I started out using TypeLits, but I was running into problems with GHC's solver along these lines:
http://stackoverflow.com/questions/24734704/append-for-type-level-numbered-lists-with-typelits
However, I would not rule out the possibility that this was due to misuse my behalf. If you know a way to make it work, that would be exactly the kind of feedback I am looking for!
Also, I'd be curious if it is possible to write an instance of the general NFunctor for NList (which does not require explicit type annotations to work):
class NFunctor t (m :: Peano) (n :: Peano) where
pmap' :: (t (Succ m) a -> t m b) -> t (Succ n) a -> t n b
zmap' :: (t m a -> t m b) -> t n a -> t n b
smap' :: (t m a -> t (Succ m) b) -> t n a -> t (Succ n) b
Cheers,
Daniel
Hi,
I have been recently playing around with GADTs, using them to keep track of how many times a functor has been applied. This provides a solution to what seems to be a long standing problem, summarized here:
https://byorgey.wordpress.com/2007/08/16/mapping-over-a-nested-functor/
If the nesting depth is part of the type, it is possible to apply fmap automatically as needed. This allows you to write fairly elegant expressions like:
data Person = Person { name :: String, age :: Integer, gender :: String, status :: String } deriving Show
let persons = fromList' [Person {name="Alice", age=20, gender="F", status="Good"}, Person {name="Bob", age=18, gender="M", status="Good"}, Person {name="Chuck", age=16, gender="M", status="Bad"}] :: NList N1 Person
persons `select` age
-- [20,18,16]
persons `groupby` gender `select` age
-- [[20],[18,16]]
persons `groupby` gender `groupby` status `select` age
-- [[[20]],[[18],[16]]]
Note that "`select` age" works regardless of nesting depth. You can also append or remove nesting levels:
persons `groupby` gender `select` age `produce` (\x -> [0..x])
-- [[[0..20]],[[0..18],[0..16]]]
persons `groupby` gender `reduce` (sumof age)
-- [20, 34]
Would this kind of thing be of interest? The code is here:
https://github.com/filonik/nlists
Please feel free to suggest improvements.
Cheers,
Daniel
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