[racket] Evaluation problem

On Sun, Sep 30, 2012 at 4:43 PM, Mikko Tiihonen
<mikko.tiihonen at tmtiihonen.fi> wrote:
> Hi!
>
> I have been going through some lambda-calculus exercises with Racket. I have stumbled on a puzzling problem where beta-reduction by hand leads  to different result from that generated by Racket.
>
>
> (define select-first
>   (lambda (first)
>     (lambda (second)
>       first)))
>
> (define select-second
>   (lambda (first)
>     (lambda (second)
>       second)))
>
> (define make-pair
>   (lambda (first)
>     (lambda (second)
>       (lambda (func)
>         (func first) second))))

I think you want an extra pair of parens in the line just above:

   ((func first) second)

Robby

>
> -----
>> ((select-first 'a) 'b)
> 'a
>> ((select-second 'a) 'b)
> 'b
>> (((make-pair 'a) 'b) select-first)
> 'b
>> (((make-pair 'a) 'b) select-second)
> 'b
> -------
>
> So, select-first returns the correct result when applied directly but returns the second argument when applied via make-pair. Also, I have checked that the implementation of make-pair gives the correct result if I do the beta-reduction by hand.
>
> I suspect that I'm missing something obvious, but I really cannot find the reason why make-pair misbehaves - you gurus on this mailing-list probably can pinpoint the problem instantly :)
>
> Best regards,
>
> -Mikko Tiihonen
>
> P.S.  Having used Racket for some months now, I find both Racket and the community around it really great!
>
>
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