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

[David McBride]

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> 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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