Generate and keep permanent salt for generating hashids for URIs

src/Data/Int/Local.hs

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+{- This file is part of Vervis.
+ -
+ - Written in 2019 by fr33domlover <fr33domlover@riseup.net>.
+ -
+ - ♡ Copying is an act of love. Please copy, reuse and share.
+ -
+ - The author(s) have dedicated all copyright and related and neighboring
+ - rights to this software to the public domain worldwide. This software is
+ - distributed without any warranty.
+ -
+ - You should have received a copy of the CC0 Public Domain Dedication along
+ - with this software. If not, see
+ - <http://creativecommons.org/publicdomain/zero/1.0/>.
+ -}
+
+module Data.Int.Local
+    ( toInts
+    , fromInts
+    )
+where
+
+import Prelude
+
+import Data.Int (Int64)
+import Data.List.NonEmpty (NonEmpty (..), (<|))
+
+modbase :: Int64
+modbase = 2 ^ 29
+
+toInts :: Int64 -> NonEmpty Int
+toInts n =
+    let (d, m) = n `divMod` modbase
+        m' = fromIntegral m
+    in  if d == 0
+            then m' :| []
+            else m' <| toInts d
+
+fromInts :: NonEmpty Int -> Int64
+fromInts = foldr (\ i n -> fromIntegral i + modbase * n) 0

src/Data/KeyFile.hs

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+{- This file is part of Vervis.
+ -
+ - Written in 2019 by fr33domlover <fr33domlover@riseup.net>.
+ -
+ - ♡ Copying is an act of love. Please copy, reuse and share.
+ -
+ - The author(s) have dedicated all copyright and related and neighboring
+ - rights to this software to the public domain worldwide. This software is
+ - distributed without any warranty.
+ -
+ - You should have received a copy of the CC0 Public Domain Dedication along
+ - with this software. If not, see
+ - <http://creativecommons.org/publicdomain/zero/1.0/>.
+ -}
+
+module Data.KeyFile
+    ( KeyFile (..)
+    , loadKeyFile
+    )
+where
+
+import Prelude
+
+import Data.ByteString (ByteString)
+import System.Directory (doesFileExist)
+
+import qualified Data.ByteString as B (readFile, writeFile)
+
+class KeyFile a where
+    generateKey :: IO a
+    parseKey    :: ByteString -> IO a
+    renderKey   :: a -> ByteString
+
+loadKeyFile :: KeyFile a => Bool -> FilePath -> IO a
+loadKeyFile setup path = do
+    e <- doesFileExist path
+    if e
+        then if setup
+                then fail $ "loadKeyFile: Initial setup but file already exists: " ++ path
+                else parseKey =<< B.readFile path
+        else if setup
+                then do
+                    k <- generateKey
+                    B.writeFile path $ renderKey k
+                    return k
+                else fail $ "loadKeyFile: File not found: " ++ path