Why isn't a tuple parameter being recognized on my function? - f#

Question

Why isn't a tuple parameter being recognized on my function?
I have the following function:
let deposit depositType ((logTransaction:string -> string -> unit), (file:string)) =
depositType |> makeInitialDeposit
|> sprintf "Deposited: %f"
|> logTransaction file
Note how the last argument of the function is a tuple:
((logTransaction:string -> string -> unit), (file:string))
I then attempt to invoke this function using the following:
let file = "c:\\myfile.txt"
(writeToFile, file) ||> deposit OneDollarBill
Yet, it's stating that it's not expecting a tuple. Instead, it's expecting:
Expecting a (string -> string -> unit) -> string -> 'a
The complete error is here:
Type mismatch. Expecting a
(string -> string -> unit) -> string -> 'a but given a
(string -> string -> unit) * string -> unit The type 'string -> string -> unit' does not match the type '(string -> string -> unit) * string'
Here's the code:
let writeToFile (filePath:string) (message:string) =
let file = new System.IO.StreamWriter(filePath)
file.WriteLine(message)
file.Close()
let makeInitialDeposit deposit =
deposit |> insert []
let deposit depositType ((logTransaction:string -> string -> unit), (file:string)) =
depositType |> makeInitialDeposit
|> sprintf "Deposited: %f"
|> logTransaction file
let file = "c:\\myfile.txt"
(writeToFile, file) ||> deposit OneDollarBill

Answer 1

||> unpacks a tuple 'a * 'b to call a normal function 'a -> 'b -> 'c; as shown, you don't want this unpacking behavior, as you have 'a * 'b -> 'c.
Either change ||> to |> so the tuple is passed directly:
(writeToFile, file) |> deposit OneDollarBill
Or change deposit to accept curried arguments rather than a tuple:
let deposit depositType (logTransaction:string -> string -> unit) (file:string) =
...
(writeToFile, file) ||> deposit OneDollarBill