Added monadErrorMaybe instance

[pseudonom]

If you all think the instance makes sense.

[garyb]

👍

[paf31]

It's not law-abiding though, because of the catch implementation. Maybe restrict e ~ Unit?

[garyb]

Good point, I didn't think of that.

[pseudonom]

not law-abiding

Yeah, it is sort of unusual behavior (though I don't see a statement of the laws anywhere). I do find it quite useful though (for combining with functions that return a Maybe or "choosing verbosity"):

big ::  forall m a. (MonadError String m, Monad m) => Number -> m Number
big n | n < 0 = throwError "Negative"
         | n < 2 = throwError "Small"
         | otherwise = pure n

bigHead :: [Number] -> Maybe Number
bigHead = big <=< head

I guess the alternative is sprinkling something like right :: Either a b -> Maybe b around.

[jdegoes]

Yeah, the Maybe instance must be restricted to Unit if it exists at all.

[paf31]

I'm happy to merge this if we can change over to Unit.

[pseudonom]

Done. @paf31

[paf31]

Sorry, we need to fix catchError too.

[pseudonom]

Oh yeah.

[paf31]

Any chance you could rebase this please?

[pseudonom]

Done.

[paf31]

👍 Thanks!