[GHC] #1323: crash when zipping two lists of the same length...

[GHC]

#1323: crash when zipping two lists of the same length...
--------------------------------------+-------------------------------------
    Reporter:  matthew at wellquite.org  |        Owner:            
        Type:  bug                    |       Status:  closed    
    Priority:  normal                 |    Milestone:            
   Component:  Compiler               |      Version:  6.6       
    Severity:  normal                 |   Resolution:  wontfix   
    Keywords:                         |   Difficulty:  Unknown   
          Os:  Linux                  |     Testcase:  Session.hs
Architecture:  x86_64 (amd64)         |  
--------------------------------------+-------------------------------------
Changes (by simonpj):

  * resolution:  => wontfix
  * testcase:  => Session.hs
  * status:  new => closed

Old description:

> I've personally not checked this against 6.6.1, but "mauke" on #haskell
> says the same crash and same line number appears in the crash report.
>
> $ ghc --version
> The Glorious Glasgow Haskell Compilation System, version 6.6
>
> $ ghc -v --make Session.hs
> Glasgow Haskell Compiler, Version 6.6, for Haskell 98, compiled by GHC
> version 6.6
> Using package config file: /usr/lib/ghc-6.6/package.conf
> hiding package hinstaller-2007.4.21 to avoid conflict with later version
> hinstaller-2007.4.24
> wired-in package base mapped to base-2.0
> wired-in package rts mapped to rts-1.0
> wired-in package haskell98 mapped to haskell98-1.0
> wired-in package template-haskell mapped to template-haskell-2.0
> Hsc static flags: -static
> *** Chasing dependencies:
> Chasing modules from: Session.hs
> Stable obj: []
> Stable BCO: []
> compile: input file Session.hs
> Created temporary directory: /tmp/ghc4588_0
> *** Checking old interface for main:Main:
> [1 of 1] Compiling Main             ( Session.hs, Session.o )
> *** Parser:
> *** Renamer/typechecker:
> *** Desugar:
>     Result size = 379
> *** Simplify:
> *** Deleting temp files:
> Deleting: /tmp/ghc4588_0/ghc4588_0.s
> Warning: deleting non-existent /tmp/ghc4588_0/ghc4588_0.s
> *** Deleting temp dirs:
> Deleting: /tmp/ghc4588_0
> ghc-6.6: panic! (the 'impossible' happened)
>   (GHC version 6.6 for x86_64-unknown-linux):
>         Pattern match failure in do expression at
> simplCore/Simplify.lhs:1540:8-21
>
> Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
>
> Now as the comments in the source say, it works fine if you replace
> newtype with data, and I am aware that there are differences in what they
> do, though I can never quite remember the fine details. So it's quite
> possibly my fault, but an error message other than a GHC crash would be
> nicer!

New description:

 I've personally not checked this against 6.6.1, but "mauke" on #haskell
 says the same crash and same line number appears in the crash report.
 {{{
 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 6.6

 $ ghc -v --make Session.hs
 Glasgow Haskell Compiler, Version 6.6, for Haskell 98, compiled by GHC
 version 6.6
 Using package config file: /usr/lib/ghc-6.6/package.conf
 hiding package hinstaller-2007.4.21 to avoid conflict with later version
 hinstaller-2007.4.24
 wired-in package base mapped to base-2.0
 wired-in package rts mapped to rts-1.0
 wired-in package haskell98 mapped to haskell98-1.0
 wired-in package template-haskell mapped to template-haskell-2.0
 Hsc static flags: -static
 *** Chasing dependencies:
 Chasing modules from: Session.hs
 Stable obj: []
 Stable BCO: []
 compile: input file Session.hs
 Created temporary directory: /tmp/ghc4588_0
 *** Checking old interface for main:Main:
 [1 of 1] Compiling Main             ( Session.hs, Session.o )
 *** Parser:
 *** Renamer/typechecker:
 *** Desugar:
     Result size = 379
 *** Simplify:
 *** Deleting temp files:
 Deleting: /tmp/ghc4588_0/ghc4588_0.s
 Warning: deleting non-existent /tmp/ghc4588_0/ghc4588_0.s
 *** Deleting temp dirs:
 Deleting: /tmp/ghc4588_0
 ghc-6.6: panic! (the 'impossible' happened)
   (GHC version 6.6 for x86_64-unknown-linux):
         Pattern match failure in do expression at
 simplCore/Simplify.lhs:1540:8-21

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}
 Now as the comments in the source say, it works fine if you replace
 newtype with data, and I am aware that there are differences in what they
 do, though I can never quite remember the fine details. So it's quite
 possibly my fault, but an error message other than a GHC crash would be
 nicer!

Comment:

 Good one!  What is happening is this.   During optimisation, GHC removes
 impossible alternatives from a case expression.  Your program has this:
 {{{
 data List :: * -> * -> * where
              Nil :: List a Zero
              Cons :: (TyNum p) => a -> List a p -> List a (Succ p)
 }}}
 where `Succ` is a '''newtype'''.  GHC 6.6 has a split personality about
 newtypes.  During typechecking it regards a newtype T and its
 representation type as distinct.  But during optimisation it regards them
 as the same.  That's WRONG here, because it makes GHC stupidly remove a
 case alterantive that is not dead.

 All this works fine in the HEAD, which regards a newtype and its rep type
 as distinct throughout (much tidier).  So I won't fix 6.6.  Meanwhile
 you'd be better defining your Succ type like this:
 {{{
   data Succ p
 }}}
 You aren't using it to carry data, and GHC allows you to define data types
 with no constructors.

 I'll add this to the test suite though!

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1323>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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