Re: [Haskell-cafe] How to implement the mean function
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.si...@gmail.com wrote: What compiler errors are you getting? -deech On Fri, Jul 1, 2011 at 12:55 AM, Ruohao Li liruo...@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 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
Re: [Haskell-cafe] How to implement the mean function
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 nathan.d.how...@gmail.comwrote: (/) 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 liruo...@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 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
Re: [Haskell-cafe] How to implement the mean function
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 jhena...@uvm.edu 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-simple-average-function 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.si...@gmail.com wrote: What compiler errors are you getting? -deech On Fri, Jul 1, 2011 at 12:55 AM, Ruohao Li liruo...@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 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] How to implement the mean function
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
