README.markdown

Parsing JSON with Purescript

I just wrote a lambda that responds to S3 events. I wrote it in Node. The JSON strcture of an S3 event is fairly complex. It is even worse when the event comes over SNS.

Handling the events in Node was error prone and testing took a while. Also, SNS will sometimes send test events that are imcomplete. This results in many failures due to missing keys. Undefined errors are not your friend.

I wanted to parse the JSON and handle error conditions more gracefully.

In this example, I'm only interested in the bucket and key of the S3 object. The parser wades through the other data and builds an S3 data type. If the bucket and key aren't present, or the JSON is malformed, then an error is returned as a Left. If the errors aren't handled then the program won't compile.

example.json

{
  "Records": [
    {
      "eventVersion": "2.0",
      "eventTime": "1970-01-01T00:00:00.000Z",
      "requestParameters": {
        "sourceIPAddress": "127.0.0.1"
      },
      "s3": {
        "configurationId": "testConfigRule",
        "object": {
          "eTag": "d41d8cd98f00b204e9800998ecf8427e",
            "key": "photos/1483/02671d5188895686ce28f1d13530322a/original/cave_scenery.png",
          "size": 1024
        },
        "bucket": {
          "arn": "arn:aws:s3:::foo",
          "name": "foo",
          "ownerIdentity": {
            "principalId": "EXAMPLE"
          }
        },
        "s3SchemaVersion": "1.0"
      },
      "responseElements": {
        "x-amz-id-2": "FMyUVURIY8/IgAtTv8xRjskZQpcIZ9KG4V5Wp6S7S/JRWeUWerMUE5JgHvANOjpD",
        "x-amz-request-id": "C3D13FE58DE4C810"
      },
      "awsRegion": "us-east-1",
      "eventName": "ObjectCreated:Put",
      "userIdentity": {
        "principalId": "EXAMPLE"
      },
      "eventSource": "aws:s3"
    }
  ]
}

Main.purs

module Main where

import Prelude
import Data.JSON
import Data.JSON.S3
import Node.FS.Async
import Node.Encoding
import Data.Either
import Control.Monad.Eff.Console

decode' :: String -> Either String Records
decode' = eitherDecode

results :: Either String Records -> String
results (Left err) = show err
results (Right s3) = show s3

parse' :: String -> String
parse' = (results <<< decode')

main = do

  readTextFile UTF8 "example.json" $ \x -> do
    case x of
         Left err -> log (show err)
         Right x' -> log (parse' x')

S3.purs

module Data.JSON.S3 where

import Prelude

import Data.Array
import Data.JSON

data Records = Records (Array S3)

data S3= S3 { bucket :: String
            , key    :: String
            }

instance showS3 :: Show S3 where
  show (S3 { bucket = b, key = k }) =
    "S3 { bucket: \"" ++ b ++ "\", key: \"" ++ k ++ "\" }"

instance showRecords :: Show Records where
  show (Records xs) = "Records " ++ show xs

instance recordsFromJSON :: FromJSON Records where
  parseJSON (JObject o) = Records <$> (o .: "Records")
  parseJSON _           = fail "Records parse failed"

instance s3FromJSON :: FromJSON S3 where
  parseJSON (JObject o) = do
    s3 <- o .: "s3"
    b  <- (s3 .: "bucket") >>= (.: "name")
    k  <- (s3 .: "object") >>= (.: "key")
    return $ S3 { bucket: b, key: k }
  parseJSON _ = fail "S3 parse failed"