On Sat, Oct 17, 2009 at 3:24 PM, Andrew Coppin
Edward Z. Yang wrote:
Excerpts from Andrew Coppin's message of Sat Oct 17 15:21:28 -0400 2009:
Suppose we have
newtype Foo x instance Monad Foo runFoo :: Foo x -> IO x
What sort of things can I do to check that I actually implemented this correctly? I mean, ignoring what makes Foo special for a moment, how can I check that it works correctly as a monad.
A proper monad obeys the monad laws:
http://www.haskell.org/haskellwiki/Monad_Laws
You can probably cook up some quickcheck properties to test for these, but really you should be able to convince yourself by inspection that your monad follows these laws.
I'm reasonably confident it works, but not 100% sure...
newtype Foo x = Foo (M -> IO x)
In this case it is trivial, Foo = ReaderT M IO which is a monad. If you want, you can verify for yourself that this is a monad. It isn't, however, uncommon for custom monads to be (equivalent to) stacks of monad transformers.