[GHC] #12809: TYPE 'UnboxedTupleRep is still a lie

5 Nov 2016

      #12809: TYPE 'UnboxedTupleRep is still a lie
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           Reporter:  goldfire       |             Owner:  goldfire
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The new scheme for describing kinds of types with values with `TYPE ::
 RuntimeRep -> Type` is a large improvement on the old way of using kind
 `#` for all unlifted types. In particular, the new scheme differentiates
 between the kind of `Int#` and `Float#`, for example, because these have
 different calling conventions. This is Good.

 But there is still a lie left in the whole scheme: `UnboxedTupleRep`,
 which covers all unboxed tuples. Of course, unboxed tuples of different
 arities and contents have different calling conventions, so these should
 be distinguished at the kind level.

 Simon and I have cooked up a new scheme to handle this, summarized in
 these definitions:

 {{{#!hs
 TYPE :: RuntimeRep -> Type    -- highly magical, just as before
 type RuntimeRep = [UnaryRep]  -- this bit is the new part
 data UnaryRep = PtrRepLifted  -- like the old RuntimeRep type
               | PtrRepUnlifted
               | IntRep
               | ...
 type Lifted = '[PtrRepLifted]  -- a very common case
 type Type = TYPE Lifted        -- good old Type
 }}}

 The `UnaryRep` type is the big sum of all possible representations, just
 like the `RuntimeRep` of today. It drops `VoidRep` and `UnboxedTupleRep`,
 however.

 The interpretation of this is that the kinds now include a ''list'' of
 unary representation forms. A "unary representation" corresponds to what
 we might expect to store in one machine register at runtime. Unboxed
 tuples naturally have a variable number of associated unary reps: this is
 precisely what an unboxed tuple means. It also baldly states that the
 unary unboxed tuple is identical (at runtime) to the thing in the tuple
 (which is correct) and also allows us to remove the runtime distinction
 between `(# #)` and `Void#`, which now both have kind `TYPE '[]`. (Indeed,
 perhaps we should just say `type Void# = (# #)`.)

 This will not be backward compatible with GHC 8.0. But I'm OK with this,
 as any user access to these features requires importing internal modules,
 and it seems quite painful to try to come up with a migration story here
 for an experimental feature.

 Patch will be written this weekend, with any luck.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12809>
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