Editing Compatibility Modules (section)

= Worked example =

Let's work out how this will play out for our fictitious <code>parseInt</code> example above. base 4.6 contains the Parse module which is about to change, so we must create a package base-compat-4_6 containing a single module, defined as such:

<pre>module Parse.Compat (module Parse) where

import Parse</pre>

This can be uploaded to Hackage, and users can immediately transition to using Parse.Compat anywhere they would have used Parse. Eventually, it comes time for the new GHC release, so we need to update base-compat-4_6 to support the new Parse API using conditional compilation. This would look something like:

<pre>{-# LANGUAGE CPP #-}
#if MIN_VERSION_base(4, 7, 0)
module Parse.Compat (module Parse, parseInt) where

import Parse hiding (parseInt)
import qualified Parse

parseInt :: String -> Int
parseInt = Parse.parseInt 0
#else
module Parse.Compat (module Parse) where

import Parse
#endif</pre>

This can be uploaded to Hackage before the GHC release is made, and then users of the compatibility package will be completely insulated from the GHC upgrade.

In sum:

* Initial release of base-compat-4_6 version 1.0.0 would only depend on base 4.6, and be a pass-through shim.
* When we're about to make the GHC 7.8 release, we at that point release base-compat-4_6 version 1.0.1, which includes base 4.7 support.
** The only change here would be if someone in the community volunteers to add the base 4.7 support earlier in order to ease GHC HEAD testing.
* The only changes to be made to this package in the future would be:
** Bug fixes.
** Support for future base versions.
* At no point would there ''ever'' be a breaking API change of this package. In other words, the versions released will always be <= 1.1.