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haskell / mtl

The Monad Transformer Library
http://www.haskell.org/haskellwiki/Monad_Transformers
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Add laws for MonadError and add tryError #62

Open Lysxia opened 5 years ago

Lysxia commented 5 years ago

tryError is used to state an additional law about the interaction of catchError with (>>=).

For reference, the 5 laws other than that one can be found in the paper Just do it: Simple Monadic Equational Reasoning, Section 5 (modulo a change from catch :: m a -> m a -> m a to catchError :: m a -> (e -> m a) -> m a).

Lysxia commented 5 years ago

I also cut the original comment on this class. I find the first sentence quite hard to read, and the rest is not quite relevant.

It is common to use Either String

IMO Either is a very niche instance of MonadError.

You can also define your own error type and\/or use a monad type constructor other than @'Either' 'String'@ or @'Either' 'IOError'@. > In these cases you will have to explicitly define instances of the 'MonadError' > class.

You don't need to define additional instances if you just want to vary your exception type from String or IOError (I would assume this is a remnant of the old MonadError).

(If you are using the deprecated "Control.Monad.Error" or "Control.Monad.Trans.Error", you may also have to define an 'Error' instance.)

The old MonadError has been deprecated for so long, I would assume people who have to maintain such old code should be able to follow the changelog.

Lysxia commented 5 years ago

See #5 . With #61, the remaining effects to document are Writer and Cont.

Lysxia commented 5 years ago

Please also note that everything I'm doing here is open for questions and discussion.

Lysxia commented 5 years ago

This probably needs a @since annotation but I don't know what version.