def hello := "world"

structure Config where
  useASCII : Bool := false
  currentPrefix : String := ""

def ConfigIO (α : Type) : Type :=
  Config → IO α

instance : Monad ConfigIO where
  pure x := fun _ => pure x
  bind result next := fun cfg => do
    let v ← result cfg
    next v cfg

#print ConfigIO
#check (Monad ConfigIO)
#check Monad.mk

def ConfigIO.run (action : ConfigIO α) (cfg : Config) : IO α :=
  action cfg

def currentConfig : ConfigIO Config :=
  fun cfg => pure cfg

def locally (change : Config → Config) (action : ConfigIO α) : ConfigIO α :=
  fun cfg => action (change cfg)

def runIO (action : IO α) : ConfigIO α :=
  fun _ => action

def usage : String :=
  "Usage: doug [--ascii]
Options:
\t--ascii\tUse ASCII characters to display the directry structure"

def configFromArgs : List String → Option Config
  | [] => some {}
  | ["--ascii"] => some {useASCII := true}
  | _ => none

inductive Entry where
  | file : String → Entry
  | dir : String → Entry

def toEntry (path : System.FilePath) : IO (Option Entry) := do
  match path.components.getLast? with
  | none => pure (some (.dir ""))
  | some "." | some ".." => pure none
  | some name =>
    pure (some (if (← path.isDir) then .dir name else .file name))

def Config.preFile (cfg : Config) :=
  if cfg.useASCII then "|--" else "├──"

def Config.preDir (cfg : Config) :=
  if cfg.useASCII then "|  " else "│  "

def Config.fileName (cfg : Config) (file : String) : String :=
  s!"{cfg.currentPrefix}{cfg.preFile} {file}"

def Config.dirName (cfg : Config) (dir : String) : String :=
  s!"{cfg.currentPrefix}{cfg.preDir} {dir}/"

/--
Modify `cfg` on entering a directory.
-/
def Config.inDirectory (cfg : Config) : Config :=
  {cfg with currentPrefix := cfg.preDir ++ " " ++ cfg.currentPrefix}

def showFileName (file : String) : ConfigIO Unit := do
  runIO (IO.println ((← currentConfig).fileName file))

def showDirName (dir : String) : ConfigIO Unit := do
  runIO (IO.println ((← currentConfig).dirName dir))

def doList [Applicative f] : List α → (α → f Unit) → f Unit
  | [], _ => pure ()
  | x :: xs, action =>
    action x *>
    doList xs action

partial def dirTree (path : System.FilePath) : ConfigIO Unit := do
  match ← runIO (toEntry path) with
  | none => pure ()
  | some (.file name) => showFileName name
  | some (.dir name) =>
    showDirName name
    let contents ← runIO path.readDir
    locally (·.inDirectory)
      (doList contents.toList fun d =>
        dirTree d.path)

def main (args : List String) : IO UInt32 := do
  match configFromArgs args with
  | some config =>
    (dirTree (← IO.currentDir)).run config
    pure 0
  | none =>
    IO.eprintln s!"Didn't understand argument(s) {" ".intercalate args}\n"
    IO.eprintln usage
    pure 1