Re: letrec semantics

[Taylan Kammer <taylan.kammer@gmail.com>]

On 28.11.2022 09:33, Alexander Asteroth wrote:
> 
>> 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
> 

Hi Alex,

The only reason the first example returns 7 is because Guile *happens* to
bind a to 7 before it binds b to the value of a.  The code could have as
well returned another value (IIRC Guile uses #<unspecified>) had Guile
decided to evaluate the arms of the letrec in a different order.

As per the part of the standard you quoted:

> the 〈init〉s are evaluated [...] (in some unspecified order)

That's what the "unspecified order" in the parentheses is referring to.

On the other hand, in your second code example, there's a strict order
in which the various expressions will be evaluated.  In the moment you
type in (set! b a), the value of a has not yet been changed to 7.

If you want to use letrec but with a specific order of evaluation of the
arms, then you can use the letrec* variant:

> (letrec* ((a 7) (b a)) b)
> $1 = 7

This will definitely work, without relying on chance or an implementation
detail of Guile.

-- 
Taylan