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-- Minimal demonstration of syb-with-class. Here we have a class Size
-- with a base instance. Then we override it for lists, and
-- demonstrate how gsize uses the list behavior on lists and the base
-- behavior otherwise.
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE UndecidableInstances #-}
{-# OPTIONS -Wall #-}
import Data.Generics.SYB.WithClass.Basics (Data, gmapQ, Proxy)
import Data.Generics.SYB.WithClass.Context (Sat(dict))
import Data.Generics.SYB.WithClass.Instances ()
import Test.HUnit (assertEqual, Counts(..), runTestTT, showCounts, Test(TestCase))
class Size a where
gsize :: a -> Int
-- Dictionary type and Sat instance
data SizeD a = SizeD { gsizeD :: a -> Int }
instance Size t => Sat (SizeD t) where
dict = SizeD { gsizeD = gsize }
instance {-# OVERLAPPABLE #-} Data SizeD t => Size t where
-- The base behavior of gsize for any instance of Data
gsize t = 1 + sum (gmapQ (error "urk" :: Proxy SizeD) (gsizeD dict) t)
instance Size a => Size [a] where
-- Override base gsize behavior for lists
gsize [] = 0
gsize (x:xs) = gsize x + gsize xs
main :: IO ()
main = do
counts <- runTestTT (let expected = (2,1)
actual = (gsize ['a', 'b'], gsize 'x') in
TestCase (assertEqual "sample" expected actual))
case counts of
Counts {errors = 0, failures = 0} -> return ()
_ -> error (showCounts counts)
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