However, if we try to apply this method to large programs we run into problems: we need to defined instances for a large number of combinations of keys/values. We could generate a large number of instances, hoping that these will be enough for most users' needs, using Template Haskell or CPP. However, the potential number of instances is very large, about a hundred if you consider only Prelude types and tens of thousands if you include tuples. We cannot add instances for types not defined in base without adding a dependency on all libraries which data types we want to add instances for.