On 12/17/07, Evan Laforge
I'm sure there's a trivial explanation for this, but here's something that I've always kind of wondered about: Given a single constructor type like "data X = X A B C" can't that be transformed into "newtype X = X (A, B, C)"? There must be some difference, because if there weren't we could transform all single constructor types that way, and dispense with newtype entirely.
Strictness. In newtype X = X A, the A field is strict. In data X = X A, the A field is lazy. So the compiler can't just turn all single-constructor "data" types into "newtypes". (To generalize, if you were going to allow newtypes like "newtype X = X (A, B, C)", the tuple would be unboxed, and you'd have the same strictness/laziness distinction.) This is explained in section 4.2.3 of the H98 Report: http://www.haskell.org/onlinereport/decls.html Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "Do we learn from our mistakes? I surely hope not / Takes all the fun out of making them again."--Trout Fishing In America