patch applied (ghc): Improve RULE matching a bit more

[Simon Peyton Jones]

Sat Jun 24 09:04:21 PDT 2006  simonpj at microsoft.com
  * Improve RULE matching a bit more
  
  Consider this example (provided by Roman)
  
  	foo :: Int -> Maybe Int -> Int
  	foo 0 (Just n) = n
  	foo m (Just n) = foo (m-n) (Just n)
  
  SpecConstr sees this fragment:
  
  	case w_smT of wild_Xf [Just A] {
  	  Data.Maybe.Nothing -> lvl_smf;
  	  Data.Maybe.Just n_acT [Just S(L)] ->
  	    case n_acT of wild1_ams [Just A] { GHC.Base.I# y_amr [Just L] ->
  	    $wfoo_smW (GHC.Prim.-# ds_Xmb y_amr) wild_Xf
  	    }};
  
  and correctly generates the rule
  
  	RULES: "SC:$wfoo1" [0] __forall {y_amr [Just L] :: GHC.Prim.Int#
  					  sc_snn :: GHC.Prim.Int#}
  	  $wfoo_smW sc_snn (Data.Maybe.Just @ GHC.Base.Int (GHC.Base.I# y_amr))
  	  = $s$wfoo_sno y_amr sc_snn ;]
  
  BUT we must ensure that this rule matches in the original function!
  Note that the call to $wfoo is
  	    $wfoo_smW (GHC.Prim.-# ds_Xmb y_amr) wild_Xf
  
  During matching we expand wild_Xf to (Just n_acT).  But then we must also
  expand n_acT to (I# y_amr).  And we can only do that if we look up n_acT
  in the in-scope set, because in wild_Xf's unfolding it won't have an unfolding
  at all. 
  
  Happily, fixing the bug is easy: add a call to 'lookupRnInScope' in the 
  (Var v2) case of 'match'.
  

    M ./compiler/basicTypes/VarEnv.lhs -1 +4
    M ./compiler/specialise/Rules.lhs -4 +48