On 12/17/07, Evan Laforge
Oops, nevermind, I just saw the other thread and link to http://www.haskell.org/haskellwiki/Newtype. Ok, so that seems like a pretty subtle diffenence... I'm assuming the rationale behind differentiating between a single constructor data and newtype is so that data types don't suddenly change their behaviour around undefined when they have only one constructor. I would find example y3 surprising if I came across it in real code!
It's not that subtle if you think about what newtype is for. Newtype is like "type", except that you're not just declaring a type synonym, but asking the typechecker to check that you don't use the synonym interchangeably with the type it's standing in for. Types declared with newtype and with type are supposed to act exactly the same way at runtime. In order to act exactly the same way at runtime, if you write newtype X = X A, X _|_ has to be indistinguishable from _|_ at runtime. In other words, the data constructor X has to be strict. In types declared with "data", constructors are lazy -- if they weren't, you wouldn't be programming in Haskell. Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "People. Can't live with 'em, can't legally set fire to 'em." -- Sheree Schrager