Question

I’m having trouble deriving a Typeable1 instance for a date structure in Haskell.

This is my code:

    {-# LANGUAGE StandaloneDeriving #-}
    {-# LANGUAGE DeriveDataTypeable #-}

    import Data.Typeable (Typeable,Typeable1)

    newtype FooM m a = Foo { unFoo :: (a -> Bar m) -> Bar m }
    newtype Bar m = Atom (m (Maybe (Bar m)))
    type Baz m = Waldo (FooM m ())
    type Waldo a = a

    data Qux m = Qux {
        baz :: Baz m
      , num :: Int
    } -- deriving Typeable1 [1]

    -- deriving instance Typeable1 Qux [2]

Uncommenting the first comment [1] gives this error:

    Cannot derive well-kinded instance of form `Typeable1 (Qux ...)'
          Class `Typeable1' expects an argument of kind `* -> *'
        In the data type declaration for `Qux'

And uncommenting [2] gives this error:

    Kind mis-match
    The first argument of `Typeable1' should have kind `* -> *',
    but `Qux' has kind `(* -> *) -> *'
    In the stand-alone deriving instance for `Typeable1 Qux'

My question is: How can I add a Typeable/Typeable1 instance of Qux, please?

Answer 1

You can’t make Qux an instance of Typeable1, but in modern GHC, you should be able to derive an instance of Typeable, which is now kind-polymorphic enough to handle such higher-kinded types, making Typeable1 and its ilk unnecessary.

Outdated answer, kept because it was the accepted answer when the question was asked: Unfortunately, you can’t: the Typeable hierarchy doesn’t have any typeclasses for things of kind (* -> *) -> *. This may be fixed at some future time now that GHC is starting to support kind polymorphism.