Simon Marlow
From Section 4.2.3, from the Haskell report:
A declaration of the form
newtype cx => T u1 ... uk = N t
introduces a new type whose representation is the same as an existing type. The type (T u1 ... uk) renames the datatype t. It differs from a type synonym in that it creates a distinct type that must be explicitly coerced to or from the original type. Also, unlike type synonyms, newtype may be used to define recursive types. The constructor N in an expression coerces a value from type t to type (T u1 ... uk). Using N in a pattern coerces a value from type (T u1 ... uk) to type t. These coercions may be implemented without execution time overhead; newtype does not change the underlying representation of an object.
*blink*
I stand corrected (for the second time today, duh, maybe I should check facts before trusting my memory next time...).
I have no idea why the report does say that though. Seems very odd, there's no need to mention the representation. Indeed, the language provides no way (absent unsafeCoerce) for a programmer to determine what the representation is, so how should we interpret that paragraph? An invisible requirement or an implementation hint?
Or as a hint that we should introduce a ``pseudo-polymorphic pseudo-function'' safeCoerce :: a -> b which can only be used if a and b have the same representation, as mandated by the language definition? (Or if the compiler can derive that for the type language currently used...) Wolfram