CPR Analysis bug in ghc 5.00.1

[Artur =?x-user-defined?q?Zaw=B3ocki?=]

Hello, 
I've found this one when compiling pretty-printing combinators of
Swierstra & Azero:

$ ghc -c -O UU_Pretty.hs 
$ ghc -v -c -O UU_Pretty_ext.hs

Glasgow Haskell Compiler, Version 5.00.1, for Haskell 98, compiled by
GHC version 4.08.2
Using package config file: /usr/local/lib/ghc-5.00.1/package.conf
Hsc static flags: -static -fignore-asserts -ffoldr-build-on
-fdo-eta-reduction -fdo-lambda-eta-expansion -fcase-merge -flet-to-case
-flet-no-escape
*** Ineffective C pre-processor
echo '{-# LINE 1 "UU_Pretty_ext.hs" #-}' > /tmp/ghc1712.hspp && cat
UU_Pretty_ext.hs >> /tmp/ghc1712.hspp

*** Checking old interface for hs = Just "UU_Pretty_ext.hs", hspp = Just
"/tmp/ghc1712.hspp":
Compiling UU_Pretty_ext    ( UU_Pretty_ext.hs, UU_Pretty_ext.o )
... 
(skipped output from various passes, and then...) 

*** Constructed Product analysis:
Removing: /tmp/ghc1712.hc
Warning: can't remove tmp file /tmp/ghc1712.hc
Removing: /tmp/ghc1712.hspp
ghc-5.00.1: panic! (the `impossible' happened, GHC version 5.00.1):
        CPR Analysis tried to take the lub of a function and a tuple


I've extracted and simplified few offending definitions from original
modules and ended up with the following files (in fact, the ghc output
above resulted from compiling these simplified files):

UU_Pretty.hs, containing

------------------------------
module UU_Pretty where

infixr 2 >-<

newtype PP_Doc = PPDoc T_PPS

class Show a => PP a where
  pp     :: a   -> PP_Doc
  pp     = undefined
 
instance PP PP_Doc where
  pp     = undefined

instance Show PP_Doc where
  show _ = undefined

empty :: PP_Doc
empty = PPDoc (sem_PPS_Above undefined undefined)

(>-<) :: (PP a, PP b) => a -> b -> PP_Doc
u >-< l  = PPDoc (sem_PPS_Above ppu ppl)
  where (PPDoc ppu) = pp u
        (PPDoc ppl) = pp l

type T_PPS = Int -> (Int, Int, Int, Int) 

sem_PPS_Above :: T_PPS -> T_PPS -> T_PPS
sem_PPS_Above upper lower _
 = let  (_, a1, _, a3) = upper 0
        (_, b1, b2, b3) = lower 0
   in   (0, a1 + b1, b2, a3 + b3) 
------------------------------

and UU_Pretty_ext.hs, with just

------------------------------
module UU_Pretty_ext where

import UU_Pretty

vlist :: PP a => [a] -> PP_Doc
vlist    = foldr  (>-<) empty
------------------------------


Yes, I know it's a known bug, but maybe this posting will help to track
it down someday.


--
Artur Zawlocki
[EMAIL PROTECTED]


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