Re: [Haskell-cafe] Type inference problem with division (/)

30 Oct 2007


      On 10/30/07, noa  wrote:
...
Hi!
I have the following function:
theRemainder :: [String] -> [String] -> Double
theRemainder xs xt = sum( map additional (unique xs) )
        where
        additional x = poccur * (inf [ppos,pneg]) --inf takes [Double]
                where
                xsxt = zip xs xt
                pi = countPos xr -- countPos returns an Int
                ni = (length xr) - pi
                len = length xs
                len2 = length xr
                ppos = pi/len2 -- THESE ARE THE PROBLEM
                pneg = ni/len2 -- THESE ARE THE PROBLEM
                poccur = (pi+ni)/len
                xr = (filter ((\y -> (fst y)==x)) (xsxt))
And I am getting this error message with ghc:
matrix.hs:54:31:
    Couldn't match expected type `Double' against inferred type `Int'
    In the expression: ppos
    In the first argument of `inf', namely `[ppos, pneg]'
    In the second argument of `(*)', namely `(inf [ppos, pneg])'
How can I change the declaration of ppos nad pneg so they are treated as
Double for the inf function?
ppos = pi/len2; pi and len2 are both Ints, so dividing them gives you
an Int. To convert to a Double, write ppos = fromIntegral (pi/len2).
(Type :t fromIntegral in ghci to see what else fromIntegral can be
used for.)

Cheers,
Tim

-- 
Tim Chevalier * catamorphism.org * Often in error, never in doubt
"After three days without programming, life becomes meaningless."  --
James Geoffrey