Combining uses_from_macos with on_macos?

[gwerbin]

Hello all,

I am working on the Idris 2 formula, which currently specifies:

uses_from_macos "zsh" => :build, since: :mojave

This is not strictly correct. The actual logic should be something like this:

• If on Mac, use the system-provided Zsh when available (starting with Mojave), otherwise use Homebrew Zsh.
• If on Linux, don't use Zsh at all, for anything.

That is, Zsh is only a build dependency on Mac, and is not required at all on Linux.

I tried to write:

if on_macos
  uses_from_macos "zsh" => :build, since: :mojave
end

But this is apparently not allowed:

Error: 1 problem in 1 formula detected
idris2:
  * 19: col 5: `on_macos` cannot include `uses_from_macos`. Only `depends_on`, `patch`, `resource`, `deprecate!`, `disable!`, `conflicts_w
ith`, and `keg_only` are allowed.

Is there some workaround to enable Zsh-from-Macos as a build dependency on Mac systems, without forcing Linux users to have Zsh when it's entirely unnecessary for them? Is there any technical reason why this shouldn't be allowed?

[gromgit]

If on Mac, use the system-provided Zsh when available (starting with Mojave), otherwise use Homebrew Zsh.

Why must you use system Zsh on supported macOS? If you relax that restriction, the solution is simple:

on_macos do
  depends_on "zsh" => :build
end

If you don't intend to support macOS before Mojave, it's even simpler: Drop the dependency entirely, and Homebrew will automatically use system Zsh on all platforms.

[gwerbin]

I see no reason to require an external dependency when a perfectly good one is already available on the user's system.

Likewise, I see no reason to arbitrarily drop support for a platform, because the package manager just so happens to not support that specific combination of features.

[carlocab]

You can also do

on_macos do
  depends_on "zsh" => :build if MacOS.version < :mojave
end

[gwerbin]

@carlocab that makes sense! Is that pretty much equivalent to what uses_from_macos does?

[carlocab]

Yes.

uses_from_macos "zsh", since: :mojave

is a no-op on macOS Mojave or newer, and becomes

depends_on "zsh"

on Linux and macOS High Sierra or older.

[gwerbin]

As per @carlocab , the workaround is:

on_macos do
  depends_on "zsh" => :build if MacOS.version < :mojave
end