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Today's Topics:
1. A question regarding the syntax behind the parenthesis and
type class. (=?utf-8?B?5riF5769?=)
2. Re: A question regarding the syntax behind the parenthesis
and type class. (David McBride)
----------------------------------------------------------------------
Message: 1
Date: Wed, 4 Apr 2018 22:08:04 +0800
From: "=?utf-8?B?5riF5769?=" <1625143...@qq.com>
To: "=?utf-8?B?SGFza2VsbOmXruetlA==?=" <beginners@haskell.org>
Subject: [Haskell-beginners] A question regarding the syntax behind
the parenthesis and type class.
Message-ID: <tencent_7d0f03a1f41b2d2d487d299b6a0f17cbc...@qq.com>
Content-Type: text/plain; charset="utf-8"
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 -> 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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Message: 2
Date: Wed, 4 Apr 2018 10:20:40 -0400
From: David McBride <toa...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] A question regarding the syntax
behind the parenthesis and type class.
Message-ID:
<can+tr4277p43qza9fifljyd1yvvj3khjqinferk2xm--gpd...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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 <1625143...@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
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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