Re: [Caml-list] Mutual recursion propagates individual recursion. Why?

[Gabriel Scherer <gabriel.scherer@gmail.com>]

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  let x1 = e1 and x2 = e2 and ... and xn = en in body

Has the effect that the x1,x2,..,xn are bound "simultaneously" in body, and
not before. Unlike what "let x1 = e1 in let x2 = e2 in ..." does, x1 is not
visible in e2, etc. This is rarely useful when programming, but extremely
useful when meta-programming, as it allows you to evaluate several
different pieces of code in the same scope independently, without risk of
variable shadowing.

For the record I don't find your feature suggestion particularly tempting.
Mutual recursion is more expressive than single-recursion, and I'm not sure
what would be the point of allowing the former and restricting the latter
-- the horse is already out of the barn. Instead of

  let rec fac = function
    | 0 -> 1
    | n -> n * fac (n - 1)

I can write

  let rec fac = function
    | 0 -> 1
    | n -> n * f (n - 1)
  and f n = fac n

turning any self-recursion into mutual-recursion.

I'm not sure I understand your point about accidental value recursion. Do
you have an example?

Note that it is possible to use recursive modules as a way to have
recursion between phrases (structure items) without explicitly using "rec".
It's a bad idea in most situations, because using recursive modules makes
you rely on more complex (and accordinly more fragile) features of the
language.

On Mon, Mar 2, 2015 at 7:25 AM, Jordan W <jordojw@gmail.com> wrote:

> (Note: When trying any of these examples, make sure to kill/restart
> your top level between each examples - non-recursive bindings that
> should fail will appear to work because they use existing bindings in
> the environment).
>
> My understanding is that self-recursion in OCaml is introduced via the
> `let rec` binding keyword pair.
>
>     let rec x a = x a
>
>
> A sequence of let bindings are made *both* mutually recursive, *and*
> individually self-recursive via a combination of `let rec` and the
> `and` keyword.
>
>    (* Notice how y is made self recursive as well *)
>    let rec x a = (x a + y a) and y a = (x a + y a);;
>
> The `and` keyword by itself is not sufficient to introduce mutual
> recursion, and not sufficient to introduce self-recursion for any of
> the bindings joined by the `and`.
>
>     (* Does not work *)
>     let x a = x a and y a = (x a + y a)
>     (* Does not work *)
>     let x a = y a and y a = x a
>
>
> My questions are:
> 1. Is there any effect to having the `and` keyword, without a `let
> rec` that initiates the let binding sequence?
> 2. Is there any way to introduce mutual recursion without also
> introducing self-recursion on *all* of the bindings?
>
> I would like self-recursion to be independent from mutual recursion.
> It would be nice to be able to create several mutually recursive
> bindings that are not individually self-recursive. I imagine the
> syntax to accomplish this would require each binding to be opened with
> "let" or "let rec" which would be totally reasonable.
>
>     (* Three mutually recursive functions that are not self-recursive *)
>     let rec thisOneIsSelfRecursive x = ... and
>     let thisOneIsNotSelfRecursive y = ... and
>     let rec thisOneIsAlsoSelfRecursive z = ...;
>
> This becomes more desirable when one of the mutually recursive
> bindings is a non-function value that you did not want to make
> self-recursive by accident (which causes cycles).
>
> Jordan
>
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