Proposal: add laws to MonadError

[David Feuer]

Sorry, I mangled that. I meant

catchError (m >>= f) h = catchError (Right <$> m) (pure . Left) >>=
  either h ((`catchError` h)  . f)


On Fri, Sep 9, 2022, 8:49 PM David Feuer <david.feuer at gmail.com> wrote:

> I agree. These are still insufficient for much reasoning, however. I would
> intuitively expect that
>
> catchError (m >>= f) h = catchError (Right <$> m) (pure . Left) >>=
>   either throwError ((`catchError` h)  . f)
>
> But I have no idea whether all "reasonable" instances obey that.
>
> Is there anything useful to say about the case when the argument to
> mapError is sufficiently nice (a monad morphism with some extra property,
> for instance?
>
> On Fri, Sep 9, 2022, 5:43 PM Alexandre Esteves <
> alexandre.fmp.esteves at gmail.com> wrote:
>
>> I ran into a scenario where the use of MonadError would only be valid if
>>   catchError (pure a) h = pure a
>> was a law, so I looked up the laws in
>> https://hackage.haskell.org/package/mtl-2.3/docs/Control-Monad-Error-Class.html#t:MonadError
>> but surprisingly found none.
>>
>> One would expect to see
>>   1. catchError (pure a) h = pure a
>>   2. catchError (throwError e) h = h e
>>   3. throwError e >>= f = throwError e
>>
>> which would rule out silly instances like
>>   instance MonadError () Maybe where
>>     throwError ()        = Nothing
>>     catchError _ f = f ()
>>
>> Searching for "monad error laws" gives me no haskell results, only
>> https://typelevel.org/blog/2018/04/13/rethinking-monaderror.html which
>> suggests the same laws.
>>
>> I propose adding these 3 laws to MonadError haddocks.
>> AFAICT the IO/Maybe/Either/ExceptT instances in
>> https://hackage.haskell.org/package/mtl-2.3/docs/src/Control.Monad.Error.Class.html%20
>> all obey the laws.
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>
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