[Haskell-cafe] instance Monad (Except err)

[Tony Morris]

Yes, however, ap is not equal to (<*>) for the given Applicative. I
don't believe such a monad is possible.

Neil Brown wrote:
> Martijn van Steenbergen wrote:
>> Hello,
>>
>> Mr. McBride and mr. Paterson define in their Applicative paper:
>>
>>> data Except e a = OK a | Failed e
>>> instance Monoid e => Applicative (Except e) where ...
>>
>> Sometimes I'd still like to use >>= on Excepts but this "feels" wrong
>> somehow, because it doesn't use monoids nicely like the Applicative
>> instance does. Are there any good reasons such a Monad instance
>> shouldn't be defined? Does it violate any laws, for example?
> Isn't the Except type just Either by another name?  OK = Right, Failed
> = Left.  Therefore the monad is just the same as the Either monad, and
> is useful as an error monad:
>
> instance Monad (Except e) where
>  (OK x) >>= f = f x
>  Failed e >>= _ = Failed e
>  return = OK
>
> This obeys all the monad laws.
>
> Thanks,
>
> Neil.
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-- 
Tony Morris
http://tmorris.net/