[GHC] #6078: GHC panic encountered for unlifted let bindings

[GHC]

#6078: GHC panic encountered for unlifted let bindings
--------------------------------+-------------------------------------------
 Reporter:  SimonMeier          |          Owner:                         
     Type:  bug                 |         Status:  new                    
 Priority:  normal              |      Component:  Compiler               
  Version:  7.4.1               |       Keywords:  unlifted, bang patterns
       Os:  Unknown/Multiple    |   Architecture:  Unknown/Multiple       
  Failure:  Compile-time crash  |       Testcase:                         
Blockedby:                      |       Blocking:                         
  Related:  4498                |  
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Comment(by michalt):

 Thanks for reporting! I can reproduce this with HEAD. When pattern
 matching
 unlifted types one must use the bang, have a look at: #2806. So the code
 should
 use the ```!```.

 Now what goes wrong with GHC is that it does not reject the program but
 goes on
 and tries to compile it (and only tries to warn about it, see #3278). As
 far as
 I can see (which might be wrong), there are a few things happening:
  * the binding ```ip1p``` gets generalized and so the pattern is
 represented by
    ```AbsBinds```
  * the fix for #4498 makes bang pattern bindings monomorphic (but
 obviously only
    works if there are bang patterns, so it doesn't apply here; see
    ```decideGeneralisationPlan```
 source:compiler/typecheck/TcBinds.lhs#L1175)
  * this causes the error when ```dsStrictBind``` is called
    source:compiler/deSugar/DsExpr.lhs#L159
 I guess the right fix would be to reject the program -- there's a note in
 source:compiler/typecheck/TcBinds.lhs#L1260
 {{{
 -- This should be a checkTc, not a warnTc, but as of GHC 6.11
 -- the versions of alex and happy available have non-conforming
 -- templates, so the GHC build fails if it's an error:
 ; warnUnlifted <- woptM Opt_WarnLazyUnliftedBindings
 ; warnTc (warnUnlifted && not bang_pat && lifted_pat)
 }}}
 but the comment seems to still apply to current HEAD, i.e. the build fails
 on
 code generated by Happy and Alex. If I have time, I'll try to have a look
 if it
 would be difficult to fix them.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6078#comment:1>
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