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

[Tim Chevalier]

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.)

Cheers,
Tim

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