Re: [Haskell-cafe] casting numerical types

2 Apr 2006

      On Sun, Apr 02, 2006 at 10:53:02AM +0100, Malcolm Wallace wrote:
...
To: haskell-cafe@haskell.org
From: Malcolm Wallace 
Date: Sun, 2 Apr 2006 10:53:02 +0100
Subject: Re: [Haskell-cafe] casting numerical types
Matthias Fischmann  writes:
...
avg :: (FractionalOrIntegral a) => [a] -> a
   avg xs = sum (map fromFractionalOrIntegral xs) / (fromIntegral (length xs))
Your condition is probably too strong.  For one thing, there is no need
to convert every element of the list being summed, just its result.  Also,
does the final result need to be the same type as the elements?
avg :: (Num a, Fractional b) => [a] -> b
    avg xs = fromRational (sum xs % toInteger (length xs))
you are right, your version looks better, and i managed to make ghc
eat these variants:

  avg' :: (RealFrac a, Fractional b) => [a] -> b
  avg' xs = fromRational (round (sum xs) % toInteger (length xs))

  avg'' :: (RealFrac a, Fractional b) => a -> [a] -> b
  avg'' epsilon xs = fromRational ((1 % toInteger (length xs)) * approxRational (sum xs) epsilon)

But the problem is that (%) requests integral types, and I do not want
to round my input.  Also, I have the same problem as above that
Integral types are not RealFrac, so these do not accept [Int] as
input.  When I try yours, however, I get:

    Couldn't match the rigid variable `a' against `Integer'
      `a' is bound by the type signature for `avg'
      Expected type: a
      Inferred type: Integer
    In the application `toInteger (length xs)'
    In the second argument of `(%)', namely `toInteger (length xs)'

So do I need 'toNum' now?

m.