Using associated data types to create unpacked data structures
bos at serpentine.com
Wed Aug 11 12:25:03 EDT 2010
On Wed, Aug 11, 2010 at 9:03 AM, Johan Tibell <johan.tibell at gmail.com>wrote:
> 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.
This is more or less the type-level version of the "I have to write how many
boilerplate instances?" problem that dogs typeclasses. I agree that it's
quite painful, and that it effectively keeps type families from being nearly
as practically useful as they could be. Like you, I'd love to see an
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