Given the following two modules:

Fold.hs:

module Fold (Tree, fold') where

data Tree a = Leaf | Node a !(Tree a) !(Tree a)

-- Strict, pre-order fold.
fold' :: (a -> b -> a) -> a -> Tree b -> a
fold' f = go
  where
    go z Leaf = z
    go z (Node a l r) = let z'  = go z l
                            z'' = f z' a
                        in z' `seq` z'' `seq` go z'' r
{-# INLINE fold' #-}

FoldTest.hs:

module FoldTest (sumTree) where

import Fold

sumTree :: Tree Int -> Int
sumTree = fold' (+) 0

I'd expect that the accumulator z used in go to be an unboxed Int#. However, it's boxed:

sumTree1 :: Int
sumTree1 = I# 0

sumTree_go :: Int -> Fold.Tree Int -> Int
sumTree_go =
  \ (z :: Int) (ds_ddX :: Fold.Tree Int) ->
    case ds_ddX of _ {
      Fold.Leaf -> z;
      Fold.Node a l r ->
        case sumTree_go z l of _ { I# z' ->
        case a of _ { I# a# ->
        sumTree_go (I# (+# z' a#)) r
        }
        }
    }

sumTree :: Fold.Tree Int -> Int
sumTree =
  \ (eta1_B1 :: Fold.Tree Int) ->
    sumTree_go sumTree1 eta1_B1

Given this definition of fold'

fold' :: (a -> b -> a) -> a -> Tree b -> a
fold' f = go
  where
    go z _ | z `seq` False = undefined
    go z Leaf = z
    go z (Node a l r) = go (f (go z l) a) r
{-# INLINE fold' #-}

I get the core I want. However, this version isn't explicit in that the left branch (i.e. go z l) should be evaluated before f is called on the result. In other words, I think my first definition is the one that correctly expresses the evaluation order, yet it results in worse core.