[Haskell-cafe] instance Monad (Except err)
tonymorris at gmail.com
Mon May 4 05:22:04 EDT 2009
Martijn van Steenbergen wrote:
> 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?
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
I'm sure you could, but then ap /= (<*>).
This seems related to a question that I once asked
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