function signature makes a difference. in this example, try swapping the signature of function convertToReq it seems implicit conversion happens depending on signature (and usual constraints), yes module Phantom where data ReqTime = ReqTime data AckTime = AckTime -- convertToReq :: Order AckTime -> Order ReqTime -- builds convertToReq :: Order AckTime -> Order AckTime -- error convertToReq o = o{price = 1} data Order a = Order { price ::Double, volume ::Int, timestamp ::Int } main :: Order ReqTime main = expectReq conv where ack = initAckTime conv = convertToReq ack initAckTime:: Order AckTime initAckTime = Order { timestamp = 0, price = 2.1, volume = 3 } expectReq::Order ReqTime -> Order ReqTime expectReq o = o