[Haskell-cafe] Type inference problem with division (/)
shachaf at gmail.com
Tue Oct 30 21:19:49 EDT 2007
On 10/30/07, Tim Chevalier <catamorphism at gmail.com> wrote:
> On 10/30/07, noa <noandrews at gmail.com> 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
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