[Hol-info] A question about the definition of the function in HOL4

[A. <956...@qq...>]

Hi,guys
     I am working with HOL4.  
     I defined a function in HOL4,like following
     - fun ccdd a b c d  = if ((a = b) /\ (c = d)) then true else false;
     It responded that:
     ! Toplevel input:
    ! fun ccdd a b c d  = if ((a = b) /\ (c = d)) then true else false;
    !                         ^^^^^^^
    ! Type clash: expression of type
    !   bool
    ! cannot have type
    !   'a -> 'b
     I can't understand it.
     Who know the reason?
         Thanks!