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 > ==== > > > > > >
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