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

27 Aug 2015


      I am attached to the data structure as it's what our Thrift message
spits out and has to be mapped that way for the down stream consumers.


On 08/27/2015 01:45 PM, David McBride wrote:
...
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 mailto: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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