Seqのappendの疑問

p21.scala

def insertAtErrorL[A](e: A, i: Int, ls: Seq[A]): Seq[A] = ls.take(i) :+ e ++ ls.drop(i)
// scala> <console>:7: error: value ++ is not a member of type parameter A
//        def insertAtErrorL[A](e: A, i: Int, ls: Seq[A]): Seq[A] = ls.take(i) :+ e ++ ls.drop(i)
//                                                                                  ^

def insertAtErrorR[A](e: A, i: Int, ls: Seq[A]): Seq[A] = ls.take(i) ++ e +: ls.drop(i)
// scala> <console>:1: error: left- and right-associative operators with same precedence may not be mixed
//        def insertAtErrorR[A](e: A, i: Int, ls: Seq[A]): Seq[A] = ls.take(i) ++ e +: ls.drop(i)
//                                                                             ^

def insertAtL[A](e: A, i: Int, ls: Seq[A]): Seq[A] = (ls.take(i) :+ e) ++ ls.drop(i)
// scala> insertAtL: [A](e: A, i: Int, ls: Seq[A])Seq[A]

def insertAtR[A](e: A, i: Int, ls: Seq[A]): Seq[A] = ls.take(i) ++ (e +: ls.drop(i))
// scala> insertAtR: [A](e: A, i: Int, ls: Seq[A])Seq[A]

def insertAt[A](e: A, i: Int, ls: Seq[A]): Seq[A] = ls.take(i) ++ Seq(e) ++ ls.drop(i)
// scala> insertAt: [A](e: A, i: Int, ls: Seq[A])Seq[A]


// 左結合でinsertAtErrorLなら成立するんじゃないん？関係無い？
// まぁエラーの通り，優先度同じだから分からないって事か，確かに…
// ずれてたの修正．
// というか，値を末尾に付けようというのがナンセンスなのかな．