Re: Proposal: add laws to MonadError

Hi, I went over an old mail, sorry for the late reply. David's law does not hold: GHCi, version 8.8.4: https://www.haskell.org/ghc/ :? for help Prelude> :m Control.Monad.Except Control.Monad.Writer Prelude Control.Monad.Except Control.Monad.Writer> let lhs f h m = catchError (m >>= f) h Prelude Control.Monad.Except Control.Monad.Writer> let rhs f h m = catchError (Right <$> m) (pure . Left) >>= either h ((`catchError` h) . f) Prelude Control.Monad.Except Control.Monad.Writer> let f a = WriterT (Left ()) Prelude Control.Monad.Except Control.Monad.Writer> let h e = WriterT (Right (True , [()])) Prelude Control.Monad.Except Control.Monad.Writer> let m = WriterT (Right (False , [(), ()])) Prelude Control.Monad.Except Control.Monad.Writer> lhs f h m WriterT (Right (True,[()])) Prelude Control.Monad.Except Control.Monad.Writer> rhs f h m WriterT (Right (True,[(),(),()])) Regards, Härmel Kontakt David Feuer () kirjutas kuupäeval L, 10. september 2022 kell 02:57:

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 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@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.h... 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.Cla... all obey the laws. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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