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

[Ryan Ingram]

On Tue, Aug 26, 2008 at 1:19 AM, wren ng thornton <wren at freegeek.org> wrote:
> It should also be noted that the overhead for newtypes is not *always*
> removed. In particular, if we have the following definitions:
>
>    data    Z   = Z
>    newtype S a = S a
>
> We must keep the tags (i.e. boxes) for S around because (S Z) and (S (S Z))
> need to be distinguishable. This only really comes up with polymorphic
> newtypes (since that enables recursion), and it highlights the difference
> between strict fields and unpacked strict fields. Typically newtypes are
> unpacked as well as strict (hence no runtime tag overhead), but it's not
> guaranteed.

Is this true?  (S Z) and (S (S Z)) only need to be distinguished
during typechecking.

This would be different if it was some sort of existential type:
> newtype N = forall a. Num a => N a
but GHC at least disallows existential boxes in newtypes.

  -- ryan