Foldable の foldLeft が型推論のためにああいうシグネチャになってる理由(引数の順番とシグネチャ)

gistfile1.txt

scala> trait Foldable1[F[_]] { def foldLeft[A, B](fa: F[A], z: B)(f: (B, A) => B): B }
warning: there was one feature warning; re-run with -feature for details
defined trait Foldable1

scala> val list = new Foldable1[List] { def foldLeft[A, B](fa: List[A], z: B)(f: (B, A) => B) = fa.foldLeft(z)(f) }
list: Foldable1[List] = $anon$1@5884a914

scala> list.foldLeft(List(1,2,3), "")((x, y) => x + y.toString)
res0: String = 123

scala> trait Foldable2[F[_]] { def foldLeft[A, B](fa: F[A], z: B, f: (B, A) => B): B }
warning: there was one feature warning; re-run with -feature for details
defined trait Foldable2

scala> val list = new Foldable2[List] { def foldLeft[A, B](fa: List[A], z: B, f: (B, A) => B) = fa.foldLeft(z)(f) }
list: Foldable2[List] = $anon$1@393881f0

scala> list.foldLeft(List(1,2,3), "", (x, y) => x + y.toString)
<console>:14: error: missing parameter type
       list.foldLeft(List(1,2,3), "", (x, y) => x + y.toString)
                                       ^
<console>:14: error: missing parameter type
       list.foldLeft(List(1,2,3), "", (x, y) => x + y.toString)
                                          ^

scala> list.foldLeft(List(1,2,3), "", (x, y: Int) => x + y.toString)
<console>:14: error: missing parameter type
       list.foldLeft(List(1,2,3), "", (x, y: Int) => x + y.toString)
                                       ^

scala> list.foldLeft(List(1,2,3), "", (x: String, y: Int) => x + y.toString) // 型書かないといけなくなる
res4: String = 123