Here's a little quirk I ran into recently. While making a little vector data type in class Num (code below), I didn't implement an instance of "fromInteger" (thinking I didn't need it). Well as you can probably guess, it turns out I did need it, and subsequently got a run time exception. Which surprised me a little, since it seems like it could have been caught at compile time. (Although I did ignore a warning). This has probably been answered a thousand times, but could someone point me in the direction of documentation explaining why it compiled? Thanks, Greg Buchholz --uncomment out "fromInteger" to get a working program instance Num Vec where (V a b c) + (V x y z) = (V (a+x) (b+y) (c+z)) (V a b c) - (V x y z) = (V (a-x) (b-y) (c-z)) --fromInteger 0 = V 0.0 0.0 0.0 instance Eq Vec where (V a b c) == (V x y z) = (a==x) && (b==y) && (c==z) data Vec = V !Double !Double !Double deriving Show main = print $ sum [(V 1.0 2.0 3.0), (V 4.0 5.0 6.0)]