Support || (or) in build-depends

[tibbe]

Currently build-depends don't support "or" between package constraints. All the package constraints are implicitly and-ed together. This means that users have to encode or-constraints using flags, which is verbose and can be more error prone (e.g. users might set the flag to manual).

Consider the planned split of network into network and network-uri, where the Network.URI module is moved out of network into network-uri. Users that want to use that module need to specify the following dependency:

flag network-uri
  description: Get Network.URI from the network-uri package.
  default: True

library
  if flag(network-uri)
    build-depends:
      network >= 2.6 && network-uri >= 2.6
  else
    build-depends:
      network < 2.6

What if we could instead write

library
  if flag(network-uri)
    build-depends:
      network >= 2.6 && network-uri >= 2.6 || network < 2.6

(The syntax probably need to be different, as build-depends already supports ||, with a different meaning.)

The implementation could still be in terms of flags.

[tibbe]

See also haskell/aeson#177 (comment).

[hvr]

Just as a side-note: this doesn't help with expressing that a package conflicts (i.e. is incompatible) with another package (as it always has to depend on the package one wants to exclude)

[dcoutts]

We should be careful. We originally designed alternation with flags so that the builder has full control over alternatives. With anonymous || choices we might loose that control unless we can carefully find the space of choices where it is benign (which probably coincides with the "if (package > x)" style conditions that we've been considering elsewhere).

[ezyang]

Came here from #3526. Maybe a better way of doing this is to be less general than an or-dependency. Instead, we should support a version test in a conditional. if depend(network >= 2.6): ... else: ...

This solves @dcoutts problem because these are controllable by the builder by construction.