Question

I find myself writing loops (recursive functions) occasionally within assignments. It makes for awkward code like this:

let value =
  let rec loop a =
    if ... then a
    else loop a.B
  loop a

I know I could move the loop outside the let binding, but it’s only purpose is to compute the bound value.

So I thought I might abstract the loop into a separate function:

let loop f a =
  let rec aux a =
    match f a with
    | Some b -> aux b
    | None -> a
  aux a

then I could do:

let value = a |> loop (fun a -> if ... then None else Some a.B)

Maybe that’s better–at least it looks like more like assignment than a function definition. Here are my questions:

1. Is a recursive function in a let binding something of a code smell?
2. Is there a better way to refactor this?
3. If not, could my loop function be generalized further, or somehow improved?

Answer 1

These questions are a bit subjective, but here are my answers:

1. No
2. I think what you’ve got is okay.

3. Here’s how I’d do it:

   let rec loop guard step init =
       if guard init then init
       else loop guard step (step init)
   
   let value = a |> loop (fun a -> ...) (fun a -> a.B)