Efficiently add trailing newline to JSON ByteString emitted by Aeson when writing to file

Question

By default, Aeson encode or encodePretty do not append a trailing newline to the ByteString. I.e., when viewing the resulting file in vim I see the [noeol] indicator.

What is a performant/idiomatic way to append a newline to the file?

Here's an SSCCE for Main.hs:

{-# LANGUAGE DeriveGeneric #-}

import           Data.Aeson
import           Data.Aeson.Encode.Pretty
import           GHC.Generics
import qualified Data.ByteString.Lazy as L
import qualified Data.ByteString.Internal  as BS

data ExampleRecord =
  ExampleRecord {
       field1 :: String
     , field2 :: String
       } deriving (Show, Generic)

instance ToJSON ExampleRecord

main = L.writeFile "foo.json" $ encodePretty (ExampleRecord "a" "b")

with accompanying myproj.cabal file:

name:                myproj
version:             0.1.0.0
build-type:          Simple
cabal-version:       >=1.10

executable emit-json
  main-is:             Main.hs
  build-depends:
      base >=4.8 && <4.9
    , aeson
    , aeson-pretty
    , bytestring
  default-language:    Haskell2010

I am able to append a newline to foo.json by using the snoc function from ByteString, changing the last line in Main.hs to this:

main = L.writeFile "foo.json" $ encodePretty (ExampleRecord "a" "b") `L.snoc` BS.c2w '\n'

However, the snoc documentation describes this as an O(n) operation. Should there be a way to append this newline in constant time?