chore(users/Profpatsch): bring reverse-haskell-deps into shape

Add a cabal file and move into subdir.
Use MyPrelude & fix a few linter warnings.

Change-Id: I19d5ba47be789fc24f8e02ee8721f73c706ae3e9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8465
Reviewed-by: Profpatsch <mail@profpatsch.de>
Autosubmit: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

users/Profpatsch/reverse-haskell-deps/ReverseHaskellDeps.hs

@@ -0,0 +1,76 @@
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE MultiWayIf #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+
+module Main where
+
+import Data.ByteString qualified as ByteString
+import Data.Either
+import Data.List qualified as List
+import Data.Maybe
+import Data.Text (Text)
+import Data.Text qualified as Text
+import Data.Text.Encoding qualified
+import MyPrelude
+import Numeric.Natural
+import Text.HTML.TagSoup qualified as Tag
+import Text.Nicify
+import Text.Read qualified as Read
+
+parseNat :: Text -> Maybe Natural
+parseNat = Read.readMaybe . textToString
+
+printNice :: Show a => a -> IO ()
+printNice = putStrLn . nicify . show
+
+type Tag = Tag.Tag Text
+
+main = do
+  reverseHtml <- readStdinUtf8
+  printNice $ List.sortOn snd $ packagesAndReverseDeps reverseHtml
+  where
+    readStdinUtf8 = bytesToTextUtf8Lenient <$> ByteString.getContents
+
+-- | reads the table provided by https://packdeps.haskellers.com/reverse
+-- figuring out all sections (starting with the link to the package name),
+-- then figuring out the name of the package and the first column,
+-- which is the number of reverse dependencies of the package
+packagesAndReverseDeps :: Text -> [(Text, Natural)]
+packagesAndReverseDeps reverseHtml = do
+  let tags = Tag.parseTags reverseHtml
+  let sections = Tag.partitions (isJust . reverseLink) tags
+  let sectionName [] = "<unknown section>"
+      sectionName (sect : _) = sect & reverseLink & fromMaybe "<unknown section>"
+  let sectionNames = map sectionName sections
+  mapMaybe
+    ( \(name :: Text, sect) -> do
+        reverseDeps <- firstNaturalNumber sect
+        pure (sectionPackageName name sect, reverseDeps) :: Maybe (Text, Natural)
+    )
+    $ zip sectionNames sections
+  where
+    reverseLink = \case
+      Tag.TagOpen "a" attrs -> findMaybe attrReverseLink attrs
+      _ -> Nothing
+
+    attrReverseLink = \case
+      ("href", lnk) ->
+        if
+            | "packdeps.haskellers.com/reverse/" `Text.isInfixOf` lnk -> Just lnk
+            | otherwise -> Nothing
+      _ -> Nothing
+
+    sectionPackageName :: Text -> [Tag] -> Text
+    sectionPackageName sectionName = \case
+      (_ : Tag.TagText name : _) -> name
+      (_ : el : _) -> sectionName
+      xs -> sectionName
+
+    firstNaturalNumber :: [Tag] -> Maybe Natural
+    firstNaturalNumber =
+      findMaybe
+        ( \case
+            Tag.TagText t -> parseNat t
+            _ -> Nothing
+        )