let f x = sprintf "[%A]" x
let g x = sprintf "{%A}" x

(*  So you have two functions (string -> string)
    Now, you want to do let result = f g "hi"
    However, f doesn't take a function string -> string, it takes a string.
    So you need to tell the compiler about your precedence.

    Normal "idiomatic" ways: *)

f (g "input")
"input" |> g |> f

(*  Crazier ways: *)

f <| g "input"
(f >> g) "input"