Problem with Infinite Lists

[Frank Seaton Taylor]

This is a bit off topic, but...


Warning: contains evangelism from a number theorist.

The Fibonacci sequence should start with 0 and 1 rather than 1 and 1. 
Doing so makes it adhere to the following property:

   all_fib !! (gcd m n) == gcd (all_fib !! m) (all_fib !! n)

for m, n nonnegative integers. With the exception that Haskell 
misdefines gcd 0 0 as an error rather than 0.

---Frank

On Wednesday, Sep 3, 2003, at 04:30 US/Eastern, Wamberto Vasconcelos 
wrote:

> Folks
>
> I am new in this forum, so apologies if this has been asked before. If 
> this is
> the case, please point me to the direction of the answer and I won't 
> bother you
> again!
>
> Also, could you make sure you CC your answers to me? I am not sure how 
> long it
> takes for my subscription to be activated and I don't want to miss out 
> on the
> action :-)
>
> The Problem
>
> I have defined this function to calculate the Fibonacci numbers:
>
> all_fib :: [Float]
>
> all_fib = 1:(1:(add_fib all_fib))
>           where add_fib (x:y:rs) = (x + y):(add_fib (y:rs))
>
> Which seems to work:
>
> Main> take 20 all_fib
> [1.0,1.0,2.0,3.0,5.0,8.0,13.0,21.0,34.0,55.0,89.0,144.0,233.0,377.0,
>  610.0,987.0,1597.0,2584.0,4181.0,6765.0]
>
> However, when I tried
>
> Main> filter even (take 20 all_fib)
> ERROR - Illegal Haskell 98 class constraint in inferred type
> *** Expression : filter even (take 20 all_fib)
> *** Type       : Integral Float => [Float]
>
> What is going on here?
>
> Thanks in advance for any help/hint.
>
> -- 
> Wamberto Vasconcelos, PhD          wvasconcelos@acm.org
> Department of Computing Science    http://www.csd.abdn.ac.uk/~wvasconc
> University of Aberdeen, Aberdeen AB24 3UE, Scotland, UK
> Phone +44 (0)1224 272283           Fax +44 (0)1224 273422
>
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