Re: [Haskell-cafe] How to implement the mean function

1 Jul 2011

      For mean xs = sum xs / fromIntegral (length xs), I got the following:

test.hs:8:10:
    Could not deduce (Fractional a)
      from the context (Num a, Fractional b)
      arising from a use of `/' at test.hs:8:10-42
    Possible fix:
      add (Fractional a) to the context of the type signature for `mean'
    In the expression: sum xs / fromIntegral (length xs)
    In the definition of `mean':
        mean xs = sum xs / fromIntegral (length xs)

test.hs:8:10:
    Couldn't match expected type `b' against inferred type `a'
      `b' is a rigid type variable bound by
          the type signature for `mean' at test.hs:7:27
      `a' is a rigid type variable bound by
          the type signature for `mean' at test.hs:7:13
    In the expression: sum xs / fromIntegral (length xs)
    In the definition of `mean':
        mean xs = sum xs / fromIntegral (length xs)

And the div way will do integer division, which is not what I want.

On Fri, Jul 1, 2011 at 2:07 PM, Nathan Howell wrote:
...
(/) operates on a Fractional instance... but length returns an Int, which
is not a Fractional.
You can convert the Int to a Fractional instance:
mean xs = sum xs / fromIntegral (length xs)
or try an integer division:
mean xs = sum xs `div` length xs
-n
On Thu, Jun 30, 2011 at 10:55 PM, Ruohao Li  wrote:
...
Hi guys,
I just started learning some Haskell. I want to implement a mean function
to compute the mean of a list. The signature of the function is:
mean :: (Num a, Fractional b) => [a] -> b
But when I implement this simple function, the compiler keep whining at me
on type errors. I know this is wrong:
mean xs = sum xs / length xs
But how to get it right? Thanks.
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