我不知道为什么ReasonML会引发此错误

Question

我正在尝试在ReasonML中创建一个程序来执行以下操作：

scanRight(add, [1, 1 , 2, 3, 5], 0);

/* Intermediate Steps
[(add 1 (add 1 (add 2 (add 3 (add 5 0))))),
(add 1 (add 2 (add 3 (add 5 0)))),
(add 2 (add 3 (add 5 0))),
(add 3 (add 5 0)),
(add 5 0),
0] */

/* Final Output */
[12, 11, 10, 8, 5, 0]

这是我编写的过程：

let rec scanRight: (('a, 'b) => 'b, list('a), 'b) => list('b) = (proc, lst, base) =>
switch(lst){
    |[] => [base]
    |[hd, ... tl] => List.flatten([proc(hd, scanRight(proc, tl, base)), scanRight(proc, tl, base)])
};

但是，它一直给我这个错误：

  2 │ switch(lst){
  3 │     |[] => [base]
  4 │     |[hd, ... tl] => **List.flatten([proc(hd, scanRight(proc, tl, base)),
       scanRight(proc, tl, base)])**
  5 │ };
  6 │

  This has type:
    list('a)
  But somewhere wanted:
    list(list('a))
  The type variable 'a occurs inside list('a)

我不确定在list(list('a))的确切位置，因为我的类型签名指示输出应为list('a)类型。

编辑：当我取出List.flatten()时会引发此错误：

  1 │ let rec scanRight: (('a, 'b) => 'b, list('a), 'b) => list('b) = **(proc, 
      lst, base) =>
  2 │ switch(lst){
  3 │     |[] => [base]
  4 │     |[hd, ... tl] => [proc(hd, scanRight(proc, tl, base)), scanRight(pr
      oc, tl, base)]
  5 │ };**

  This has type:
    (('a, list('b)) => list('b), list('a), list('b)) => list(list('b))
  But somewhere wanted:
    (('a, list('b)) => list('b), list('a), list('b)) => list('b)
  The type variable 'b occurs inside list('b)