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

On 10/30/07, Tim Chevalier wrote:

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.)

You can't divide Ints with (/) at all -- they aren't Fractional. You'll probably want to either fromIntegral both pi and len2 or use div for integer division. (Also, pi is a bit of a confusing name; you may want to consider using another one.) Shachaf