Re: [Haskell-beginners] converting a json encoded radix tree to a haskell data type

28 Aug 2015

      I was trying this but ran into a bit of trouble.  Are you super attached to
that data structure?  I would expect a radix tree as you've described it to
look more like this:

data RadixTree = Node [(Text, RadixTree)] | Leaf Times
data Times = Times (Maybe Int) (Maybe Int)

In which case it is much easier to write the json instances.  From there
you shouldn't have too much of a problem writing a recursive function to do
the rest, without dealing with all the aeson stuff at the same time.
Here's what I ended up with (I think it could be cleaned up a bit).

import Control.Monad
import Data.Text as T
import Data.Aeson
import Data.HashMap.Strict as HM
import Data.Vector as V hiding (mapM)

data RadixTree = Node [(Text, RadixTree)] | Leaf Times deriving Show
data Times = Times (Maybe Int) (Maybe Int) deriving Show

instance FromJSON RadixTree where
  parseJSON (Object o) = do
    let els = HM.toList o
    contents <- mapM (\(t,v) -> do v' <- parseJSON v; return (t, v'))
(HM.toList o)
    return $ Node contents
  parseJSON a@(Array _) = Leaf <$> parseJSON a
  parseJSON _ = mzero

instance FromJSON Times where
  parseJSON (Array v) | (V.length v) >= 2 =
    let v0 = v V.! 0
        v1 = v V.! 1
    in Times <$> parseJSON v0 <*> parseJSON v1
  parseJSON _ = mzero

{-
tree2things :: RadixTree -> [(Text, (Maybe Int, Maybe Int))]
tree2things (Node xs) = _
tree2things (Leaf t) = _
-}

On Thu, Aug 27, 2015 at 11:30 AM, Adam Flott <adam@adamflott.com> wrote:
...
On 08/27/2015 11:18 AM, Karl Voelker wrote:
...
On Thu, Aug 27, 2015, at 08:04 AM, Adam Flott wrote:
...
data Things = MkThings {
  thing   :: TL.Text,
  times :: ThingTimes
  } deriving (Show, Eq, Typeable)
data ThingTimes = MkThingtimes {
  ml :: V.Vector Times
  } deriving (Show, Eq, Typeable)
data Times = MkTimes {
  t1 :: Maybe Int32,
  t2 :: Maybe Int32
  } deriving (Show, Eq, Typeable)
-- radix.json --
{
    "a" : {
        "b" : [ 1, 2 ],
        "c" : {
            "d" : [ 3, null ]
        }
    },
    "a2" : { "b2" : [ 4, 5 ] }
}
-- radix.json --
It looks like your input file has Things nested inside Things, but your
data types don't allow for that. Is that intentional? What value is that
example input supposed to parse to?
Vector [
    MkThings "ab" (MkThingTimes (Vector [ Just 1, Just 2 ])),
    MkThings "abcd" (MkThingsTimes (Vector [ Just 3, Nothing))
    MkThings "a2b2" (MkThingTimes (Vector [ Just 4, Just 5 ])) ]
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Re: [Haskell-beginners] converting a json encoded radix tree to a haskell data type

David McBride