Allow to pull extra-deps from other resolvers

[zudov]

It's often that a project uses an resolver x but also needs some dependencies which aren't in the x (but are in the y, or z). At the moment the only option is to just specify it in extra-deps with an explicit version. It would be nice if it was possible to pull them from some other resolver.

This syntax comes to mind (and would probably explain the intention better than text above):

- resolver: lts-3.9
- deps:
  - extra: # same as current `extra-deps`
    - package-which-isnt-present-in-any-of-resolvers-0.1.2.3
  - nightly-2015-10-16:
    - package-which-is-present-in-nightly

package-which-is-present-in-nightly doesn't need to contain an explicit version, it would just pick up the one from nightly-2015-10-16.

[borsboom]

I'm a bit unclear on what concrete benefit this would really provide. Is it so hard to provide the exact version number in this case?

[zudov]

Let's imagine a situation where we use lts resolver, but want some other dependencies which are not present in lts but are in the nightly. Currently we end up with something like that:

extra-deps:
  - distributed-process-0.5.5.1
  - distributed-process-async-0.2.2
  - distributed-process-client-server-0.1.3.1
  - distributed-process-execution-0.1.2.1
  - ... many more

When I add those extra-deps, I don't have much guarantees that they would work fine with my project. However, if I pick the same versions as in the latest nightly release I can get the guarantee that they would at least compile against each other. So for every item in extra-deps I have to (1) check the version in some nightly release; (2) put that version in my extra-deps. I'd much rather just write:

- deps:
  - nightly-2015-10-16:
    - distributed-process
    - distributed-process-async
    - distributed-process-client-server
    - distributed-process-execution
    - ...many more

Now, let's imagine that some time passed and I want to bump these versions. Again I would have to open the stackage page of the latest resolver and check a version of every package that I have in my extra-deps. I would much rather prefer to do:

-    - nightly-2015-10-16
+    - nightly-2015-10-22

[zudov]

@borsboom So, answering your question.

It's not hard to provide the exact version number. But if you have more than a few dependencies and want them to be of the same versions as in some nightly it becomes repetitive and error-prone work which could be easily automatized, and I don't see any downsides of such feature (please tell me if you do).

[mgsloan]

The main downside I can think of for this is just that it's added code / configuration complexity.

Extensible snapshots seem to be related: #863

I'm on the wall about whether we should have something like this, but maybe it could be part of snapshot extension.

[borsboom]

Related: #1265 (Unify extra-deps and extensible snapshots)

[mgsloan]

I don't think this is going to be implemented, closing.