Add modifyError :: MonadError e' m => (e -> e') -> ExceptT e m a -> m a

[akrmn]

I recently came to this as a handy way to adapt a function from one MonadError to another. I couldn't find anything with this signature on hoogle, so I thought I'd add it here. I'm not 100% sure about the name, so any suggestions are welcome. I hope the docs are clear enough.

[akrmn]

hi @ekmett @hvr I'd really appreciate getting some feedback on this, who would be the right person for that? thanks!

[ekmett]

A couple of thoughts:

I have no real objection to adding something with this functionality.

However, the name doesn’t quite follow mtl/transformer conventions where lifts are used in a stylized manner to raise operations from lower monads in the stack (and are supplied by transformers rather than mtl).

[akrmn]

I have no real objection to adding something with this functionality.

thanks!

However, the name doesn’t quite follow mtl/transformer conventions where lifts are used in a stylized manner to raise operations from lower monads in the stack (and are supplied by transformers rather than mtl).

I think this would have to be defined in mtl, though, since the type has to mention MonadError, which is defined here. Regarding the name, I absolutely agree, naming things is hard indeed. Perhaps

• exceptToError by analogy with exceptToMaybeT (though the Error suffix doesn't quite convey that the return value is m a for any MonadError _ m)?

• exceptToMonadError like above, but makes a bit clearer what it does (though I don't think I've ever seen the word Monad in a function name)

• withError' by analogy with withError, the ' indicating that the error type can change?

• rethrow could also work, there's no analogy with existing functions but the name is memorable and perhaps what users would try to look for.

• rethrowExceptT like above, but indicating the input type

I really don't have a strong opinion regarding the name, so I'll be happy to have any other suggestions

[emilypi]

@akrmn I like this functionality. Can I suggest modifyError or translateError (or some variation thereof)? Then you have the intuition withError = modifyError (f :: e -> e).

[chessai]

Can this be renamed to modifyError? Naming is really the only thing keeping it unmerged.

[akrmn]

@emilypi @chessai thanks for the suggestion, I'll rename it to modifyError.

[emilypi]

Needs a resolution and a changelog entry and we're good

[akrmn]

hey @emilypi thanks for taking this forward

[akrmn]

I've just added the changelog entry, sorry I forgot to do it earlier

[emilypi]

Thanks for bearing with us @akrmn and thanks for the PR :)