[MLton] Minor simplification to NumericLiteral

[Vesa Karvonen]

I noticed that the Wiki version of the make function requires
unnecessarily many operations.  In particular, op < and x2s can be
removed.  The only difference is that the Fail message may look different
compared to the Wiki version.  However, the Fail message is now arguably
more consistent, because the values are now always printed as decimal
integers.  Here is the simplified Num module:

structure Num =
   struct
      fun make (op *, op +, i2x) iBase =
          let
             val xBase = i2x iBase
          in
             Fold.fold
                ((i2x 0,
                  fn (i, x) =>
                     if 0 <= i andalso i < iBase then
                        x * xBase + i2x i
                     else
                        raise Fail (concat
                                       ["Num: ", Int.toString i,
                                        " is not a valid\
                                        \ digit in base " ^
                                        Int.toString iBase])),
                 fst)
          end

      fun I  ? = make (op *, op +, id) ?
      fun II ? = make (op *, op +, IntInf.fromInt) ?
      fun W  ? = make (op *, op +, Word.fromInt) ?

      fun ` ? = Fold.step1 (fn (i, (x, step)) =>
                               (step (i, x), step)) ?

      val a = 10
      val b = 11
      val c = 12
      val d = 13
      val e = 14
      val f = 15
   end

where

  fun fst (x, _) = x

I'll update the Wiki with this simpler Num module shortly.

-Vesa Karvonen