Editing Newtype (section)

== The short version ==

The restriction to one constructor with one field means that the new type and the type of the field are in direct correspondence:

<haskell>
State :: (s -> (s, a)) -> State s a
runState :: State s a -> (s -> (s, a))
</haskell>

or in mathematical terms they are ''isomorphic''. This means that after the type is checked at compile time, at run time the two types can be treated essentially the same, without the overhead or indirection normally associated with a data constructor. So if you want to declare different type class instances for a particular type, or want to make a type abstract, you can wrap it in a <hask>newtype</hask> and it'll be considered distinct to the type-checker, but identical at runtime. You can then use all sorts of deep trickery like phantom or recursive types without worrying about GHC shuffling buckets of bytes for no reason.