Can we have an eitherReader convenience function?

[nh2]

For parsing simple arguments, I often define functions of type parseFoo :: String -> Either String a, which are used in multiple places, and then I want to use them with reader.

Could we have a convenience function like this?

eitherReader str2either = reader (either (Left . ErrorMsg) Right . str2either)

This would not be necessary if the fail instance of Either String was not an exception, but it is and fail isn't a realy thing in Monad either (no pun intended).

Thanks :)

[pcapriotti]

Yes, that would be a useful addition. Can you submit a patch?

[nh2]

I just realized that my implementation doesn't work.

Weirdly enough, it does work on a successful parse; if the parse fails, however, and returns Left, then my program just exits with status one instead of printing the error message.

This implementation, using fail from the Monad instance, works as expected in the case of failure:

eitherReader str2either = reader (either fail return . str2either)

It's very unclear to me what's going on here, I believe the two should really do the same.

(This is like in #47, but this time, it's on the 0.5.2.1, not on master.)

[pcapriotti]

I'll look into it, thanks for the report.

[nh2]

I think it comes from the

(Just r, ErrorMsg _) -> return r

here (at least the error message I expect is contained in that.

[nh2]

Thanks!