Because you are not showing x or pi, you are showing (pi * x * x).  And the type of the (*) operator is (*) :: Num a => a -> a -> a.  So if pi is Floating => a, then x must also be the same type as pi, and thus must be have the same Floating constraint.

Keep in mind that Floating is a Fractional, and Fractional is a Num, so you know that if you have a Floating it is already an instance of Num, so you don't have to worry about that constraint.

On Wed, Apr 4, 2018 at 10:08 AM, 清羽 <1625143974@qq.com> wrote:
this is the original question:

  1. Write a function showAreaOfCircle which, given the radius of a circle, calculates the area of the circle,

    showAreaOfCircle 12.3  ⇒ "The area of a circle with radius 12.3cm is about 475.2915525615999 cm^2"

    Use the show function, as well as the predefined value pi :: Floating a => a to write showAreaOfCircle.

the type signature works for this function is: showAreaOfCircle :: (Show a, Floating a) => a -> a -> String

my question is : I don't quite understand the syntax behind the parenthesis and the type class Floating a. Since pi is not a function, and hence the entire function only takes one argument which belongs to the type class Show, why should i put a Floating a in the type signature in the first place?(i come up with this type signature because of the error message and the suggestion stated above) Thanks

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