On Monday 31 January 2011 18:29:59, michael rice wrote: > I'm mapping a function over a list of data, where the mapping function > is determined from the data. > > g f l = map (g l) l
g f l = map (f l) l probably > > So > > g serialize "prolog" -> [4,5,3,2,3,1] > > But I'm having typing problems trying to do a similar thing with a > function that statistically normalizes data. > > See: > http://people.revoledu.com/kardi/tutorial/Similarity/Normalization.html# >Statistic > > So > > g normalize [2,5,3,2] -> > [-0.7071067811865475,1.414213562373095,0.0,-0.7071067811865475] > > Is my typing for normalize too loose. You can omit the type signatures and see what the compiler infers as the type. In this case, > normalize :: (Num a, Num b) => [a] -> a -> b > normalize l = let (total,len) = sumlen l > avg = total/len > stdev = sqrt $ ((/) (len-1)) $ sum $ map ((** 2.0) . > (subtract avg)) l in ((/) stdev) . (subtract avg) In the final result, I suppose it should be (/ stdev) and not ((/) stdev) [the latter is (stdev /), i.e. \x -> stdev / x]. by sumlen's type, len has an Integral type. You want to use (/) to divide, which gives a Fractional constraint, (/) :: Fractional a => a -> a -> a Since it is not sensible for a type to be a member of both, the Fractional and Integral classes, you should convert len to the appropriate type with fromIntegral. For stdev, you call sqrt :: Floating a => a -> a and (**) :: Floating a => a -> a which means the list elements must have a type belonging to Floating (you could replace the (** 2.0) with (^ 2), which would probably be better, but the Floating constraint remains due to the sqrt). Finally, the resulting function is \x -> (x - avg) / stdev, hence x must have the same type as abg and stdev, and the final result has the same type. Altogether, normalize :: Floating a => [a] -> a -> a normalize l = let (total, len0) = sumlen l len = fromIntegral len0 avg = total/len stdev = sqrt $ sum [(x-avg)^2 | x <- l] / (len-1) in (/ stdev) . subtract avg but that gives nonsense if you pass a complex-valued list, so it might be better to restrict the type to normalize :: RealFloat a => [a] -> a -> a > Should I be using Floating rather than Num? You have to, and one number type only (well, you could use two or three types if you compose with conversion functions, realToFrac for example). _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
