Splitting Network.URI from the network package

[Johan Tibell]

On Mon, Aug 4, 2014 at 6:21 PM, Michael Snoyman <michael at snoyman.com> wrote:
> The idea is that users of Network.URI will be able to add a dependency
along
> the lines of:
>
>     build-depends: network >= 2.5 && < 2.7, network-uri >= 2.5 && < 2.7
>
> and work with both the pre- and post-split versions of the package,
without
> any conditional compilation or cabal flags. I'm sensitive to adding any
> requirements of cabal flags, since (1) it clutters cabal files quite a
> bit[1], and (2) we just went through some painful cabal-install dependency
> solver issues around flags.

I agree that using flags to encode OR is a bit heavy weight. We should
really support || in build-depends. For completeness, here's what a
flag-based solution looks like. Assuming that network-uri-2.6 has the URI
module and network-2.6 no longer has it we get this flag setup:

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

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

If the user wants something else from network (e.g. Network.Socket) it
would be

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

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

Lets see if I understood your scheme correctly. We'd have

 * network-2.5: Has URI module.
 * network-uri-2.5: Doesn't have URI module. Depends on < network-2.6 (i.e.
network-2.5 in this example).
 * network-2.6: Doesn't have URI module.
 * network-uri-2.6: Has URI module. Depends on network >= 2.6 (i.e. network
2.6 in this example).

Given

build-depends: network >= 2.5 && < 2.7, network-uri >= 2.5 && < 2.7

legal combinations of the above packages are

 * network-2.5 and network-uri-2.5, which gives URI through network, and
 * network-2.6 and network-uri-2.6, which gives URI through network-uri.

There are two implications of this, which are slightly strange:

 * There's no point in anyone depending on only network-uri-2.5 (as it
doesn't expose anything).
 * network-uri-2.6 and later will have to have a lower dependency on
network forever, even though network-uri doesn't use anything from network
(this is reflected when you discuss the upper bound in the context of the
PVP later).

I don't know if there's any other implications. Can anyone think of any?

-- Johan
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