[Haskell-cafe] Stacking monads

Brandon S. Allbery KF8NH allbery at ece.cmu.edu
Fri Oct 3 19:34:24 EDT 2008

On 2008 Oct 3, at 16:12, Andrew Coppin wrote:
> Brandon S. Allbery KF8NH wrote:
>> On Oct 3, 2008, at 15:10 , Andrew Coppin wrote:
>>> Again, it looks like MonadPlus == Monad + Monoid, except all the  
>>> method names are different. Why do we have this confusing  
>>> duplication?
>> Because typeclasses aren't like OO classes.  Specifically:  while  
>> you can specify what looks like class inheritance (e.g. "this Monad  
>> is also a Monoid" you can't override inherited methods (because  
>> it's a Monad, you can't specify as part of the Monad instance the  
>> definition of a Monoid class function).  So if you want to define  
>> MonadPlus to look like a Monad and a Monoid, you have to pick one  
>> and *duplicate* the other (without using the same names, since  
>> they're already taken by the typeclass you *don't* choose).
> I was thinking more, why not just delete MonadPlus completely, and  
> have any function that needs a monad that's also a monoid say so in  
> its context? (Obviously one of the answers to that is "because it  
> would break vast amounts of existing code".)

It also touches on some Haskell98 braindamage.  (Look up "MonadZero".)

brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH

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