[Haskell-beginners] lens-aeson and error messages

[Thomas Koster]

Hi Alexey,

On Fri, Jun 12, 2015 at 7:11 AM, Thomas Koster <tkoster at gmail.com> wrote:
> 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 <rasen.dubi at gmail.com> wrote:
> 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