Hello – I wanted to add some comments. mean is as you describe.
mean1 as defined can take a list of any Real type, and return any Fractional type. These types need not be the same, and it’s up to the *caller* of mean1 to say what types they want to give and get returned, e.g. the following is possible:
:m +Data.Ratio
:m +Data.Complex
mean1 [6%5, 2%3] :: Complex Double
0.9333333333333333 :+ 0.0
This may not be what you were expecting, and maybe you want something like this:
mean2 :: (Fractional a) => [a] -> a
mean2 xs = sum xs / genericLength xs
(and note the realToFrac is then unces
From: Beginners on behalf of Joe King
Sent: Saturday, May 8, 2021 10:39:50 AM
To: beginners@haskell.org
Subject: [Haskell-beginners] Function to compute the mean
Greeetings I am new here and pretty new to Haskell.
I was wondering what are the relative advanatges/disadvatnages of specifying a mean function in these two ways:
mean :: [Double] -> Double
mean xs = sum xs / fromIntegral (length xs)
and
mean1 :: (Real a, Fractional b) => [a] -> b
mean1 xs = realToFrac (sum xs) / genericLength xs
I understand that mean1 has the advantage that it can be called with lists of any Real type, so would work with things like
foo :: [Int]
foo = [1,2,3]
mean foo
-- type mismatch error
mean1 foo
-- no error
But suppose that I know I will only ever use lists of Double, is there still any advantage (or disadvantage of using mean1). For example is there any performance benefit by using mean in that case since mean1 has additional function evaluation.
Are there any other considerations ?
Thanks in advance
JK
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