GetIn.hs

{-# LANGUAGE OverloadedStrings #-}

module GetIn where

import Prelude             hiding ( lookup )

import Control.Monad              ( (<=<)
                                  , join
                                  )
import Data.Aeson                 ( Value( Array
                                         , Object
                                         )
                                  , decode
                                  )
import Data.HashMap.Strict        ( lookup )
import Data.Maybe                 ( fromMaybe )
import Data.Text                  ( Text
                                  , unpack
                                  )
import Data.Vector                ( (!?) )
import Text.Read                  ( readMaybe )

getIn :: [Text] -> Value -> Maybe Value
getIn []     v = Just v
getIn (k:ks) v = case v of
  Object m -> maybe Nothing (getIn ks) . lookup k $ m
  Array a  -> join . fmap (getIn ks) . (a !?) <=< readMaybe . unpack $ k
  _        -> Nothing

example :: Value
example = fromMaybe (error "boom") . decode
  $ "{\"a\":{\"b\":{\"c\":[\"foo\",\"bar\",\"baz\"]}}}"

example2 :: Value
example2 = fromMaybe (error "boom") . decode
  $ "{\"a\":{\"b\":{\"c\":[\"foo\",{\"d\":7},\"baz\"]}}}"

-- λ> getIn ["a", "b", "c"] example
-- Array [String "foo",String "bar",String "baz"]

-- λ> getIn ["a", "b", "c", "1"] example
-- String "bar"

-- λ> getIn ["a", "b", "c", "1", "d"] example2
-- Number 7.0

-- λ> getIn ["a", "b", "q"] example
-- Nothing

-- λ> getIn ["a", "b", "c", "d"] example
-- Nothing