Pandoc filter to replace internal links with page references

filter.hs

import Text.Pandoc.JSON
import Data.List

main :: IO ()
main = toJSONFilter myfilter

myfilter (Link _ text (url, _))
  | '#':ref <- url
  = Span nullAttr $
    text <>
    [ Space
    , Str "(page"
    , Space
    , RawInline (Format "latex") ("\\pageref{" <> ref <> "}")
    , Str ")"
    ]
myfilter x = x