Standard ML Syntax

I'm new to Standard ML and trying to write the following code

 fun whilestat test stmt1  = 
        (fn x => if (test x) then (stmt1 x;whilestat test stmt1 ) else (x) );

The issue is that it gives me the following error

w.sml:21.6-22.82 Error: right-hand-side of clause doesn't agree with function result type [circularity]
expression:  ('Z -> 'Y) -> 'Z -> 'Z
result type:  ('Z -> 'Y) -> 'Z
in declaration:
whilestat2 = (fn arg => (fn <pat> => <exp>))

uncaught exception Error
 raised at: ../compiler/TopLevel/interact/evalloop.sml:66.19-66.27
         ../compiler/TopLevel/interact/evalloop.sml:44.55
         ../compiler/TopLevel/interact/evalloop.sml:292.17-292.20

Im just trying to emaulate a while condition where if the staement is true then it recurses, else returns the value.