Unexpected restriction in "let rec" expressions

["Loup Vaillant" <loup.vaillant@gmail.com>]

hello,

I was trying to translate this simple Haskell definition in Ocaml:

loop :: ((a,c) -> (b,c)) -> a -> b
loop f a = b
  where (b,c) = f (a,c)

However, the direct translation doesn't work:

# let loop f a =
  let rec (b, c) = f (a, c) in
    b;;
    Characters 25-31:
    let rec (b, c) = f (a, c) in
            ^^^^^^
Only variables are allowed as left-hand side of `let rec'

So I try to bypass this restriction:

# let loop f a =
  let rec couple = f (a, snd couple) in
    fst couple;;
    Characters 34-51:
    let rec couple = f (a, snd couple) in
                     ^^^^^^^^^^^^^^^^^
This kind of expression is not allowed as right-hand side of `let rec'


Any ideas about what is this restriction, an what is it for?

Thanks,
Loup