F＃函数参数类型不匹配

Question

I am using following function to make tuples of two lists in F#

let rec MyOwnZipper xs ys =
    match xs,ys with
    | [],l| l,[]  -> []
    | x1::rest, y1::yrest -> (x1, y1) :: (MyOwnZipper rest yrest) 

it works fine when calling function with lists of integers like

System.Console.WriteLine( MyOwnZipper [1; 2; 3] [4; 5; 6] )

The problem comes when I change arguments to string

 System.Console.WriteLine( MyOwnZipper [1; 2; 3] ["Hello"; "World"; "Peace"] )

I get the following error

 error FS0001: This expression was expected to have type
    int    
but here has type
    string    

exit status 1

Answer 1

This happens because of the first case of the match expression:

       | [],l| l,[]  -> []

Here, the identifier l gets bound either to the first list, or to the second list. Since an identifier cannot have two different types at once, the compiler concludes that the lists must have the same type. Therefore, if you try to call the function with differently typed lists, you get a type mismatch error, quite expectedly.

To fix the problem, break the case in two separate cases:

let rec MyOwnZipper xs ys =
    match xs,ys with
    | [],l  -> []
    | l,[]  -> []
    | x1::rest, y1::yrest -> (x1, y1) :: (MyOwnZipper rest yrest)