[GHC] #1349: Generalise the ! and UNPACK mechanism for data types, to unpack function arguments

[GHC]

#1349: Generalise the ! and UNPACK mechanism for data types, to unpack function
arguments
---------------------------------+------------------------------------------
    Reporter:  simonpj           |        Owner:              
        Type:  task              |       Status:  new         
    Priority:  normal            |    Milestone:  6.12.3      
   Component:  Compiler          |      Version:  6.6.1       
    Keywords:                    |   Difficulty:  Unknown     
          Os:  Unknown/Multiple  |     Testcase:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
---------------------------------+------------------------------------------

Comment(by tibbe):

 While working on a builder monoid for `Data.Text` I ran in to a problem
 related to lack of unpacking for continuation arguments.  Using these
 types:

 {{{
 data Buffer s = Buffer {-# UNPACK #-} !(A.MArray s Word16)
                        {-# UNPACK #-} !Int  -- offset
                        {-# UNPACK #-} !Int  -- used units
                        {-# UNPACK #-} !Int  -- length left

 newtype Builder = Builder {
      runBuilder :: forall s. (Buffer s -> ST s [S.Text])
                 -> Buffer s
                 -> ST s [S.Text]
    }
 }}}

 the following module generates bad code with lots of boxing and closures:

 {{{
 module Loop (loop) where

 import Data.Monoid (mappend, mempty)
 import Data.Text.Lazy.Builder

 loop  :: Int -> Builder
 loop n = go 0 mempty
   where
     go i acc
       | i < n     = go (i+1) (acc `mappend` singleton 'a')
       | otherwise = acc
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1349#comment:15>
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