[GHC] #13423: Exception to I/O hack in demand analyzer too broad

[GHC]

#13423: Exception to I/O hack in demand analyzer too broad
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.1
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The I/O hack in the demand analyzer triggers only for expressions that are
 not direct applications of primops (see `io_hack_reqd` in
 `stranal/DmdAnal.hs`). This seems rather arbitrary, and can produce
 surprising results. Consider

 {{{#!hs
 fish :: Int -> IORef Int -> IORef Int -> IO ()
 fish n ref next = do
   writeIORef ref n
   writeIORef next n
   x <- getLine
   case readMaybe x of
     Just True -> fish (n + 1) ref next
     _ -> pure ()
 }}}

 If `next` is bottom, I would expect this to write `n` to `ref` and then
 throw an exception. But we actually get something else. First, the demand
 signature for `fish` ends up being
 `Str=<L,U(U)><S(S),1*U(U)><S(S),1*U(U)><S,U>`, which seems wrong. Then, as
 a result, worker/wrapper produces a wrapper that immediately unboxes both
 `IORef`s, so this is operationally wrong.

 Why do I think this is wrong? The story Haskell users are told, over and
 over, is that evaluation is driven only by execution; that we never force
 anything unless we need its value to determine the next `IO` action. That
 principle is violated here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13423>
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