There is no such thing as a "linear value" per se, since linearity is on the arrow. But speaking of linear values makes sense when there's an ambient monad, and all of the restrictions you expect are enforced; see Section 2.7 (and the last paragraph of Section 6.1) of https://arxiv.org/pdf/1710.09756.pdf for details.

The general idea is that the discipline is automatically enforced via the type system, e.g. you have a library which provides a type FragileAndDangerousResource, but the only way to interact with such a thing is via (linear) functions along the lines of

    withFADR :: (FragileAndDangerousResource #-> a) -> a
    
    addToFADR :: Int -> FragileAndDangerousResource #-> FragileAndDangerousResource

In other words, linear types are purportedly useful precisely because they make it reasonable to ask for that extreme discipline on the programmer's part, because the compiler is right there to tell them when they screw it up--ideally with a helpful error message.

Would

  withFADR' :: (FragileAndDangerousResource #-> IO ()) -> IO ()

be a better type? My reasoning is if I use the linear identity function

  lid :: forall a. a #-> a

in the following

  withFADR lid

I should be able to “break out of” the managed context [1] by the types. Using

  withFADR'' :: (FragileAndDangerousResource #-> ()) -> ()

is obviously not right since nothing could be done with the `FragileAndDangerousResource`, so I think side effects are necessary.

---

[1]: I'm assuming that's what you're going for with `withFADR`.

Good observation. I believe you would use

  withFADR :: (FragileAndDangerousResource #-> Unrestricted a) -> Unrestricted a

One can't tag "linear" values in the type system but one can tag non-linear "unrestricted" values. See "borrow" in Section 7.2, page 5:26 of https://arxiv.org/pdf/1710.09756.pdf

Yeah, that seems right. I'd also believe you if you told me that the second arrow also needs to be linear, since I can't really give a good reason why it should be one way or the other off the top of my head.

I admittedly don't really have a great intuition for how to work with this machinery, since I haven't had a chance/excuse to actually play around with it, to be honest; I'm just going off of a SPJ talk on this stuff I watched many months ago, which I mostly understood at the time, and the paper associated with the GHC implementation, which I understood substantially less of.

No that's not the case. I am fairly sure that situation you describe results in a type error. The extension does in fact enforce linearity, and does not have such a trivial & universal escape hatch afaiu.

Feel free to give GHC 9.0 a try with a minimal example of your scenario!

There is a recent article about these scenarios, you may find it helpful - https://tek.brick.do/64693fb8-39b4-40a5-8762-768009eeed91