Yes.And I finally make the function error-free.Thanks ------------------ 原始邮件 ------------------ 发件人: "Joel Neely"<joel.neely@gmail.com>; 发送时间: 2018年4月4日(星期三) 晚上7:19 收件人: "The Haskell-Beginners Mailing List - Discussion of primarilybeginner-level topics related to Haskell"<beginners@haskell.org>; 主题: Re: [Haskell-beginners] I have a question in designing a 'show'function You only have a single plus sign before the second invocation of show: ++ " " + show(pi * x * x) ++ "cm^2"Don't you mean concatenation? ++ " " ++ show(pi * x * x) ++ "cm^2" Good luck, -jn- On Wed, Apr 4, 2018 at 1:37 AM, 清羽 <1625143974@qq.com> wrote: here is the question: Write a function showAreaOfCircle which, given the radius of a circle, calculates the area of the circle, <code>showAreaOfCircle 12.3</code> ⇒ <code>"The area of a circle with radius 12.3cm is about 475.2915525615999 cm^2"</code> my solution is: showAreaOfCircle :: Show a => a -> String showAreaOfCircle x = "The area of a circle with radius" ++ show x ++ "is about" ++ " " + show(pi * x * x) ++ "cm^2" but there is an error • Could not deduce (Num a) arising from a use of ‘*’ from the context: Show a bound by the type signature for: showAreaOfCircle :: forall a. Show a => a -> String at FirstStep.hs:20:1-41 Possible fix: add (Num a) to the context of the type signature for: showAreaOfCircle :: forall a. Show a => a -> String • In the first argument of ‘show’, namely ‘(pi * x * x)’ In the second argument of ‘(+)’, namely ‘show (pi * x * x)’ In the second argument of ‘(++)’, namely ‘" " + show (pi * x * x)’ I had tried to change the type declaration to Show a => Num a => a -> String i guess there must be a problem with pi. Can someone help me with the error message? Great thanks _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners -- Beauty of style and harmony and grace and good rhythm depend on simplicity. - Plato