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June 2023

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Proposal: add laws to MonadError
by Alexandre Esteves 06 Jul '23

06 Jul '23

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.… 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.Cl… all obey the laws.

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