[GHC] #5205: Control.Monad.forever leaks space

[GHC]

#5205: Control.Monad.forever leaks space
---------------------------------+------------------------------------------
    Reporter:  akio              |        Owner:  igloo                  
        Type:  bug               |       Status:  new                    
    Priority:  highest           |    Milestone:  7.2.1                  
   Component:  libraries/base    |      Version:  7.0.3                  
    Keywords:                    |     Testcase:                         
   Blockedby:                    |   Difficulty:                         
          Os:  Unknown/Multiple  |     Blocking:                         
Architecture:  Unknown/Multiple  |      Failure:  Runtime performance bug
---------------------------------+------------------------------------------
Changes (by simonpj):

  * owner:  simonpj => igloo


Comment:

 I've made 'forever' INLINABLE, and added a SPECIALISE pragma in GHC.ST.
 {{{
 commit ae10342b49b95393b09ffee8df8c847409699968
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Thu Jun 9 20:44:21 2011 +0100

     Make 'forever' inlinable (fixes Trac #5205)

     See Note [Make forever INLINABLE] in Control.Monad

 >---------------------------------------------------------------

  Control/Monad.hs |   18 ++++++++++++++++++
  GHC/ST.lhs       |    4 ++++
  2 files changed, 22 insertions(+), 0 deletions(-)
 }}}

 That fixes the bug, but only with `-O`.  Without `-O` you still get the
 leak, and I don't really think its unreasonable.  You have
 {{{
 x = forever (return ())
 }}}
 which expands to
 {{{
 x = return () >> return () >> return () >> ....etc....
 }}}
 If `(>>)` was expensive when applied to two args, then it'd be right to
 hang onto the computed result.  In the case of IO it isn't expensive, and
 it's best to recompute (and save space) but only the optimiser can reveal
 that.

 Ian: can you add a perf/ test please, then close?

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5205#comment:4>
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