Let’s say you are given a list of data points:
[7;5;12;8;5]
And you are asked to find the distance between every adjacent pair, that is:
[(7-5);(5-12);(12-8);(8-5)] = [2;-7;4;3]
It turns out that there is an elegant solution to this problem:
let rec pairs = function
| h1 :: (h2 :: _ as tail) -> (h1, h2) :: pairs tail
| _ –> []
let distances dataPoints =
dataPoints |> pairs |> List.map (fun (a, b) -> a - b)
The magic happens in the pairs function, this function takes [7;5;12;8;5] and turns it into [(7,5);(5,12);(12,8);(8,5)], that is, it creates a tuple with every member of the list and its right neighbor. The trick is that tail gets bounded to ( h2 :: _ ), so that the recursive call processes the list starting with h2. This is called a named subpattern, something I discovered in section 1.4.2.2 of F# for Scientists. You learn at least a nice thing every day!