letrec semantics

[Alexander Asteroth]

Dear all,

I know this topic has been discussed in the past. I found at least one
discussion in 2003 in guile-user@gnu.org which in the end referred to
even earlier discussions in comp.lang.scheme. But still I'm confused
about this and wonder if someone could help with this or point me to a
discussion that resolves the following issue.

In R5RS it sais about letrec:

>Semantics: The 〈variable〉s are bound to fresh locations
> holding undefined values, the 〈init〉s are evaluated in the
> resulting environment (in some unspecified order), each
> 〈variable〉 is assigned to the result of the corresponding
> 〈init〉, the 〈body〉 is evaluated in the resulting environmet [...]

As I (and others) understand 

> scheme@(guile-user)> (letrec ((b a)(a 7)) b)
> $1 = 7

should be equivalent (of course in a new scope) to:

> scheme@(guile-user)> (define b #nil)
> scheme@(guile-user)> (define a #nil)
> scheme@(guile-user)> (set! b a)
> scheme@(guile-user)> (set! a 7)
> scheme@(guile-user)> b
> $2 = #nil

but obviously it is't. Why is b assigned to a's reference rather than
it's value in letrec? ... and would it be a correct implementation of
R5RS-letrec to return #nil from the letrec above?

Cheers,
Alex