Step 0

main

foldl

FUN(f acc list ->
       case list of {
          Nil -> acc;
          Cons h t ->
             let { newAcc = THUNK(f_? acc h) } in foldl_3 f newAcc t
       })

list1

CON(Cons one nil)

list2

CON(Cons two list1)

list3

CON(Cons three list2)

main

THUNK(sum_1 list3)

nil

CON(Nil)

one

CON(I 1)

plusInt

FUN(x y ->
       case x of {
          I i ->
             case y of {
                I j ->
                   case plus# i j of {
                      x -> let { result = CON(I x) } in result
                   }
             }
       })

sum

FUN(list -> foldl_3 plusInt zero list)

three

CON(I 3)

two

CON(I 2)

zero

CON(I 0)