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Today's Topics:
1. Constructor classes and type classes. (PATRICK BROWNE)
----------------------------------------------------------------------
Message: 1
Date: Sun, 30 Oct 2016 20:56:12 +0000
From: PATRICK BROWNE <patrick.bro...@dit.ie>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] Constructor classes and type classes.
Message-ID:
<CAGFLrKfQVaQQJybMTn-pJksV1VKUXrmBks14Jkm==Zqt=bd...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
{-
I am trying to understand constructor classes and their relationship with
ordinary type classes.
I wrote the code below to help me understand the distinction. The code is
only for explanatory purposes, set operations use a tuple syntax.
I use the naming convention of 'typeVar', 'typeCons', and 'dataCons' for
type variables, type constructors, and data constructors respectively.
Question1:
Is my naming convention correct?
I am particularly concerned about SetClass3 where the super class seems to
use a type constructor but the subclass seems to use the same term as a
data constructor.
Q2:
In this case constructor classes and type classes seem to provide similar
functionality.
In general what situation are each best suited?
-}
import Data.List
data SetType typeVar = SetType [typeVar] deriving Show
class SetClass1 typeCons where
member1 :: Eq typeVar => (typeVar, typeCons typeVar) -> Bool
intersect1 :: (Eq typeVar,Show typeVar) => (typeCons typeVar, typeCons
typeVar) -> typeCons typeVar
class SetClass2 dataCons typeVar where
member2 :: (typeVar, (dataCons typeVar)) -> Bool
intersect2 :: (dataCons typeVar, dataCons typeVar) -> dataCons typeVar
class SetClass1 typeCons => SetClass3 typeCons dataVariable where
union3 :: (Eq dataVariable,Show dataVariable) => (typeCons dataVariable,
typeCons dataVariable) -> typeCons dataVariable
instance SetClass1 SetType where
member1 (x ,(SetType y)) = elem x y
intersect1 ((SetType x),(SetType y)) = SetType (intersect x y)
instance SetClass2 SetType Int where
member2 (x ,SetType y) = elem x y
intersect2 (SetType x,SetType y) = SetType (intersect x y)
instance SetClass3 SetType Int where
union3 (SetType x,SetType y) = SetType (union x y)
test1a = member1 (1, (SetType [1,2]))
test1b = intersect1 ((SetType [1,3,4]),(SetType [1,2]))
test2a = member2 (1, (SetType [1::Int]))
test2b = intersect2 ((SetType [1::Int]), (SetType [(1::Int)]))
test3a = union3 ((SetType [1::Int,2::Int,3::Int]),(SetType [4::Int,5::Int]))
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