MonadAccum instance for ParsecT

[olafklinke]

There exists an instance

(Stream s, MonadState st m) => MonadState st (ParsecT e s m)

and State is more expressive than Writer, in the sense that any State monad can implement the MonadWriter interface for monoidal state. Hence I wonder why there are no instances

(Stream s, MonadAccum w m) => MonadAccum w (ParsecT e s m)
(Stream s, MonadWriter w m) => MonadWriter w (ParsecT e s m) 

I claim that accumulating some value while parsing is a frequent use case and therefore should be supported by any parsing library. After all, ParsecT does accumulate its own errors and hints. Certainly tell a.k.a. add is easy enough to implement:

addP :: MonadAccum w m => w -> ParsecT e s m ()
addP w = ParsecT (\s _ _ whenOK _ -> add w >> (whenOK () s mempty))

and look can be implemented via lift:

lookP :: MonadAccum w m => ParsecT e s m w
lookP = lift look