为什么 MIX 依赖在语义版本之前有一个“~>”？

Question

On mix.exs you can declare dependencies like:

def deps do
  [{:plug, "~> 1.0"}]
end

Why does it need to have "~>" instead of simply the version on the second part of the tuple.

I have seen that if it gets the dependency from git, you can write the dependency like:

def deps do
  [{:plug, git: "git://github.com/elixir-lang/plug.git"}]
end

Answer 1

This fancy arrow is supported by Version module. It rounds your dependency with precision set by the digit, which is left neighbour of the most specific one.

From the docs:

 # 2.0.0 and later until 2.1.0 ">= 2.0.0 and < 2.1.0"

Since the example above is such a common requirement, it can be expressed as:

 "~> 2.0.0"

Check more examples in the Version module.

Basically it's for your convinience, because it allows you to upgrade your deps automatically whenever you do mix deps.upgrade , but it lets you control the upgrade - you could download the upgrade, which crashes your current codebase etc.