added Container.MustGet() function

[drgomesp]

Thanks for the pull request, @vgarvardt.

Although I appreciate your contribution, I'm having trouble finding a reason for a MustGet method.

We can achieve the same results with

service, err := container.Get("foo)

if err !=nil {
    panic(err)
}

I don't think the container should panic if a service is not found.

[vgarvardt]

It's just wrap for the code above to avoid code duplications - e.g. in my case missed id in container alway must panic, so MustGet saves 4 lines of code for each usage. Same as html/template.Must() and many other examples.

[drgomesp]

I understand your point, but my question (which is more of a rhetoric question) is: does the container have the responsibility of panicking if a service is not found?

For example, when you hit a web page the doesn't exist, does the response comes as 500 or 404?

[tomsquest]

@drgomesp I do think the container must panic when an asked dependency is not found, or better the asker can choose, via Get and MustGet.

• Spring DI (java) panics when a dependency is not found (context.getBean(Foo.class))
• Persistance API offer the choice: either you get() which returns possibly null, or you load() which throw an error

[vgarvardt]

For example, when you hit a web page the doesn't exist, does the response comes as 500 or 404?

For example when you hit a config loader or any other important resource that does not exist.

[vgarvardt]

Added couple tests.

[drgomesp]

Thank you, @vgarvardt!