Newtype

One frequent question is what is the difference between data and newtype? The answer has to do with the level of undefinedness that occurs in the values. The following sample code shows how three different data declarations behave with undefined present.

module Foo where

data Foo1 = Foo1 Int
data Foo2 = Foo2 !Int
newtype Foo3 = Foo3 Int

x1 = case Foo1 undefined of
     Foo1 _ -> 1		-- 1

x2 = case Foo2 undefined of
     Foo2 _ -> 1		-- undefined

x3 = case Foo3 undefined of
     Foo3 _ -> 1		-- 1

y1 = case undefined of
     Foo1 _ -> 1		-- undefined

y2 = case undefined of
     Foo2 _ -> 1		-- undefined

y3 = case undefined of
     Foo3 _ -> 1		-- 1