Safe read in Aeson parseJSON - haskell

Question

I'm using Aeson to parse json quote data from Yahoo's API. A quote might look like this:
{
"date": "2010-03-10",
"Date": "2010-03-10",
"Open": "0.37",
"High": "0.37",
"Low": "0.34",
"Close": "0.35",
"Volume": "443000",
"Adj_Close": "0.35"
}
(that's using this YQL query)
As you can see, the numbers are quoted. I can write a fromJSON implementation like this:
instance FromJSON Quote where
parseJSON (Object o) =
Quote <$> o .: "Date"
<*> o .: "Open"
<*> o .: "High"
<*> o .: "Low"
<*> o .: "Close"
<*> o .: "Volume"
parseJSON _ = mzero
which is the same as what would be derived. Unfortunately this only works if I want Open, High, Low, etc, to be a Text type. The parse fails if I try and have any of those fields as Double, say.
I can write this:
<*> (fmap read $ o .: "Open")
to get it as anything I like, but this uses read, which is a partial function. How would I get the above functionality without using a partial function?

Answer 1

First, find a safe read function. I had to hoogle it.
Second, you have to use more than just applicative to get choice. In the below, I used readMay for the safe read and made a helper function to extract the fields while applying the Read instance.
{-# LANGUAGE OverloadedStrings, NoMonomorphismRestriction #-}
import Safe
import Data.Aeson
import Data.Aeson.Types (Parser)
import Data.Text as T
import Data.Word
import Control.Applicative
import Control.Monad
data Quote = Quote Text Double Double Double Double Word64
instance FromJSON Quote where
parseJSON (Object o) = do
let readField :: (Read a) => T.Text -> Parser a
readField f = do
v <- o .: f
case readMay (T.unpack v) of
Nothing -> fail $ "Bad Field: " ++ T.unpack f
Just r -> return r
Quote <$> o .: "Date"
<*> readField "Open"
<*> readField "Close"
<*> readField "Low"
<*> readField "High"
<*> readField "Volume"
parseJSON _ = mzero