Generic _ fieds handling

[akaspin]

Example:

data Solid = Solid {
    _id :: String,
    solidStr :: String
} deriving (Show, Eq, Data, Typeable)

solidJson = "{\"_id\":\"a\",\"solidStr\":\"s\"}"

And now big headahe:

decode solidJson :: Maybe Solid

Expect:

Just (Solid {_id = "a", solidStr="s"})

But got:

Nothing

This happens only if _-fields in JSON. But I use aeson for CouchDB. And CouchDB sends "system" fields _id and _rev.

[bos]

This looks nasty. I think @basvandijk introduced the _ mangling - I'd like him to weigh in before I take the pull request.

[basvandijk]

This looks nasty. I think @basvandijk introduced the _ mangling

No, I think the _ mangling originally came from the json package. The patch: a1ee5a5 first introduced it.

I'd like him to weigh in before I take the pull request.

I never liked the mangling so I'm OK with removing it.

And if someone really wants to have automatic FromJSON and ToJSON instances with _ mangling she can use
deriveJSON and do the mangling herself.

[EduardSergeev]

Btw, the above example will work as expected if __id field is used.
The current mangling does look like a hack and should probably be removed but nevertheless it is breaking change. Shouldn't it first go through DEPRECATED pragma stage?
Anyway, I just fixed the broken hack but I have no objections to removing entire hack altogether.

[bos]

I'm going to wait a little before releasing 0.6, in case any more bits need fixing.