parser.sml

signature PARSER = sig
  eqtype tok
  type 'a result = ('a * tok list) option
  type 'a parser = tok list -> 'a result
  ...
end

functor Parser (eqtype token) : PARSER = struct
  type tok = token (* Another question: if I put `eqtype` here the compiler yells at me... *)
  
  (* Why do I need the following two lines? *)
  type 'a result = ('a * tok list) option
  type 'a parser = tok list -> 'a result
  ...
end