Implement --allow-older

[hvr]

This provides the dual flag to --allow-newer for symmetry.

The need for relaxing lower bounds was suggested by @mgsloan here.

[hvr]

Unfortunately, AllowNewer was moved into the Cabal library thereby making it part of the public API :-(

So I didn't rename the AllowNewer type to avoid API breakage, nor did I introduce a type AllowOlder as it would have resulted in code duplication for little gain.

TODO:

• documentation
• changelog
• testsuite

[23Skidoo]

Since this is targeted at master, why is it important to avoid API breakage?

Also seems to break Travis.

[hvr]

@23Skidoo If it's ok to break the API in Cabal 1.26 I'll happily rename the type to something like AllowScope or RelaxScope (any better ideas?)

and Travis fails because I didn't adapt the testsuite yet...

[23Skidoo]

I think it's fine to break the API in this case. It's unlikely that this will break any setup scripts.

[hvr]

@23Skidoo do you have any preference for how to rename the current AllowNewer type? Also I'd do this in a separate commit, maybe even in a spearate PR, as this will likely touch a lot of code.

For reference, here's the affected types that would need renaming to become less specific to Newer:

-- | Policy for relaxing upper bounds in dependencies. For example, given
-- 'build-depends: array >= 0.3 && < 0.5', are we allowed to relax the upper
-- bound and choose a version of 'array' that is greater or equal to 0.5? By
-- default the upper bounds are always strictly honored.
data AllowNewer =

  -- | Default: honor the upper bounds in all dependencies, never choose
  -- versions newer than allowed.
  AllowNewerNone

  -- | Ignore upper bounds in dependencies on the given packages.
  | AllowNewerSome [AllowNewerDep]

  -- | Ignore upper bounds in dependencies on all packages.
  | AllowNewerAll
  deriving (Eq, Read, Show, Generic)

-- | Dependencies can be relaxed either for all packages in the install plan, or
-- only for some packages.
data AllowNewerDep = AllowNewerDep PackageName
                   | AllowNewerDepScoped PackageName PackageName
                   deriving (Eq, Read, Show, Generic)

[23Skidoo]

@hvr I think that something like the following should work:

data RelaxDeps = RelaxDepsNone | RelaxDepsSome [RelaxedDep] | RelaxDepsAll

data RelaxedDep = RelaxedDep PackageName
                | RelaxedDepScoped PackageName PackageName

newtype AllowNewer = AllowNewer RelaxDeps
newtype AllowOlder = AllowOlder RelaxDeps

[dcoutts]

Seems reasonable.

[hvr]

@23Skidoo @dcoutts ok, so you want this typesafe via newtypes to reduce the risk of mixing up codepaths related to newer/older? :-)

[hvr]

@23Skidoo @dcoutts I've rebased this on top of #3476 and the testsuite still compiles... yay!

[kosmikus]

This PR is marked "solver", but the way it's implemented, it does not seem to touch solver code at all.

In principle, I'd prefer somewhat if both --allow-newer and --allow-older would actually be implemented by tweaking the solver rather than by tweaking the index, but that would be a major change.

From what I can see, this PR looks useful and ok, but it touches a lot of modules I don't know very much about.

[hvr]

rebased against latest master

[23Skidoo]

@kosmikus

In principle, I'd prefer somewhat if both --allow-newer and --allow-older would actually be implemented by tweaking the solver rather than by tweaking the index, but that would be a major change.

One problem with that is that ./Setup.hs configure --allow-newer won't be able to utilise the solver-based code path, so we'll have to maintain the current one anyway.

[23Skidoo]

LGTM, I think that this can be merged once the TODO items are implemented.

[hvr]

rebased one last time & added docs/changelog/tests... once travis passes I'm gonna merge this to get this PR out of my sight... :-)