Solved it. The problem was that in the class declaration I should include a type variable.

jeff@jeffLenovUbu:~/work/computer/dwt/haskell$ ghci -XNullaryTypeClasses
GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> data D = DConstructor
Prelude> class C a -- earlier I omitted the a; that seems to have been the problem
Prelude> instance C D

Prelude>

On Sat, Jun 13, 2015 at 12:03 AM, Jeffrey Brown <jeffbrown.the@gmail.com> wrote:

jeff@jeffLenovUbu:~$ ghci -XNullaryTypeClasses
GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> class C
Prelude> data D = DConstructor
Prelude> instance C D

<interactive>:4:10:
‘C’ is applied to too many type arguments
In the instance declaration for ‘C D’
Prelude>