Even though it already has `reduce`...

fold.io

my_fold := method(list, coll, lambda,
  if (list size == 0) then (
    return coll
  ) else (
    return fold(list rest,
                lambda call(coll, list first),
                lambda)
  )
)

# Io> fold(
# ...   list(1, 2, 3),
# ...   0,
# ...   block(x, y,
# ...     x + y
# ...   )
# ... )
# ==> 6

my_map := method(items, lambda,
  my_fold(
    items,
    list(),
    block(result, item,
      result append(lambda call(item))
    )
  )
)

# Io> map(
# ...   list(1, 2, 3),
# ...   block(x,
# ...     x * x
# ...   )
# ... )
# ==> list(1, 4, 9)