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Solutions to Minor ML homework
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1. What are the values of the following expressions:

  1. #2("bob", "paul", "susan")  ==>  "paul"

  2. tl [2, 3, 4]  ==>  [3, 4]

  3. tl([2, 3, 4])  ==>  [3, 4]

  4. implode([#"a", #"b", #"c])  ==>  "abc"

  5. hd (explode("Tom"))  ==>  #"T"

  6. tl ("Tom" :: ["Bob", "Mary"])  ==>  ["Bob", "Mary"]

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2. What is wrong with each of the following expressions?

  1. #4(3,4,5)

     This tuple doesn't have a 4th element.  Possible fixes
     are #3(3,4,5) and #4(3,4,5,6).

  2. #2([1,2,3])

     ML interprets this as #2 on a list, which isn't valid.
     Try #2(1,2,3).

  3. tl (tl (tl [1, 2]))

     Final call to tl is on an empty list, which isn't allowed.
     Remove a call to tl or add another element to the list.

  4. implode(["a", "b", "c"])
     
     Implode works on characters, not strings.  Change to
     implode([#"a", #"b", #"c"])

  5. "Tom" :: ("Bob" :: "Mary")

     The cons operator takes an element and a list of elements of
     the same type, so the 2nd cons doesn't make sense.  Try
     "Tom" :: ("Bob" :: ["Mary"]).

  6. [1, "a", #"a"]

     Elements in a list need to have the same type.  You could do
     this as a tuple:  (1, "a", #"a").

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3. Write function declarations to compute the following:

   1. Compute the length of a list

	fun len(L) = 
		if null(L) then 0
		else 1 + len(tl L);

   2. Cycle a list once.

	fun cycle(nil) = nil
	|   cycle(x::xs) = xs @ [x];

   3. Cycle a list the other way.

	fun cycleback(L) =
		if L = nil then nil
		else hd(rev(L)) :: rev(tl(rev(L)));

   4. Remove all of the odd numbers for a list of integers.

	fun odd(x) = x mod 2 = 1;

	fun remove_odd(nil) = nil
	|   remove_odd(x::xs) =
		if odd(x) then remove_odd(xs)
		else x :: remove_odd(xs);

   5. Remove all vowels from a string.

	fun isvowel(c) = 
		(c = #"a") orelse (c = #"A") orelse
		(c = #"e") orelse (c = #"E") orelse
		(c = #"i") orelse (c = #"I") orelse
		(c = #"o") orelse (c = #"O") orelse
		(c = #"u") orelse (c = #"U");

        (* remove vowels for character lists *)
	fun remove_vowels_list(nil) = nil
	|   remove_vowels_list(x::xs) =
		if isvowel(x) then remove_vowels_list(xs)
		else x :: remove_vowels_list(xs);

        (* handle string <--> character list conversion *)
	fun remove_vowels(L) = implode(remove_vowels_list(explode L));

(NOTE: the filter function could also be used for parts 4 and 5.)

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Bonus:  Read in a file and return a list of characters in it.

	open TextIO;

	fun streamToCharList(ins) =
		if endOfStream(ins) then nil
		else explode(inputLine ins) @ streamToCharList(ins);

	fun fileToCharList(filename) = 
		streamToCharList (openIn filename);