lambdas can have defines so you can replace your letrec with

((lambda ()
(define x1 v1)
(define x2 v2)
...
(define xn vn)
...
))

My macros are a bit rusty right now but ill try and put up some code in the evening

Ofcourse, no internal defines, set!, syntax-case etc allowed.

Got that, but only after trying to read the two papers. Remind me to read carefully before posting. :-P

I must admit this self reference problem is pretty interesting.

Here is another letrec (again no macros).

(letrec ([x1 v1][x2 v2]...[xn vn])
<The letrec codeblock>)

Converts to

(let
((_x1 (lambda (x1 x2 ... xn) v1))
(_x2 (lambda (x1 x2 ... xn) v2))
...
(_xn (lambda (x1 x2 ... xn) vn)))
(let
((x1 (_x1 _x1 _x2 ... _xn))
(x2 (_x2 _x1 _x2 ... _xn))
...
(x1 (_x1 _x1 _x2 ... _xn)))
<The letrec codeblock>))

And we replace all instances of x1, x2... xn
in v1 v2 ... vn with (xi x1 x2 ... xn) where i is 1, 2 ... n

I dont follow -

Q) When you do this
(let
((_x1 (lambda (x1 x2 ... xn) v1))
(_x2 (lambda (x1 x2 ... xn) v2))
what happens to all refernces to x1..xn in v* ? They will not be bound.

Also what do you mean by the last line : "replace all instances of x1, x2... xn in v1 v2 ... vn with (xi x1 x2 ... xn) where i is 1, 2 ... n"

Roshan

You replace all the references of of x1..xn in v* with function applications.

A var x1 in v1 will b replaced with the function
(x1 x1 x2 ... xn) the similar process should be applied to all the other references.

That was brilliant!

Friedman pretty much did the same thing today and it struck me that this is what you were trying to say. Hats off to you for thinking of this.

Hey could you guys put up some code for an implementation of letrec.

I dont know how to write the macros to implement my version.

Hey could you guys put up some code for an implementation of letrec.

I dont know how to write the macros to implement my version.

Well, here's code for something simpler:

> (let ([E (lambda (even odd)
(lambda (n)
(if (zero? n) #t
((odd even odd) (sub1 n))
)))]
[O (lambda (even odd)
(lambda (n)
(if (zero? n) #f
((even even odd) (sub1 n))
)))])
((O E O) 1))
#t
> (let ([E (lambda (even odd)
(lambda (n)
(if (zero? n) #t
((odd even odd) (sub1 n))
)))]
[O (lambda (even odd)
(lambda (n)
(if (zero? n) #f
((even even odd) (sub1 n))
)))])
((E E O) 1))
#f
>

The letrec is along the same lines, I'll mail the code to you.

Want to hear other challenge questions. Here are two:

1. What is wrong with this code?

(define-syntax or
(syntax-rules ()
[(_ e* ...) (cond [e*] ... [else #f])]))

2. Write macros for add1, sub1 and + that operate on binary digits. As an extension you should be able to write * and exponentiation.

Example:
(expand '(+ (1 0 1) (1 0 0 1)))

will output

(0 1 1 1)

Note:
- the list of digits is not quoted. Its in reverse order.
- Number 0 represents '()

I am still thinking about both.

hey thats the same as mine except that you removed the inner let which was pretty useless anyway. I just put it there in a vague attempt of optimization.

What say Sid now, ... Nice?

Pretty Cool.
Don't work for PLT scheme though

What is PLT scheme? is that what you use?
I think I am using only standard scheme constructs, I thought it would work on all scheme implementations [though I tried only on chez scheme and Dr scheme]

Do you know what fails?

Okay so PLT is DrScheme, I'll have to check.

Hmmm I thought you said you checked on DrScheme :-P

Btw. The code you sent me worked on Chez.

Question:
Can you call a macro recursively the way you would a function?

i thought so too :P
had too many intermediate versions of letrec, prob didnt run the final version.

yes, you can. its just text substitution, so wherever it sees the macro name it matches the pattern and replaces it with the body.

Okay, so my letr macro code runs fine on PLT.
Sid, what is your language setting? Make sure its not beginners student's level. Mine is set to EOPL 2nd edition and things work beautifully :)

[I wasn't wrong I guess - I had run it earlier too probably, I keep shuffling between the two - chez scheme and drscheme] based on which editor I want to use]

Good Service

blog spams. It cant be just me?