"Bryan O'Sullivan"
On Wed, Dec 5, 2012 at 8:16 AM, Henning Thielemann < lemming@henning-thielemann.de> wrote:
The same can be asked for
Prelude> :t either (const False) (const True) either (const False) (const True) :: Either a b -> Bool
or
isLeft = isJust . maybeLeft
We at least have evidence that isLeft and isRight are actually present in several packages. That helps this proposal squeak over my bar, so I'm +0 on it.
I think that maybeLeft and maybeRight are interesting, but the burden of proof for "these things solve a problem that actually exists" is a little higher. Consider me -0.01.
In general, I feel we have a fondness for a few too many near-trivial one-liners that make libraries bigger and harder to navigate, without really adding much expressivity.
fair enough, here's some motivation why I think that maybe{Left,Right} might be worth to be added (and to some degree maybe even their co/dual counterparts): - maybe{Left,Right} compose points-free conveniently with existing Data.Maybe primitives, e.g. - lefts = mapMaybe maybeLeft - isLeft = isJust . maybeLeft - fromLeft' default = fromMaybe default . maybeLeft - fromRight' default = fromMaybe default . maybeRight - maybeRight is used in at least two packages (I didn't have time to search for more, but if it makes a difference, I'll search Hackage for more use cases): - precis:Precis.Utils.ControlOperators.suppress - errors:Control.Error.Util.hush and also their dual (see fromRight' above) is defined there: - precis:Precis.Utils.ControlOperators.elaborate - errors:Control.Error.Util.note - from{Left,Right} are useful when working in the 'Maybe' monad or applicative functor for converting 'Either'-typed values (this also applies to the dual case of working in the 'Either' monad/app-functor and having to deal with 'Maybe' values) - IMHO, 'maybeRight' has better readability than inlining 'either (const Nothing) Just' - for me it's not so much about typing less, as more about having code that is easier to read out loud. - I often use the 'when'-like combinator 'whenJust' in monadic code: whenJust :: Monad m => Maybe a -> (a -> m ()) -> m () whenJust (Just x) a = a x whenJust _ _ = return () or simply defined as a type specialized 'forM_': whenJust = Data.Foldable.forM_ then the argument about composability with 'Maybe' stated above applies (imho): do result <- try $ foobar whenJust (maybeLeft result) $ \e -> do putStrLn $ "warning: got error during shutdown:" putStrLn $ " " ++ e return () this way I can avoid having to use a 'case of' expression with an explicit '_ -> return ()' branch for which I always struggle a bit how to indent it: do result <- try $ foobar case result of Left e -> do putStrLn $ "warning: got error during shutdown: " putStrLn $ " " ++ e _ -> return () return () - As the types in Data.Either and Data.Maybe are part of the Haskell standard library, and IMHO basic primitives such as maybe{Left,Right} should be located in those modules as well. cheers, hvr