gistfile1.txt

signature POSITIONAL_STREAM =
sig
   type elem
   type stream
   val peek : stream -> (elem * stream) option
   val getPos : stream -> pos
end

structure SubstringStream : STREAM =
struct
   type elem = char
   type stream = substring
   val peek = Substring.getc
end

structure PositionalSubstringStream : POSITIONAL_STREAM =
struct
   type elem = char
   type stream = substring * pos
   fun peek (s, p) =
       case Substring.getc s of
           NONE => NONE
         | SOME (#"\n", s') => SOME (#"\n", (s', incrLine p))
         | SOME (x, s') => SOME (x, (s', incrCol p))
   fun getPos (_, p) = p
end