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 wrote:

What compiler errors are you getting? -deech

On Fri, Jul 1, 2011 at 12:55 AM, 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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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 wrote:

Additionally, this SO question[0] is nearly identical, and provides a little more elaboration.

[0]: http://stackoverflow.com/questions/2376981/haskell-types-frustrating-a-simpl...

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 wrote: What compiler errors are you getting? -deech

On Fri, Jul 1, 2011 at 12:55 AM, 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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

==== "Computer Science is no more about computers than astronomy is about telescopes." -- Edsger Dijkstra ====

The problem is that you need to convert (length xs) to a Num, then return a Fractional. 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. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe