existential-0.1.0.0: examples/Eq.hs

{-# LANGUAGE TemplateHaskell,TypeSynonymInstances,FlexibleInstances,MultiParamTypeClasses #-}
import Control.Lens
import Data.Existential

newtype MyType = MyType { _myTypeCell :: Cell MyClass }

class (Eq a, Show a) => MyClass a where

makeFields ''MyType
    -- makeFields create an Iso' MyType (Cell Eq) called cell,
    -- i.e. MyType is made into an instance of HasCell.

instance Eq MyType where
    (==) = cellEqual' (==)

instance Show MyType where
    show = ("MyType " ++) . readCell' show

instance MyClass Int where
instance MyClass Char where
instance MyClass a => MyClass [a] where

main :: IO ()
main = do
    let xs = [makeCell (7 :: Int),makeCell "hello",makeCell [1,2,3::Int]]
        xs :: [MyType]
    print $ xs == xs
        -- prints True
    let ys = xs & traverse._Cell' .~ (8 :: Int)
        ys' = replicate 3 $ makeCell (8 :: Int)
        zs  = [makeCell (8 :: Int),makeCell "hello",makeCell [1,2,3::Int]]
        zs :: [MyType]
        zs' = xs & traverse .~ makeCell (8 :: Int)
        zs' :: [MyType]
        zs'' = xs & traverse %~ readCell' (makeCell . show)
        zs'' :: [MyType]
    print $ xs == ys
        -- prints False
    print $ ys == ys'
        -- prints False
    print $ zs == ys
        -- prints True
    print $ zs' == ys'
        -- prints True
    print zs
        -- prints [MyType 8,MyType "hello",MyType [1,2,3]]
    print zs'
        -- prints [MyType 8,MyType 8,MyType 8]
    print zs''
        -- prints [MyType "7",MyType "\"hello\"",MyType "[1,2,3]"]