Hello, As per my understanding following hold true about macroes for data nodes. CONSTR(e, s, ws): This is used to construct header for the data nodes which contain more than one constructors e.g List, Tree etc CONSTRW(s, e) and CONSTRP(s, e) : Are used to construct header for data nodes which contain single constructor e.g Pair. I just wrote a program : ---------------------------------- data MyPair a = ThisPair a a a selectThird (ThisPair 1 x y) = ThisPair 1 1 1 selectThird (ThisPair _ _ y) = ThisPair y y y ------------------------------------ When I saw the corresponding bytecode, I saw (To my surprize) CONSTR(0, 3, 0) being used to construct the data node in the function selectThird (i.e "ThisPair 1 1 1" etc). What is happening here ? If this is the case, then when exactly are the meaning of CONSTR() and CONSTRW(), CONSTRP() Regards, ------------------------------------ Arunkumar S Jadhav, Masters Student, KReSIT, IIT-Bombay, India Ph: +91-22-25764967 http://www.it.iitb.ac.in/~arunk ------------------------------------ I exist because I work.