Parsing AWS JSON w/ Purescript

README.markdown

Paring AWS JSON w/ PureScript

I'm experimenting with parsing AWS JSON with purescript.

Given the Data.JSON.S3 module provided, here's what I see in the console:

$> eitherDecode "{ \"Records\": [{ \"s3\": { \"bucket\": { \"name\": \"foo\"}, \"object\": { \"key\": \"bar.png\" } } } ] }" :: Either String Records

Left ("fromList Cons (Tuple (\"s3\") (JObject fromList Cons (Tuple (\"bucket\") (JObject fromList Cons (Tuple (\"name\") (JString \"foo\")) (Nil))) (Cons (Tuple (\"object\") (JObject fromList Cons (Tuple (\"key\") (JString \"bar.png\")) (Nil))) (Nil)))) (Nil) is not (Either a b).")

Parsing the individual S3 objects works as expected. Why doesn't this work?

example.json

{
  "Records": [
    {
      "s3": { 
        "bucket": { "name": "foo"}, 
        "object": { "key": "bar.png" }
      }
    } 
  ] 
}

S3.purs

module Data.JSON.S3 where

import Prelude

import Data.Maybe
import Data.Either
import Data.Array
import Data.JSON

data Records = Records (Array (Either String S3))

data S3 = S3 S3Children

data S3Children = S3Children Bucket S3Object

data Bucket = Bucket String

data S3Object = S3Object String

instance showS3 :: Show S3 where
  show (S3 x) =
    "S3 " ++ show x

instance showS3Children :: Show S3Children where
  show (S3Children b o) =
    "S3Children " ++ show b ++ " " ++ show o

instance showBucket :: Show Bucket where
  show (Bucket s) = "Bucket " ++ s

instance showS3Object :: Show S3Object where
  show (S3Object s) = "S3Object " ++ s

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) = S3 <$> (o .: "s3")
  parseJSON _ = fail "S3 parse failed"

instance s3ChildrenFromJSON :: FromJSON S3Children where
  parseJSON (JObject o) = S3Children <$> (o .: "bucket") <*> (o .: "object")
  parseJSON _           = fail "S3Children parse failed"

instance bucketFromJSON :: FromJSON Bucket where
  parseJSON (JObject o) = Bucket <$> (o .: "name")
  parseJSON _           = fail "Bucket parse failed"

instance s3ObjectFromJSON :: FromJSON S3Object where
  parseJSON (JObject o) = S3Object <$> (o .: "key")
  parseJSON _           = fail "S3Object parse failed"

IRC sorted me out. Instead of data Records = Records (Array (Either String S3)) I just needed data Records = Records (Array S3)

I feel like I'm still working too hard though... so I'll probably come back to this...