F＃为列表编写自定义折返函数

Question

I am learning F#. I have written custom fold function for list (with help here on StackOverflow). I am now trying to write foldback function say myOwnFoldBack. The expected output might be myOwnFoldBack (+) 0 [1; 2; 3 ] myOwnFoldBack (+) 0 [1; 2; 3 ] myOwnFoldBack (+) 0 [1; 2; 3 ] should return 6 .

Here is my code for myOwnFold

let rec myOwnFoldf s0 =
  function
  | [] -> s0
  | x::tail -> myOwnFoldf (f s0 x) tail

This works fine. Here is code for myOwnFoldBack

let rec myOwnFoldBack  f s0 =
  function
  | [] -> 0
  | x::tail -> x f (myOwnFoldBackf tail)

The error I get is:

Type mismatch. Expecting a
    'a    
but given a
    ('b -> 'a -> int) list -> int    
The resulting type would be infinite when unifying ''a' and '('b -> 'a -> int) list -> int'

Answer 1

I think I figured it out finally!

let rec myOwnFoldBack  f s0 =
  function
  | [] -> s0
  | x::tail ->    f  x (myOwnFoldBack f s0 tail)