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 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 > 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 > > > > > > > ___ 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
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 > > ___ 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
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 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 398E692F.asc Description: application/apple-msg-attachment PGP.sig Description: This is a digitally signed message part ___ 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 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
Re: [Haskell-cafe] How to implement the mean function
(/) 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
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 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
Re: [Haskell-cafe] How to implement the mean function
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
[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
