[Haskell-beginners] I have question about Haskell

[Baojun Wang]

putStr (show (polyEvaluate (coeffs) (read value :: Float) ))

Here ``polyEvaluate`` is inferred as a Show(able), However, ``polyEvaluate``
definition on the bottom returns a monad. (In haskell, meaning of return is
quite different than imperative programming language).

It's encouraged to write pure functions without effects, don't use monad
unless it's really necessary. rewrite ``polyEvaluate`` as below should make
the example compile.

polyEvaluate (coeffs) x = map (\(a, b) -> a+b) . zip coeffs . iterate (+1)
$ 0

Thanks
baojun

On Thu, Apr 21, 2016 at 8:33 PM Eunsu Kim <wntuwntu at gmail.com> wrote:

> Hi
>
> i have a problem in my code!
>
> here is my code:
>
> -- Baic I/O and Loop (50 Points)
>
> evalpoly = do putStr "What is the degree of polynomial: "
> degree <- getLine
> coeffs <- (funcOfCoeff ((read degree::Int)+1) [])
> putStr "What value do you want to evaluate at: "
> value <- getLine
> putStr "The value of the polynomial is: "
> putStr (show (polyEvaluate (coeffs) (read value :: Float) ))
> putStr "\n"
>
> --function loop to get coefficient--
>
> funcOfCoeff 0 coeffs = do --to check the degree of 0
> return coeffs --return list of coefficient
>
> funcOfCoeff degree coeffs = do
> putStr ("What is the x^" ++ show(degree-1))
> putStr " coefficient: "
> coeff <- getLine
> loop <- funcOfCoeff (degree-1) ((read coeff :: Int) : coeffs)
> return loop
>
>
> polyEvaluate (coeffs) x = do
> powers <- zip coeffs (iterate (+1) 0)
> result <- map (\(a,b)-> a+b) powers —PROBLEM IS HERE!!!!
> return result
>
> here is error message:
>
>
> in very bottom function (polyEvaluate), why is not working “result <- map
> (\(a,b) -> a+b) powers” ???
>
> in Prelude, it is working
>
>
> Thanks!
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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