Re: If newtype = data !, then why use does Haskell' need newtype

On 10/11/07, Dan Weston wrote:

I presume that the two statements below are equivalent [1]:

newtype A = MkA Int data A = MkA !Int

So does Haskell' still need newtype? It seems like a needless keyword.

newtype and data have different semantics: Haskell guarantees that an A will have the exact same run-time representation as an Int, if A is declared with the newtype as above. It doesn't guarantee that if you use data instead of newtype. It may be that GHC compiles code in the same way with either of your type declarations above, but that's an implementation-dependent choice. Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "in a recent future, this is past" -- James Keelaghan