[Haskell-cafe] instance Monad (Except err)

[Tony Morris]

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?
>
> Thanks,
>
> Martijn.
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>
I'm sure you could, but then ap /= (<*>).

This seems related to a question that I once asked
http://www.haskell.org/pipermail/haskell-cafe/2009-January/054139.html


-- 
Tony Morris
http://tmorris.net/