[Haskell-beginners] I have a question in designing a 'show' function

[Joel Neely]

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 at 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
>
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-- 
Beauty of style and harmony and grace and good rhythm depend on simplicity.
- Plato
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