Re: [Haskell-cafe] Re: ANN: First Monad Tutorial of the Season

ryani.spam:

On Mon, Aug 25, 2008 at 5:33 AM, Hans van Thiel wrote:

...

The books I use for reference, the Craft and SOE, don't seem to mention this. I have to confess, I don't really understand the difference between newtype and data. Again, an explanation would be appreciated.

A newtype has no run-time representation; it is an entity at type-checking time, nothing more. data, on the other hand, has a runtime representation.

...

data D = D Int newtype N = N Int

...

d1 = D 1 n1 = N 1 i1 = 1 :: Int

In memory at runtime, these look different; n1 looks exactly the same as i1. We can represent them both in this way:

lit = 1 :: Int# (unlifted, unboxed int) i1 = { tag = I#, content -> lit } n1 = { tag = I#, content -> lit }

But d1 is different; it looks like this:

d1 = { tag = D, content -> i1 }

There's an extra level of indirection between the "data" version and the contents. What does this mean? Well, in particular, you get different behavior in a couple of cases when dealing with "error" (or infinite loops):

Though the D tag will be represented as a bit set on the bottom of the thunk pointer in GHC (useful to remember). 'data' tags are cheap.