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-- https://hirrolot.github.io/posts/why-static-languages-suffer-from-complexity.html

inductive Fmt where
  | FArg : Fmt → Fmt
  | FChar : Char → Fmt → Fmt
  | FEnd

def toFmt : List Char → Fmt
  | '*' :: xs => .FArg <| toFmt xs
  | x :: xs => .FChar x <| toFmt xs
  | [] => .FEnd

def PrintfType : Fmt → Type 1
  | .FArg fmt => {α : Type} → [ToString α] → α → PrintfType fmt
  | .FChar _ fmt => PrintfType fmt
  | .FEnd => PLift String

def printf (fmt : String) : PrintfType <| toFmt fmt.toList :=
  let rec printfAux (fmt : Fmt) (acc : List Char) : PrintfType fmt :=
    match fmt with
    | .FArg fmt => fun x => printfAux fmt <| acc ++ (toString x).toList
    | .FChar c fmt => printfAux fmt <| acc ++ [c]
    | .FEnd => .up acc.asString
  printfAux (toFmt fmt.toList) []

#eval printf "Hello * * *" (-1) [42, 69] "Meow" |>.down