what is `@.` in Julia `--project` command line option?

Question

When starting Julia in the command line one can specify project directory. One of the options is @., resumably the current directory. What is @. in this context?

# from `julia --help`
--project[={<dir>|@.}]    Set <dir> as the home project/environment

From the docs:

If the variable is set to @., Julia tries to find a project directory that contains Project.toml or JuliaProject.toml file from the current directory and its parents.

I realise the cli --project argument parsing occurs in this C code, and apparently used by this init code, although I'm not sure of the sequence of events in between.

Specifically in initdefs.jl we have:

project = (JLOptions().project != C_NULL ?
    unsafe_string(Base.JLOptions().project) :
    get(ENV, "JULIA_PROJECT", nothing))
HOME_PROJECT[] =
    project == nothing ? nothing :
    project == "" ? nothing :
    project == "@." ? current_project() : abspath(project)

My reading of this code is that @. is just an arbitrary token, could it have been a simple . for cli? How does Julia search "the current directory and its parents."

There is similar notation used in LOAD_PATH(["@", "@v#.#", "@stdlib"]), as discussed here and here. Does @. belong to the same family of expansion as LOAD_PATH symbols?

Answer 1

家庭项目和 --project

该--project标志定义了“家庭项目”或“家庭环境”。环境由Project.toml/Manifest.toml定义，并定义哪些包可用于using/ import。

您可以设置--project为 (i) 目录（带或不带Project.toml）（ii）到 a 的路径Project.toml或（iii）到@.. (i) 和 (ii) 是不言自明的——Julia 会将位于路径上的项目视为主项目。现在对于 (iii)，如果您设置了--project=@.Julia 将尝试查找现有 Project.toml文件并将其用作主项目。考虑以下示例：

~$ tree .
.
??? A
??? Project.toml

1 directory, 1 file

whereA是一个空目录。我们可以轻松尝试（i）和（ii）：

# existing file
[~]$ julia --project=Project.toml -E 'Base.active_project()'
"~/Project.toml"

# existing directory
[~]$ julia --project=A -E 'Base.active_project()'
"~/A/Project.toml"

# non-existing directory
[~]$ julia --project=B -E 'Base.active_project()'
"~/B/Project.toml"

# non-existing file
[~]$ julia --project=B/Project.toml -E 'Base.active_project()'
"~/B/Project.toml"

请注意，在上面的最后三个示例中，该Project.toml文件不存在，但会在需要时创建（例如，当Pkg用于操作包时）。

现在，将此与 的行为进行比较@.，后者将查找现有项目文件：

# from our root directory
[~]$ julia --project=@. -E 'Base.active_project()'
"~/Project.toml"

# from inside the A directory
[~/A]$ julia --project=@. -E 'Base.active_project()'
"~/Project.toml"

在这两种情况下，我们都找到了相同的Project.toml文件。使用@.选项 Julia 首先在当前目录中查找Project.toml文件，如果未找到，则上一级到父文件夹并在那里查找，依此类推。这就是第二个例子中发生的事情；Julia 没有Project.toml在空A目录中找到文件，所以我们向上移动到父目录，并在Project.toml那里找到了我们的文件。

是的，我们可以为此选择一些其他标记，但不是.因为它已经具有意义；它是当前目录的路径，并且是与--project.

负载路径和 "@"

要加载一个包Example，我们Example在它的[deps]部分定义一个主项目是不够的；主项目还需要显示在 Julias 加载路径 ( Base.load_path()) 中。默认情况下，加载路径从["@", "@v#.#", "@stdlib"]where "@v#.#"expands 扩展到~/.julia/environments/v#.#with#替换为 Julias 主要和次要版本号，并"@stdlib"扩展到 Julias 标准库的目录。"@"扩展为 1. 活动项目（用Pkg.activate/激活pkg> activate）或 2. 家庭项目。我们可以使用以下命令检查扩展的加载路径Base.load_path()：

# without home project; @ expands to nothing
[~]$ julia -E 'Base.load_path()'
["~/.julia/environments/v1.0/Project.toml", "~/julia10/usr/share/julia/stdlib/v1.0"]

# with home project; @ expands to our specified home project
[~]$ julia --project=@. -E 'Base.load_path()'
["~/Project.toml", "~/.julia/environments/v1.0/Project.toml", "~/julia10/usr/share/julia/stdlib/v1.0"]

最后，如果我们"@"从加载路径中删除，我们是否定义了一个 home 项目并不重要：

[~]$ export JULIA_LOAD_PATH="" && julia --project=@. -E 'Base.load_path()'
String[]