brew/docs/Building-Against-Non-Homebrew-Dependencies.md

# Building Against Non-Homebrew Dependencies

## History

Originally Homebrew was a build-from-source package manager and all user environment variables and non-Homebrew-installed software were available to builds. Since then Homebrew added `Requirement`s to specify dependencies on non-Homebrew software (such as those provided by `brew cask` like X11/XQuartz), the `superenv` build system to strip out unspecified dependencies, environment filtering to stop the user environment leaking into Homebrew builds and `default_formula` to specify that a `Requirement` can be satisifed by a particular formula.

As Homebrew became primarily a binary package manager, most users were fulfilling `Requirement`s with the `default_formula`, not with arbitrary alternatives. To improve quality and reduce variation, Homebrew now exclusively supports using the default formula, as an ordinary dependency, and no longer supports using arbitrary alternatives.

## Today

If you wish to build against custom non-Homebrew dependencies that are provided by Homebrew (e.g. a non-Homebrew, non-macOS `ruby`) then you must [create and maintain your own tap](How-to-Create-and-Maintain-a-Tap.md) as these formulae will not be accepted in Homebrew/homebrew-core. Once you have done that you can specify `env :std` in the formula which will allow a e.g. `which ruby` to access your existing `PATH` variable and allow compilation to link against this Ruby.
Deprecate default_formula Requirement DSL This has been a nightmare in terms of the complexity to our dependency system and the whack-a-mole required on bugs. If a Requirement resolves to a Formula it should just use `depends_on "formula"` instead. This matches the effective behaviour all users of bottles (the vast majority of users and installs) and what we're doing in Homebrew/homebrew-core. 2018-01-14 13:27:43 +00:00			`# Building Against Non-Homebrew Dependencies`

			`## History`

			Originally Homebrew was a build-from-source package manager and all user environment variables and non-Homebrew-installed software were available to builds. Since then Homebrew added `Requirement`s to specify dependencies on non-Homebrew software (such as those provided by `brew cask` like X11/XQuartz), the `superenv` build system to strip out unspecified dependencies, environment filtering to stop the user environment leaking into Homebrew builds and `default_formula` to specify that a `Requirement` can be satisifed by a particular formula.

			As Homebrew became primarily a binary package manager, most users were fulfilling `Requirement`s with the `default_formula`, not with arbitrary alternatives. To improve quality and reduce variation, Homebrew now exclusively supports using the default formula, as an ordinary dependency, and no longer supports using arbitrary alternatives.

			`## Today`

			If you wish to build against custom non-Homebrew dependencies that are provided by Homebrew (e.g. a non-Homebrew, non-macOS `ruby`) then you must [create and maintain your own tap](How-to-Create-and-Maintain-a-Tap.md) as these formulae will not be accepted in Homebrew/homebrew-core. Once you have done that you can specify `env :std` in the formula which will allow a e.g. `which ruby` to access your existing `PATH` variable and allow compilation to link against this Ruby.