eford:
So, to answer your question, the only computational overhead with a data declaration is the extra memory and time to store and process the constructor tags. It usually isn't noticeable, but sometimes the difference can be important.
Is there also potentially overhead due to laziness? I mean, in this case:
newtype Fast = Fast Int
If you have a Fast value, you know you've got a real Int, not a thunk, right? But here:
data Slow = Slow Int
Doesn't a Slow value remain unevaluated until you pattern match out the Int?
Also, remember that GHC is an optimising, strictness-analysing compiler, making 'data' cheap or non-existent. Tags are also represented as bits in pointers to heap values, so 'tags' are very cheap, if they're still needed (e.g. when testing on Just/Nothing). E.g. Slow = Slow { unSlow :: Int } deriving Show bigger :: Slow -> Slow bigger (Slow n) = Slow (n * 52) small = Slow 7 main = print (unSlow $ bigger small) Is compiled too: lvl :: String lvl = $wshowSignedInt 0 364 ([] @ Char) So there's no runtime overhead. -- Don