GCCT Wrapper Example.hs

-- a Wrapper is a pair of *something*, and if that something is an Int, an additional boolean
data Wrapper a =
    Wrapper { get  :: a
            , test :: (a ~ Int) => Bool }
 
-- We can construct a Wrapper from something of any type
makeWrapper :: a -> Wrapper a
makeWrapper x = Wrapper x (x == 0) -- Whoa! We're comparing it to zero even if it might not be an Int!
 
-- We can construct a Wrapper, as noted above, at various type indices…
foo :: Wrapper String
foo = makeWrapper "foo"
bar :: Wrapper Int
bar = makeWrapper 5
 
-- And we can destruct it with both destructors if it has an Int type index...
get  bar == 5
test bar == False
 
-- But we can only use one of the destructors if it was not...
get  foo == "foo"
--test foo -- type error: can’t unify String with Int