Question

Consider this OCaml code:

let coupe_inter i j cases =
  let lcases = Array.length cases in
  let low,_,_ = cases.(i)
  and _,high,_ = cases.(j) in
  low,high,
  Array.sub cases i (j-i+1),
  case_append (Array.sub cases 0 i) (Array.sub cases (j+1) (lcases-(j+1)))

Why the expression let ... and ... in is used in place of a let ... in let ... in sequence (like F# force you to do)? This construct seems quite frequent in OCaml code.
Thanks!

Answer 1

let x = a and y = b in c has the effect of defining x and y “simultaneously”. This means that the order of evaluation (a after or before b) is unspecified (you must not assume that a will be evaluated before), and that x is not bound in b and y not bound in a, they are only available in c.

I rarely use this construction, because I have been bitten in the past by the evaluation order thing. I often use the recursive variant of it, let rec ... and ... in ... (where all variable bound are available everywhere), however, to define mutually recursive functions.

let rec even n = (n = 0) || odd (n - 1)
and odd n = (n <> 0) && even (n - 1)