Re: [Haskell-cafe] do we have something like isDefined or isNull in Haskell?

15 Jun 2006

      Vladimir,

I think you forgot to put Haskell-cafe as a recipient of this email,
so first I'll repost what you wrote.

On 6/15/06, Vladimir Portnykh  wrote:
...
many thanks. i have the follwoing code:
module MyType (DataContainer(..)) where
import Maybe
data DataContainer =
    MyType {
            date :: [Maybe Double],
                        weight ::[Maybe Double],
                        height ::[Maybe Double]
           }
            deriving (Eq, Ord, Show)
myType =  MyType {
            date =
[38548,38562,38576,38590,38604,38618,38632,38646,38660,38674],
                        weight = [],
                        height = []
                        }
reportProblem = if isJust (height myType  !! (length (date myType) - 1))
then 0 else 1
it failed to compiled saying "No instance for (Num (Maybe Double)) arising
from the literal 38674 .
After reading your code for a while, I still don't know if I quite
understand.  Can you explain exactly what you are trying to do?

My thought is that maybe you just want a list of objects, since you
are indexing height at the same index as date.  Then you just want a
list of the structures that Duncan described.

myObjects = [default {weight = 3.2}, default {weight = 7.9, date=38458},...]

If not, hopefully somebody a little more knowledgable than I can help you.

Bryan

Re: [Haskell-cafe] do we have something like isDefined or isNull in Haskell?

Bryan Burgers