# A question regarding the syntax behind the parenthesis and type class. Classic List Threaded 2 messages Reply | Threaded
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## A question regarding the syntax behind the parenthesis and type class.

 this is the original question: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 -> Stringmy 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_______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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## Re: A question regarding the syntax behind the parenthesis and type class.

 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 wrote:this is the original question: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 -> Stringmy 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_______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners _______________________________________________ Beginners mailing list [hidden email] http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners