[plt-scheme] Behind the scenes of letrec

[John Clements]

On Feb 27, 2008, at 6:00 PM, Marco Morazan wrote:

> Dear All,
>
> I am considering different alternatives for implementing letrec. As an
> example consider the following:
>
>  (letrec ((a (lambda (b) (* 2 b)))
>           (b (lambda (c) (a c)))
>           (c (lambda (q) (b q))))
>    (c 5))
>
> We can implement it by transforming letrec into let and use  
> assignment:
>
> (let ((a '())
>        (b '())
>        (c '()))
>    (let ((x (lambda (b) (* 2 b)))
>          (y (lambda (c) (a c)))
>          (z (lambda (q) (b q))))
>      (set! a x)
>      (set! b y)
>      (set! c z))
>    (c 5))
>
> I am aware that Dybvig et. al. proposed a more sophisticated
> transformation to let with assignments.
>
> Alternatively, we could transform the letrec into an application
> expression and create a new local function (i.e. within the same scope
> of the letrec expression) such as:
>
> new local function:
>
> (define (new)
>    (define a (lambda (b) (* 2 b)))
>    (define b (lambda (c) (a c)))
>    (define c (lambda (q) (b q)))
>    (c 5))
>
> new application expression:
>
>  (new)
>
>> From a purist's perspective, the latter is attractive given the lack
> of assignment. What are the reasons for preferring the former
> transformation?

How are you planning to implement the local defines?

John

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