let prefixFun a b = a + "" + b;;
let exclaim s = s + "!";;
let bigHello = prefixFun >> exclaim;;

let bigHello = prefixFun >> exclaim;;
----------------------------^^^^^^^

stdin(28,29): error FS0001: Type mismatch. Expecting a
'(string -> string) -> 'a'
but given a
    'string -> string'
The type 'string -> string' does not match the type 'string'