This is a modified version of haskell-indentation-mode.  Currently
the semantic parser is not changed, but the UI of tab completion is
reworked.  Most notably:

 - DEL and C-d is not mapped: if you want to indent backwards, you can
   use S-TAB.  This means no random jumping on backspace anymore.

 - TAB steps to the right as before, but when the end is reached, it
   starts going to the left instead of wrapping around.

 - TAB stays inside the code, instead of going to the beginning of the
   line.  As in pyhton-mode and perl-mode.

 - Region indentation common case is supported: TAB and S-TAB is simply
   moving the whole region to the left/right by 1-column.  Can be
   pressed repeatedly.

 - The current indentations are shown as underscores in the current
   line.  So you have some visual indication on what's gonna happen.
   Also useful while hacking on the parser and want to see the results.
   Can be turned off by setting hi2-show-indentations to nil in your
   init file or calling hi2-disable-show-indentations from the buffer.
   If there are collisions with other overlay hacking modes
   (e.g. fill-column-indicator), try to turn off
   hi2-show-indentations-after-eol.

 - The buffer is not changed when indentation is not changed (so there
   are no undo points created and no dirty flag in the buffer if
   pressing TAB had no effect).

 - The code for all this is somewhat commented and cleaned.

There is ongoing development on making this better and I invite
everyone to participate (report bugs, fix issues, etc.):
  https://github.com/errge/hi2

Once this is stable and better in every way than the default
haskell-indentation-mode, I'm happy to merge it with haskell-mode,
but until this is moving fast, I want to keep it separate.