On Sep 17, 2009, at 12:04 AM, Brian Mastenbrook wrote:
> You could imagine the REPL in this model as an ever nested
> sequence of LET forms:
>
> (let ()
> *first user input here*
> (let ()
> *second user input here*
> (let ()
> *third user input here*
> ...)))
>
> In this case, all of the bindings returned by any individual form
> entered at the REPL are established in parallel, and multiple
> individual forms establish bindings sequentially.
Ikarus does this pretty much (which some would consider broken, so,
spare me!). There is only two minor things it does differently:
1. If *user input here* contains a reference to an unbound variable
(in any outer scope), it does what amounts to injecting a dummy
definition to that variable giving it some #<unbound> value.
2. If *user input here* contains a new definition of a variable
that was previously defined in the repl, the definition is turned
into a set!.
So, if you type:
(define (f) --- g ---)
(define (g) --- f ---)
that would be equivalent to:
(let ()
(define g)
(define (f) --- g ---)
(let ()
(set! g (lambda () --- f ---))
(let ()
...)))
while typing:
(begin
(define (f) --- g ---)
(define (g) --- f ---))
is equivalent to:
(let ()
(define (f) --- g ---)
(define (g) --- f ---)
(let ()
...))
I believe Chez Scheme (7.9) and Larceny (0.97, in "err5rs" mode)
both do what amounts to the same thing.
Aziz,,,
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