[Haskell-cafe] Are constructors strict?

[Jan Christiansen]

On 21.01.2011, at 11:52, Daryoush Mehrtash wrote:

> Do I have to have MonadPlus m or would any other Monad class work  
> the same way?


Not all monad instances satisfy

   undefined >>= return . Just = undefined

if that's what you are asking for. For example, consider the identity  
monad.

   instance Monad Identity where
     return = Identity
     m >>= k = k (runIdentity m)

Then we have

      undefined >>= return . Just
   = undefined >>= Identity . Just
   = Identity (Just undefined)
   /= undefined

If >>= performs pattern matching on its first argument like most  
instances do then you get undefined >>= return . Just = undefined.

I think that the monadplus laws

   mplus m n >>= f = mplus (m >>= f) (n >>= f)

called (Ldistr) in the paper and

   mzero >>= f = mzero

called (Lzero) in the paper imply

   undefined >>= return . Just = undefined

At least if you have mzero /= mplus m n which is quite desirable. I  
don't think that this holds for continuation based monads. But  
Sebastian will most certainly know better as he is one of the authors  
of the paper.

Cheers, Jan