id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
3698,Bad code generated for zip/filter/filter loop,rl,,"Here is the program:
{{{
zip_filter :: (Num a, Ord a) => a -> [a] -> [a] -> [a]
zip_filter x as bs = zipWith (+) (filter (<x) as) (filter (<x) bs)
}}}
GHC generates this:
{{{
poly_z_smp = \ (@ a_aiz) _ -> GHC.Types.[] @ a_aiz

T.zip_filter =
  \ (@ a_aiz) $dNum_ajm $dOrd_ajn eta_B3 eta_B2 eta_B1 ->
    letrec {
      go_smr :: [a_aiz] -> [a_aiz] -> [a_aiz]
      go_smr =
        \ (ds_ak5 :: [a_aiz]) ->
          case ds_ak5 of _ {
            [] -> poly_z_smp @ a_aiz;
            : y_aka ys_akb ->
              let {
                r_smt :: [a_aiz] -> [a_aiz]
                r_smt = go_smr ys_akb } in
              case GHC.Classes.< @ a_aiz $dOrd_ajn y_aka eta_B3 of _ {
                GHC.Bool.False -> r_smt;
                GHC.Bool.True ->
                  \ (ds_alR :: [a_aiz]) ->
                    case ds_alR of _ {
                      [] -> GHC.Types.[] @ a_aiz;
                      : y_alW ys_alX ->
                        GHC.Types.:
                          @ a_aiz (GHC.Num.+ @ a_aiz $dNum_ajm y_aka y_alW) (r_smt ys_alX)
                    }
              }
          }; } in
    go_smr
      eta_B2
      (GHC.List.filter
         @ a_aiz
         (\ (ds_djz :: a_aiz) ->
            GHC.Classes.< @ a_aiz $dOrd_ajn ds_djz eta_B3)
         eta_B1)
}}}

Eta-expanding `go_smr` would result in much better code.",bug,new,low,7.6.2,Compiler,6.13,,,,Unknown/Multiple,Unknown/Multiple,Runtime performance bug,,,,,