A format for expressing an ordered list of integers is to use a comma separated list of either

 individual integers
 Or a range of integers denoted by the starting integer separated from the end integer in the range by a dash, '-'. (The range includes all integers in the interval including both endpoints)
 The range syntax is to be used only for, and for every range that expands to more than two values.
Example

The list of integers:

 -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20
Is accurately expressed by the range expression:

 -6,-3-1,3-5,7-11,14,15,17-20
(And vice-versa).

Task

 Create a function that takes a list of integers in increasing order and returns a correctly formatted string in the range format. 
 Use the function to compute and print the range formatted version of the following ordered list of integers. (The correct answer is: 0-2,4,6-8,11,12,14-25,27-33,35-39).


    0,  1,  2,  4,  6,  7,  8, 11, 12, 14,
   15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
   25, 27, 28, 29, 30, 31, 32, 33, 35, 36,
   37, 38, 39

 Show the output of your program.


Related task

   Range expansion


procedure main()
 
   R := [  0,  1,  2,  4,  6,  7,  8, 11, 12, 14, 
          15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 
          25, 27, 28, 29, 30, 31, 32, 33, 35, 36, 
          37, 38, 39 ]
 
   write("Input list      := ",list2string(R))
   write("Extracted sting := ",s := range_extract(R)  | "FAILED")
end
 
procedure range_extract(R)         #: return string/range representation of a list of unique integers
local s,sep,low,high,x
 
   every if integer(x:= !R) ~= x then fail                  # ensure all are integers, 
   R := sort(set(R))                                        # unique, and sorted
 
   s := sep := ""
   while s ||:= sep || ( low := high := get(R) ) do {       # lower bound of range
      sep := ","
      while high := ( R[1] = high + 1 ) do get(R)           # find the end of range
      if high > low+1 then s ||:= "-" || high               # - record range of 3+
      else if high = low+1 then push(R,high)                # - range of 2, high becomes new low
      }
   return s
end
 
procedure list2string(L)          #: helper to convert list to string
local s
 
   every (s := "[ ") ||:= !L || " "
   return s || "]"
end