[Haskell-cafe] what happens to ()'s from Core?

John Lato jwlato at gmail.com
Wed Oct 5 19:38:53 CEST 2011


I'm working on a small EDSL, and I think I've finally managed to get
GHC to compile it to good core.  Basically, it allows for the creation
of expressions like:

> g = 0.5*x + 0.1*y

which is then compiled to a tuple (related work: CCA, stream fusion)

> exists s. (s, s -> Double -> (s,Double))

I also have a function 'mapAccumL :: (V.Unbox a, V.Unbox b) => (s -> a
-> (s,b)) -> s -> V.Vector a -> V.Vector b'.  Basic usage would be
similar to:

> import qualified Data.Vector.Unboxed as V
> main = do
>   let (gs, gf) = $(compile [] g)
>       ys = mapAccumL gf gs $ V.enumFromTo (1::Double) 5
>  print ys

For 'g' as above, I currently get 's :: (((), ()), Double)', which is
expected.  GHC produces the following core for the inner loop, which
looks pretty good to me:

letrec {
      $s$wa_s2OL [Occ=LoopBreaker]
        :: ()
           -> ()
           -> GHC.Prim.Double#
           -> GHC.Prim.Int#
           -> GHC.Prim.State# (Control.Monad.Primitive.R:PrimStateST s_a1Y9)
           -> (# GHC.Prim.State# s_a1Y9, () #)
      [LclId, Arity=5, Str=DmdType LLLLL]
      $s$wa_s2OL =
        \ _
          (sc2_s2Oq :: GHC.Prim.Double#)
          (sc3_s2Or :: GHC.Prim.Int#)
             :: GHC.Prim.State#
                  (Control.Monad.Primitive.R:PrimStateST s_a1Y9)) ->
          case GHC.Prim.<# sc3_s2Or rb1_a2EV of _ {
            GHC.Types.False -> (# sc4_s2Os, GHC.Unit.() #);
            GHC.Types.True ->
              let {
                x#_a2aI [Dmd=Just L] :: GHC.Prim.Double#
                [LclId, Str=DmdType]
                x#_a2aI =
                          rb2_a2EW (GHC.Prim.+# rb_a2EU sc3_s2Or))
                    (GHC.Prim.*## sc2_s2Oq 0.1) } in
                (GHC.Prim.+# sc3_s2Or 1)
                    @ (Control.Monad.Primitive.PrimState (GHC.ST.ST s_a1Y9))
                     `cast` (GHC.Prim.State#
(Control.Monad.Primitive.TFCo:R:PrimStateST <s_a1Y9>))
                             :: GHC.Prim.State#
(Control.Monad.Primitive.R:PrimStateST s_a1Y9)
(GHC.ST.ST s_a1Y9)))))
                 `cast` (GHC.Prim.State#
                           (Control.Monad.Primitive.TFCo:R:PrimStateST <s_a1Y9>)
                         :: GHC.Prim.State#
(GHC.ST.ST s_a1Y9))
(Control.Monad.Primitive.R:PrimStateST s_a1Y9)))
          }; } in

So my question is, what happens to the ()'s after this stage?  Since
they're not used, and also expressed as literals in core (both in the
recursive case and the original call site of $s$wa_s2OL, is the
backend smart enough to get rid of them completely?

Thanks for any advice,
John L.

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