[Haskell-beginners] Function to compute the mean

[David James]

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 <beginners-bounces at haskell.org> on behalf of Joe King <joeking1809 at yahoo.com>
Sent: Saturday, May 8, 2021 10:39:50 AM
To: beginners at haskell.org <beginners at 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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