[Haskell-cafe] JSON querying

[Sergey Mironov]

2013/2/17 Oliver Charles <ollie at ocharles.org.uk>:
> On 02/17/2013 03:01 PM, Sergey Mironov wrote:
>>
>> Hi folks. Hackage contains several JSON packages but as far as I see,
>> they all provide 'static' conversion from JSON format to Haskell data
>> type. Is there a method of converting object containing optional filed
>> 'a' to for example Maybe a.
>
> Assuming you have some sort of 'path' to the key in question, aeson-lens
> might be exactly what you want:
> http://hackage.haskell.org/package/aeson-lens
>
> I use aeson-lens to turn a list of strings of the form ["foo", "bar", "baz"]
> into a query into first the 'foo' object, then the 'bar' object, then the
> 'baz' object, using the following:
>
> pathToLens :: Functor f
>            => [T.Text]
>            -> (Maybe Value -> f (Maybe Value))
>            -> Maybe Value
>            -> f (Maybe Value)
> pathToLens [p] = key p
> pathToLens ps = let ps' = filter (not . T.null) ps
>                 in key (head ps') . (foldl (.) id . map pathElem $ tail ps')
>   where
>     pathElem p = maybe (key p) nth (readMay $ T.unpack p)
>
> So as you can see - I turn any arbitrary path into a query into JSON, with a
> chance of failure.
>
> Hope this helps!
> - Ollie
>
>
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Good news, thanks a lot!

Sergey