# [Haskell-cafe] How to implement the mean function

Ruohao Li liruohao at gmail.com
Fri Jul 1 08:47:23 CEST 2011

```Thanks for the SO link, change the Num a constraint to Real a and using
realToFrac then it just works.

On Fri, Jul 1, 2011 at 2:11 PM, Jack Henahan <jhenahan at uvm.edu> wrote:

> Additionally, this SO question[0] is nearly identical, and provides a
> little more elaboration.
>
> [0]:
>
> On Jul 1, 2011, at 2:07 AM, Ruohao Li wrote:
>
> > For mean xs = sum xs / length xs, I got the following:
> >
> > test.hs:8:10:
> >     No instance for (Fractional Int)
> >       arising from a use of `/' at test.hs:8:10-27
> >     Possible fix: add an instance declaration for (Fractional Int)
> >     In the expression: sum xs / length xs
> >     In the definition of `mean': mean xs = sum xs / length xs
> >
> > test.hs:8:10:
> >     Couldn't match expected type `b' against inferred type `Int'
> >       `b' is a rigid type variable bound by
> >           the type signature for `mean' at test.hs:7:27
> >     In the expression: sum xs / length xs
> >     In the definition of `mean': mean xs = sum xs / length xs
> >
> > test.hs:8:19:
> >     Couldn't match expected type `a' against inferred type `Int'
> >       `a' is a rigid type variable bound by
> >           the type signature for `mean' at test.hs:7:13
> >     In the second argument of `(/)', namely `length xs'
> >     In the expression: sum xs / length xs
> >     In the definition of `mean': mean xs = sum xs / length xs
> > On Fri, Jul 1, 2011 at 2:00 PM, aditya siram <aditya.siram at gmail.com>
> wrote:
> > What compiler errors are you getting?
> > -deech
> >
> > On Fri, Jul 1, 2011 at 12:55 AM, Ruohao Li <liruohao at gmail.com> 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.
> > > _______________________________________________
> > > Haskell-Cafe mailing list
> > >
> > >
> >
> > _______________________________________________
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>
> ====
> "Computer Science is no more about computers than astronomy is about
> telescopes."
> -- Edsger Dijkstra
> ====
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