[Haskell-cafe] Re: what is inverse of mzero and return?

21 Jan 2005

      In article ,
 "S. Alexander Jacobson"  wrote:
...
I assume there is a standard name for this 
class/function:
instance Foo [] where
     foo [] = mzero
     foo (x:_) = return x
instance Foo (Maybe x) where
     foo Nothing = mzero
     foo Just x = return x
I don't believe so. I had to write my own classes to do this sort of 
thing.

This is also a good opporunity to point out an ambiguity in the standard 
MonadPlus class. All instances satisfy these:

  mplus mzero a = a
  mplus a mzero = a

But only some instances (such as []) satisfy this:

  (mplus a b) >>= c = mplus (a >>= c) (b >>= c)

Other instances (IO, Maybe) satisfy this:

  mplus (return a) b = return a

I think mplus should be separated into two functions. This code shows 
the difference a bit more clearly:

  do
    b <- mplus (return True) (return False)
    if b then mzero else return ()

For the first kind this is the same as "return ()", for the second kind 
it's the same as "mzero".

-- 
Ashley Yakeley, Seattle WA