Luke Palmer wrote:
On Mon, Jan 19, 2009 at 3:58 AM, Patai Gergely <patai_gerg...@fastmail.fm>wrote:
However, there are other type classes that are too general to assign
such concrete uses to. For instance, if a data structure can have more
than one meaningful (and useful) Functor or Monoid instance,
As a side curiosity, I would love to see an example of any data structure
which has more than one Functor instance. Especially those which have more
than one useful functor instance.
There are plenty of data structures which are multi-functors--- namely
any type constructor with multiple arguments. The choice of order for
arguments to the type constructor is arbitrary, so you can trivially
choose the order you need for the Functor instance you want.
Depending on intensional vs extensional definitions for types, some may
argue that these are "different" types due to currying and kinding
issues, but those concerns only highlight the limitations of not having
type-level functions like `flip`, rather than having any category
theoretic basis.
--
Live well,
~wren
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