[Haskell-cafe] Typeclasses vs simple functions?

---------- Forwarded message ---------- From: Lyndon Maydwell Date: Tue, Sep 15, 2009 at 10:03 PM Subject: Re: [Haskell-cafe] Typeclasses vs simple functions? To: Olex P I think it depends on what is going to be using the functions, data. As far as I can tell, type classes seem to be used in code designed for reuse. So if it is just for a small part of a project that is hidden behind an interface or something, then it's probably fine to just use data/functions without type classes. Keep in mind that these are the musings of someone who only recently began using Haskell. If someone else has a better explanation, then I'd be interested as well :) On Tue, Sep 15, 2009 at 9:57 PM, Olex P wrote:

Well... How this:

instance Encircled Geometry where        perimeter (Sphere _ r) = Nothing        perimeter (Circle _ r) = Just $ 2.0 * pi * r

differs from this:

perimeter :: Geometry -> Maybe Double perimeter (Sphere _ r) = Nothing perimeter (Circle _ r) = Just $ 2.0 * pi * r

and from this:

perimeter :: Geometry -> Double perimeter (Sphere _ r) = 0.0 perimeter (Circle _ r) = 2.0 * pi * r

The latter is even simpler because there is no need in extraction of Double value from Maybe. So the question is still there: do I need a type class?

On Tue, Sep 15, 2009 at 12:21 PM, Olex P wrote:

...

Sure! I completely forgot about Maybe. The only one question is is it good from the point of view of ordinary user who doesn't know about such things like functional programming, monads etc. Imagine average user who is looking into a spreadsheet and sees values 0.1, 1.4, Nothing... From other side it seems to be logical. Why not. Thanks for the idea :)

On Tue, Sep 15, 2009 at 12:05 PM, Lyndon Maydwell wrote:

...

I think the problem is that you want to compose a list with no indication of weather one member can have a perimeter or not. I'm not sure if this is a good solution or not, but I immediately think to make all Geometry objects instances of a class that return a Maybe value for the perimeter:

e.g.

---

import Data.Maybe

data Geometry = Sphere Position Radius | Circle Position Radius deriving (Show)

type Position = (Double, Double) type Radius = Double type Height = Double

class Encircled x where        perimeter :: x -> Maybe Double

instance Encircled Geometry where        perimeter (Sphere _ r) = Nothing        perimeter (Circle _ r) = Just $ 2.0 * pi * r

list = [Sphere (1,1) 1, Circle (2,2) 2]

main = (print . catMaybes . map perimeter) list

--- [12.566370614359172]

On Tue, Sep 15, 2009 at 6:29 PM, Olex P wrote:

...

Hey guys,

It's a dumb question but I'd like to know a right answer... Let's say we have some geometry data that can be Sphere, Cylinder, Circle and so on. We can implement it as new data type plus a bunch of functions that work on this data:

data Geometry = Sphere Position Radius                         | Cylinder Position Radius Height                         | Circle Position Radius                         deriving (Show)

perimeter (Sphere _ r) = 0.0 perimeter (Cylinder _ r h) = 0.0 perimeter (Circle _ r) = 2.0 * pi * r

Perimeter doesn't make sense for Sphere or Cylinder. So we could define a type class for objects that have perimeter and make an instance of it only for Circle (data Circle = Circle Position Radius). Make sense. But these three functions above have desired behaviour. If user has a list of objects like [Sphere, Circle, Circle, Cylinder] he would like to calculate perimeters of each object using map perimerer list (in this case we also have to modify Geometry data type). So we could make instances of "perimeter" type class for all objects and return zero in case if perimeter doesn't make sense. Same as previous version but with typeclasses and with additional constructors (constructors for each type of object + constructors in Geometry data). Looks a bit overcomplicated. Any reasons to use type classes in this case? Maybe there is something I'm missing?

Cheers, -O

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