Initially feeding a parser with empty string results in non-obvious parse

[bgamari]

While writing a parser Fold for @Gabriel439's foldl package I stumbled upon this interesting inconsistency (full example here),

A.eitherResult
$ flip A.feed ""
$ flip A.feed "123"
$ A.parse A.double "1.3"
== 1.3123

A.eitherResult
$ flip A.feed ""
$ flip A.feed "123"
$ flip A.feed "1.3"
$ A.parse A.double ""
== 1.3

Given that empty strings are a bit special as they indicate the end of the stream it's not entirely clear that this is a bug but it is quite surprising. Why does the initial empty string affect the resulting parse? Is this considered buggy behavior?

[bos]

Smaller repro:

{-# LANGUAGE OverloadedStrings #-}

import Data.ByteString
import Control.Applicative
import qualified Data.Attoparsec.ByteString.Char8 as A

wat :: A.Parser ByteString
wat = do
  "a"
  "b" <|> ("empty" <$ "")

[bgamari]

Thanks!