{-# OPTIONS -fglasgow-exts #-} module HList (tests) where {- This module illustrates heterogeneously typed lists. -} import Test.HUnit import Data.Typeable -- Heterogeneously typed lists type HList = [DontKnow] data DontKnow = forall a. Typeable a => DontKnow a -- The empty list initHList :: HList initHList = [] -- Add an entry addHList :: Typeable a => a -> HList -> HList addHList a l = (DontKnow a:l) -- Test for an empty list nullHList :: HList -> Bool nullHList = null -- Retrieve head by type case headHList :: Typeable a => HList -> Maybe a headHList [] = Nothing headHList (DontKnow a:_) = cast a -- Retrieve tail by type case tailHList :: HList -> HList tailHList = tail -- Access per index; starts at 1 nth1HList :: Typeable a => Int -> HList -> Maybe a nth1HList i l = case (l !! (i-1)) of (DontKnow a) -> cast a ---------------------------------------------------------------------------- -- A demo list mylist = addHList (1::Int) $ addHList (True::Bool) $ addHList ("42"::String) $ initHList -- Main function for testing tests = ( show (nth1HList 1 mylist :: Maybe Int) -- shows Just 1 , ( show (nth1HList 1 mylist :: Maybe Bool) -- shows Nothing , ( show (nth1HList 2 mylist :: Maybe Bool) -- shows Just True , ( show (nth1HList 3 mylist :: Maybe String) -- shows Just "42" )))) ~=? output output = ("Just 1",("Nothing",("Just True","Just \"42\"")))