Since nobody replied so far, I'm also forwarding to cafe.
Petr
---------- Forwarded message ----------
From: Petr Pudlák
Date: 2014-01-29
Subject: incorrect MonadPlus law "v >> mzero = mzero"?
To: "libraries@haskell.org"
Hi,
this law apparently fails for a MonadPlus instance that has more than one
possible failure value. Consider:
runIdentity . runErrorT $
((ErrorT . Identity $ Left "failure") >> mzero :: ErrorT String
Identity ())
evaluates to `Left "failure"`, which isn't equal to ErrorT's mzero `Left
""`.
This isn't just the case of ErrorT, it fails for any MonadPlus with
multiple failure values. For example
lift (tell "foo") >> mzero :: MaybeT (Writer String) ()
is again distinct from mzero.
Actually, no monad transformer with a MonadPlus instance can satisfy the
law, because the first part in front of `>> mzero` can introduce side
effects in the underlying monad.
I'm not sure what should be the proper solution. Perhaps to change the laws
to:
return x >> mzero = mzero
(v >> mzero) >>= f = (v >> mzero)`
That is, if an expression ends with `mzero`, it behaves like `mzero`.
Petr
