syb-0.7.2.2: tests/Typecase1.hs

{-# LANGUAGE DeriveDataTypeable #-}

module Typecase1 (tests) where

{-

This test demonstrates type case as it lives in Data.Typeable.
We define a function f that converts typeables into strings in some way.
Note: we only need Data.Typeable. Say: Dynamics are NOT involved.

-}

import Test.Tasty.HUnit

import Data.Typeable
import Data.Maybe

-- Some datatype.
data MyTypeable = MyCons String deriving (Show, Typeable)

--
-- Some function that performs type case.
--
f :: (Show a, Typeable a) => a -> String
f a = (maybe (maybe (maybe others
              mytys (cast a) )
              float (cast a) )
              int   (cast a) )

 where

  -- do something with ints
  int :: Int -> String
  int a =  "got an int, incremented: " ++ show (a + 1)

  -- do something with floats
  float :: Double -> String
  float a = "got a float, multiplied by .42: " ++ show (a * 0.42)

  -- do something with my typeables
  mytys :: MyTypeable -> String
  mytys a = "got a term: " ++ show a

  -- do something with all other typeables
  others = "got something else: " ++ show a


--
-- Test the type case
--
tests = ( f (41::Int)
        , f (88::Double)
        , f (MyCons "42")
        , f True) @=? output

output = ( "got an int, incremented: 42"
         , "got a float, multiplied by .42: 36.96"
         , "got a term: MyCons \"42\""
         , "got something else: True")