arrow analogs of lambda case and multi-way if

[jeltsch]

GHC has now support for lambda case and multi-way if (see ticket #4359). It would be good if arrow variants of these features would be implemented. There are three things that should be considered:

Lambda case expressions should have analog proc expressions where

proc case pat_1 -> cmd_1
          ...
          pat_n -> cmd_n

desugars to

proc fresh -> case fresh of
                  pat_1 -> cmd_1
                  ...
                  pat_n -> cmd_n

Lambda case expressions should also have analog arrow commands where

\ case pat_1 -> cmd_1
       ...
       pat_n -> cmd_n

desugars to

\ fresh -> case fresh of
               pat_1 -> cmd_1
               ...
               pat_n -> cmd_n

Multi-way if expressions should have analog arrow commands where

if | cond_1 -> cmd_1
     ...
     cond_n -> cmd_n

desugars to

case () of
    _ | cond_1 -> cmd_1
      ...
      | cond_n -> cmd_n

Identifiers pat_i, cond_i, and cmd_i denote patterns, boolean expressions, and arrow commands, respectively.

Bug #7071 has to be fixed before starting with this ticket.

[simonpj]

To me this all looks plausible. Do work with Dan on #7071.