[I think this thread would be more appropriate on haskell-café, so I'm redirecting it] | addToSPair :: Char -> (String, String) -> (String, String) | addToSPair c (xs, ys ) = (c:xs, ys ) | | --------------------------------------------------------------- | | g1 is similar to f1, g2 similar to f2. | | f1 and f2 are evaluated immediately to 'a'; | | g1 evaluates to 'a' after a long computation; | g2 evaluates to Fail: <<loop>> | | As g2 is similar to f2, should it have a value 'a' ? | In | head $ fst $ addToSPair 'a' bottomSPair Try using lazy pattern matching, thus addToSPair :: Char -> (String, String) -> (String, String) addToSPair c ~(xs, ys ) = (c:xs, ys ) The only change is the "~" in the defn of addToSPair. Now g2 will have value 'a'. Simon