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

[Adam Flott]

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 <adam at adamflott.com
> <mailto:adam at 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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