hackage-update.hs

import Control.Monad
import Data.Foldable
import Data.Functor
import Data.List
import Data.Maybe
import Data.Time.Clock
import System.Directory
import System.Process

main = listPackages >>= filterM needsUpdate >>= traverse_ update

listPackages = listDirectory "hackage" <&> mapMaybe (dropSuffix ".nix")

needsUpdate pkg = do
  now <- getCurrentTime
  before <- getModificationTime (pkgFile pkg)
  return $ diffUTCTime now before > nominalDay

pkgFile pkg = "hackage/" <> pkg <> ".nix"

update pkg = cabal2nix pkg >>= writeNixFile pkg

cabal2nix pkg = readCreateProcess (proc "cabal2nix" ["cabal://" <> pkg]) ""

writeNixFile pkg expr = writeFile (pkgFile pkg) expr

dropSuffix sfx x
  | sfx `isSuffixOf` x = Just $ take (length x - length sfx) x
  | otherwise          = Nothing