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]))

This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie

Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie