[Haskell-cafe] Typing problem
daniel.is.fischer at googlemail.com
Mon Jan 31 19:38:22 CET 2011
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
> 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.
> g normalize [2,5,3,2] ->
> 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
(**) :: 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
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).
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