[Haskell-beginners] A question regarding the syntax behind the parenthesis and type class.

[David McBride]

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 at 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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