They can be a bit unwieldy when generating pairs, it may be tempting to construct the pairs from some function over their progression, like the following pair-of-ints-in-range-descending:

let all_pairs_desc a b : (int * int) Seq.t =
let upper = (max a b)-1 in
let lower = min a b in
let rec all_pairs_aux a b = fun () ->
if b >= lower
then Seq.Cons ((a, b), all_pairs_aux a (b-1))
else begin
if a > lower
then Seq.Cons ((a-1, a-1), all_pairs_aux (a-1) (a-2))
else Seq.Nil
end
in all_pairs_aux upper upper

..but we can make it more elegant than that, using sequences-of-sequences and flat_map. Following your `count` example of counting pairs with no common denominator, using the `gcd` function from above:

let coprime (x, y) = (gcd x y == 1) in
let int_range from until =
Seq.ints from |> Seq.take_while (fun x -> x <= until)
in
let seq_ij i =
int_range (i/3+1) (i/2) |> Seq.map (fun j -> (i, j))
in
int_range 5 10000
|> Seq.flat_map seq_ij
|> Seq.filter coprime
|> Seq.length

]]> https://curiousprogrammer.wordpress.com/2009/02/27/pretty-printing-xml/#comment-34384 Mon, 10 Mar 2025 18:23:21 +0000 http://curiousprogrammer.wordpress.com/?p=276#comment-34384 Lovvely post

(condition-case err
…
(‘error (…)))

Is wrong, and should instead be

(condition-case err
…
(error (…)))

I.e. remove the quote.