Drastic Prelude changes imminent

[Johan Tibell]

I re-read the goals* of the proposal:
https://wiki.haskell.org/Foldable_Traversable_In_Prelude

"One goal here is that people should be able to use methods from these
modules without the need of a qualified import -- i.e. to prevent clash in
the namespace, by resolving such in favor of the more generalized versions.
Additionally, there are some new methods added to the Foldable class
because it seems to be the "right" thing."

Before FTP, I would have to write this to use Foldable/Traversable:

    import qualified Data.Foldable as F

The import is qualified as to not collide with the Prelude.

If I have code that needs to be compatible with more than one GHC release
(I typically need compatibility with the last 3 major releases), what do I
have to write post-FTP? Since I cannot rely on the Prelude being
generalized (because I might be compiling with a pre-FTP compiler) I need
to either write:

    #if MIN_VERSION_base(x,y,z)
    -- Get Foldable etc from Prelude
    #else
    import Data.Foldable (...)
    import Prelude hiding (...)  -- the same
    #endif

Which is terrible. Alternatively I can write

    import qualified Data.Foldable as F

but now nothing is gained over the pre-FTP state. Only after 3+ years (at
the current GHC release phase) I can drop that one extra import. One out of
perhaps 20. That seems quite a small gain given that we will then have made
Data.List a very confusing module (it's essentially Data.Foldable under a
different name), broken some code due to added type ambiguity, and also
removed one of the simpler ways to remove that ambiguity, which would have
been to import one of the monomorphic list functions that no longer exist.

* People tell me that there are other goals, but they aren't really stated
clearly anywhere.

-- Johan
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