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
Am Mittwoch, 3. September 2003 12:50 schrieb Steffen Mazanek:
Hello,
all_fib :: [Float]
You define "all_fib" to return a list of "Float", but "even" does only work for numbers whose type is an instance of the class Integral, e.g. Int.
HTH and ciao, Steffen
And I see no reason to define it as a list of Float because all Fibonacci numbers are natural numbers. So better write: all_fib :: [Integer] Wolfgang
On Wednesday 03 September 2003 10:30, Wamberto Vasconcelos wrote:
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?
"even" wants an integral type for its argument, but you are applying it to a list of floats. Konrad.
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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participants (5)
-
Frank Seaton Taylor -
Konrad Hinsen -
Steffen Mazanek -
Wamberto Vasconcelos -
Wolfgang Jeltsch