Hi Alexey,
On Fri, Jun 12, 2015 at 7:11 AM, Thomas Koster
tl;dr - I am using aeson and have an unusual JSON data structure to parse, so I have implemented the parseJSON function using the prisms and traversals in the lens-aeson package. Unfortunately, the only error message I ever get from my parser is "mempty". How can my parser give better error messages when using lens-aeson?
instance FromJSON Row where parseJSON v = Row <$> (v ^. nth 0 . to parseJSON) <*> (v ^. nth 1 . to parseJSON) <*> (v ^. nth 2 . to parseJSON)
On 12 June 2015 at 16:28, Alexey Shmalko
I would come with implementation similar to this:
instance FromJSON Row where parseJSON = withArray "Row" $ \ a -> case Vector.length a of 3 -> Row <$> parseJSON (a ! 0) <*> parseJSON (a ! 1) <*> parseJSON (a ! 2) _ -> fail "Invalid Row."
Thanks for your response. I was so caught up in trying to do things "the Haskell, functional way", with pattern matching, that I totally overlooked the plain old (!) operator! I have decided that Row is not complex enough to need lens-aeson and will follow your recommendation. I might add this (.!) operator to my toolkit though, analogous to (.:), to remove the need for those "parseJSON"s. (.!) :: FromJSON a => Array -> Int -> Parser a I wonder why this is missing from aeson... I am still interested if an answer to the original question exists, should I need lens-aeson at some later time. Thanks, -- Thomas Koster