SML - help with a function

Question

Declare a function of

shortLong: string list -> int * int

, so the result of a call short long ts becomes (k, l) where k is the number of texts in ts with six or fewer characters, and l is the number of texts in ts with seven or peripheral characters. Try to express the short-long in such a way that the argument "not" be run two, but only once.

Answer 1

You can use explode to convert a string to a character list. Do this recursively on each item in string list (or use foldl).

Answer 2

Use foldl starting with (0,0) and then at each iteration use the size function to find out the length of the string and then add 1 to the first or second item of the tuple accordingly.

Edit:

Now that you mention it, an easier way than using foldl is to use List.partition. You can use size to write a function that returns true iff a string has 6 or fewer characters. You can then use partition with that function, to get a list of strings with 6 or fewer characters and another one with the strings with more than 6 characters.

You can then use length on both lists to get their length and then return a tuple containing the two lengths.

Edit2:

Here's the code resulting from the comments over multiple lines for readability:

fun shortLong list =
  let
    val (longs, shorts) = List.partition (fn k => size k > 6) list
  in
    (length shorts, length longs)
  end