Re: [Haskell-cafe] GADTs and Exponentiated Functors

Just a quick follow up to this, unless I am mistaken GHC TypeLits does not actually export constructors (Zero, Succ) for the Nat kind, does it? If it did that, of course I could just use those... FYI, I have cleaned up the example a bit, adding some applicative magic, it now looks like: data Person = Person { name :: String, age :: Integer, gender :: String, status :: String } deriving Show 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 <$$> groupby (gender) <$$> orderby (age) <$$> select (age + 1) <$$> reduce (sum) -- [21,36] Or, if you are feeling more adventurous, you can do thing like: let abg = persons <$$> groupby (gender) <$$> select (age) in ((/) <$> (abg <$$> select(realToFrac)) <***> (abg <$$> reduce(mean))) [[1.0],[1.0588235294117647,0.9411764705882353]] All operations select/reduce/produce/filterby/orderby/groupby work on arbitrarily nested lists, making this very composable. Cheers, Daniel ________________________________ From: Daniel Filonik Sent: Monday, March 7, 2016 3:13 AM To: amindfv@gmail.com Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] GADTs and Exponentiated Functors 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-l... 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 ________________________________ From: amindfv@gmail.com Sent: Monday, March 7, 2016 2:38 AM To: Daniel Filonik Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] GADTs and Exponentiated Functors Interesting! What's the reason you redefine the Piano numbers and hide the import of the ones from GHC. TypeLits? Tom El 6 mar 2016, a las 07:11, Daniel Filonik mailto:d.filonik@hdr.qut.edu.au> escribió: 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.orgmailto:Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe