #:key outputs #:allow-other-keys)
  ;; Run the 'configure' script.  Install to output "out".
  (let ((out (assoc-ref outputs "out")))
    (invoke "./configure"
            (string-append "--prefix=" out))))

(define* (build #:allow-other-keys)
  ;; Compile.
  (invoke "make"))

(define* (check #:key (test-target "check") (tests? #true)
                #:allow-other-keys)
  ;; Run the test suite.
  (if tests?
      (invoke "make" test-target)
      (display "test suite not run\n")))

(define* (install #:allow-other-keys)
  ;; Install files to the prefix 'configure' specified.
  (invoke "make" "install"))

(define %standard-phases
  ;; The list of standard phases (quite a few are omitted
  ;; for brevity).  Each element is a symbol/procedure pair.
  (list (cons 'unpack unpack)
        (cons 'configure configure)
        (cons 'build build)
        (cons 'check check)
        (cons 'install install)))

This shows how %standard-phases is defined as a list of symbol/procedure pairs (see Pairs in GNU Guile Reference Manual). The first pair associates the unpack procedure with the unpack symbol—a name; the second pair defines the configure phase similarly, and so on. When building a package that uses gnu-build-system with its default list of phases, those phases are executed sequentially. You can see the name of each phase started and completed in the build log of packages that you build.

Let’s now look at the procedures themselves. Each one is defined with define*: #:key lists keyword parameters the procedure accepts, possibly with a default value, and #:allow-other-keys specifies that other keyword parameters are ignored (see Optional Arguments in GNU Guile Reference Manual).

The unpack procedure honors the source parameter, which the build system uses to pass the file name of the source tarball (or version control checkout), and it ignores other parameters. The configure phase only cares about the outputs parameter, an alist mapping package output names to their store file name (see Packages with Multiple Outputs). It extracts the file name of for out, the default output, and passes it to ./configure as the installation prefix, meaning that make install will eventually copy all the files in that directory (see configuration and makefile conventions in GNU Coding Standards). build and install ignore all their arguments. check honors the test-target argument, which specifies the name of the Makefile target to run tests; it prints a message and skips tests when tests? is false.

The list of phases used for a particular package can be changed with the #:phases parameter of the build system. Changing the set of build phases boils down to building a new alist of phases based on the %standard-phases alist described above. This can be done with standard alist procedures such as alist-delete (see SRFI-1 Association Lists in GNU Guile Reference Manual); however, it is more convenient to do so with modify-phases (see modify-phases).

Here is an example of a package definition that removes the configure phase of %standard-phases and inserts a new phase before the build phase, called set-prefix-in-makefile:

(define-public example
  (package
    (name "example")
    ;; other fields omitted
    (build-system gnu-build-system)
    (arguments
     '(#:phases (modify-phases %standard-phases
                  (delete 'configure)
                  (add-before 'build 'set-prefix-in-makefile
                    (lambda* (#:key outputs #:allow-other-keys)
                      ;; Modify the makefile so that its
                      ;; 'PREFIX' variable points to "out".
                      (let ((out (assoc-ref outputs "out")))
                        (substitute* "Makefile"
                          (("PREFIX =.*")
                           (string-append "PREFIX = "
                                          out "\n")))
                        #true))))))))

The new phase that is inserted is written as an anonymous procedure, introduced with lambda*; it honors the outputs parameter we have seen before. See Build Utilities, for more about the helpers used by this phase, and for more examples of modify-phases.

Keep in mind that build phases are code evaluated at the time the package is actually built. This explains why the whole modify-phases expression above is quoted (it comes after the ' or apostrophe): it is staged for later execution. See G-Expressions, for an explanation of code staging and the code strata involved.