Rewrite mercurial (hg) commit dates/timestamps with haskell

change-dates.hs

{-|
  How to use:

  ```
  hg export -g -r 1: | runhaskell ~/projects/haskell/mercurial/change-dates.hs > ../patch.txt
  # rm the same commits, e.g. with histedit
  hg import ../patch.txt
  ```
-}

import Data.Time.Clock.POSIX
import Data.List
import Text.Regex.Posix

patternToMatch = "# Date ([0-9]+) ([-+][0-9]+)"

delta = 2

matchPattern :: String -> String -> [String]
matchPattern pattern line = getAllTextMatches (line =~ pattern) :: [String]

groupsPattern :: String -> String -> (String,String,String,[String])
groupsPattern pattern line = line =~ pattern :: (String,String,String,[String])

groupsOnlyPattern :: String -> String -> [String]
groupsOnlyPattern pattern line = let (_, _, _, xs) = groupsPattern pattern line in xs

isTimeStamp :: String -> Bool
isTimeStamp = isPrefixOf "# Date "

replaceTimeStamp timestamp line =
  (timestamp - delta, "# Date " ++ show timestamp ++ " " ++ timeZone)
  where
    _:timeZone:[] = groupsOnlyPattern patternToMatch line

processLine :: String -> (Int, [String]) -> (Int, [String])
processLine line (time, lines) =
  (newTime, replacedLine:lines)
    where
      (newTime, replacedLine) = if isTimeStamp line then replaceTimeStamp time line else (time, line)

process :: Int -> String -> String
process time = unlines . snd . foldr processLine (time, []) . lines

main = do
  unixTime <- round <$> getPOSIXTime
  interact $ process unixTime