Aeson-Lens

output.log

% ./dist/build/test-aeson-lens/test-aeson-lens
"127.0.0.1"
"27017"

test-aeson-lens.cabal

-- Initial test-aeson.cabal generated by cabal init.  For further
-- documentation, see http://haskell.org/cabal/users-guide/

name:                test-aeson-lens
version:             0.1.0.0
-- synopsis:
-- description:
license:             MIT
-- license-file:        LICENSE
author:              cosmo0920
-- maintainer:
-- copyright:
category:            Text
build-type:          Simple
cabal-version:       >=1.8

executable test-aeson-lens
  main-is: test-aeson.hs
  -- other-modules:
  build-depends:       base       >=4.5 && < 5
                     , aeson      >= 0.6
                     , lens       >= 3.9 && < 4
                     , aeson-lens >= 0.5
                     , text       >= 0.11
                     , bytestring >= 0.9
  extensions: OverloadedStrings

test-aeson-lens.hs

import Data.Aeson (decode, Value)
import Data.Maybe
import qualified Data.ByteString.Lazy as B
import qualified Data.Text as T
import Control.Lens
import Data.Aeson.Lens

main :: IO ()
main = do
  fstr <- B.readFile "test.json"
  let v = decode fstr :: Maybe Value
  let hostVal = v ^. key (T.pack "host") :: Maybe String
      portVal = v ^. key (T.pack "port") :: Maybe String
  putStrLn $ show $ fromJust hostVal
  putStrLn $ show $ fromJust portVal

test.json

{"host":"127.0.0.1","port":"27017"}