Toying with haskell MultiParamTypeClasses

MultiparamTypeClasses example.hs

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE AllowAmbiguousTypes #-}

data  TestItem b n  = TestItem {itemName::b, itemValue::n}   

class (Num n,IsString b) => PieChartItem  a b n  where
  getValue :: a -> n 
  getName ::  a -> b
  
instance (Num n,IsString b) => PieChartItem (TestItem b n) b n where
     --getValue (TestItem b n) = n
     --getName (TestItem b n) = b
     getValue ti = itemValue ti
     getName ti = itemName ti

MultiparamTypeClasses example1.hs

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE OverloadedStrings #-}

import Data.String
import qualified  Data.Text as T
import qualified Data.Text.IO as I
import Data.Monoid

data  TestItem b n  = TestItem {itemName::b, itemValue::n}

class (Num n)=> PieChartClass a n where
  getValue ::(Num n, PieChartClass a n) => a -> n

class (IsString n)=> HasName a n where
  getName ::(IsString n, HasName a n) => a -> n

instance (Num n) => PieChartClass (TestItem b n) n where
     getValue ti = itemValue ti

instance (IsString b) => HasName (TestItem b n) b where
     getName ti = itemName ti

testIO::IO()
testIO = do
   let itemFloat::(TestItem String Float) = TestItem "Jack" 56.6
   let itemInt::(TestItem String Int) = TestItem "Bob" 32

   let itemFloatT::(TestItem T.Text Float) = TestItem "Jack" 56.6
   let itemIntT::(TestItem T.Text Int) = TestItem "Bob" 32

   let valueFloat::Float = getValue itemFloat
   let valueInt::Int = getValue itemInt

   let name::String = getName itemFloat
   let nameT::T.Text = getName itemFloatT

   putStrLn $ "float value " ++ (show valueFloat)
   putStrLn $ "int value " ++ (show valueInt)

   putStrLn ("string value " ++ name)
   I.putStrLn (("text value "::T.Text) <> nameT)
   putStrLn "_"

TypeClass constraint.hs

data  TestItem n = TestItem {itemName::String, itemValue::n}   deriving Show

class (Num n)=> PieChartClass a n where
  getValue ::(Num n, PieChartClass a n) => a -> n
  
class  HasName a where
  getName :: a -> String 

instance Num n => PieChartClass (TestItem n) n where
     getValue ti = itemValue ti

instance HasName (TestItem n) where
    getName ti = itemName ti

getPP::(Num n, PieChartClass a n) => a -> n
getPP pci = getValue pci

let itemFloat::(TestItem Float) = TestItem "Jack" 56.6
let itemInt::(TestItem Int) = TestItem "Bob" 34
let valueFloat::Float = getValue itemFloat
let valueInt::Int = getValue itemInt
let name = getName itemFloat
putStrLn $ show valueFloat
putStrLn $ show valueInt
putStrLn name