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2021-07-19 08:48:19 <guest719> why `traverse` would immediatly exit when it meet a Left a?
2021-07-19 08:48:51 <guest719> traverse (\i -> if odd i then Right () else Left i) [1..10]
2021-07-19 08:49:42 <lars8> > traverse (\i -> if odd i then Right () else Left i) [1..10]
2021-07-19 08:49:44 <lambdabot> Left 2
2021-07-19 08:51:41 <lars8> > sequence [Left 1, Right 2]
2021-07-19 08:51:42 <lambdabot> Left 1
2021-07-19 08:51:46 <lars8> @src sequence
2021-07-19 08:51:46 <lambdabot> sequence [] = return []
2021-07-19 08:51:46 <lambdabot> sequence (x:xs) = do v <- x; vs <- sequence xs; return (v:vs)
2021-07-19 08:51:46 <lambdabot> --OR
2021-07-19 08:51:46 <lambdabot> sequence xs = foldr (liftM2 (:)) (return []) xs
2021-07-19 08:52:00 <lars8> does that help?
2021-07-19 08:52:50 <lars8> traverse f is basically sequence . map f
2021-07-19 08:53:22 <guest719> but why it meet Left a will early exit?
2021-07-19 08:53:48 <lars8> because that's what liftM2 does
2021-07-19 08:54:04 <guest719> @src liftM2
2021-07-19 08:54:04 <lambdabot> liftM2 f m1 m2 = do
2021-07-19 08:54:04 <lambdabot> x1 <- m1
2021-07-19 08:54:05 <lambdabot> x2 <- m2
2021-07-19 08:54:05 <lambdabot> return (f x1 x2)
2021-07-19 08:54:23 <lars8> > liftM2 (+) (Right 1) (Right 2)
2021-07-19 08:54:25 <lambdabot> Right 3
2021-07-19 08:54:35 <lars8> > liftM2 (+) (Right 1) (Left 2)
2021-07-19 08:54:36 <lambdabot> Left 2
2021-07-19 08:54:48 <dminuoso> guest719: `Either a`, like `Maybe`, models computations with exceptions.
2021-07-19 08:55:07 <dminuoso> So a `Left err` or `Nothing` has the same semantics as an exception, it shortcircuits the entire computation
2021-07-19 08:55:12 <lars8> if any argument is Left, entire computation is Left
2021-07-19 08:55:17 <guest719> oh, Left 2 will break the computation chain in >>=
2021-07-19 08:55:21 <dminuoso> Yes.
2021-07-19 08:55:52 <guest719> dminuoso and why it's that? define in >>= ?
2021-07-19 08:55:55 <dminuoso> guest719: Yes.
2021-07-19 08:56:26 <dminuoso> The applicative and monadic interface of `Maybe` and `Either s` simply models exceptions.
2021-07-19 08:56:27 <guest719> dminuoso except Left a and Nothing, is there other would break computation chain?
2021-07-19 08:56:30 <dminuoso> There's no deeper reason than that.
2021-07-19 08:56:37 → Erutuon joins (~Erutuon@user/erutuon)
2021-07-19 08:56:39 <dminuoso> Sure there are others
2021-07-19 08:56:46 <guest719> for examples?
2021-07-19 08:57:06 <dminuoso> ExceptT
2021-07-19 08:57:19 <dminuoso> Though that's just an `Either s` in disguise
2021-07-19 08:57:36 <guest719> I can't help when you said Exception, I always think about run-time Exceptions
2021-07-19 08:57:48 <dminuoso> Yes, these are pretty much like runtime exceptions!
2021-07-19 08:57:51 <dminuoso> Consider:
2021-07-19 08:57:53 <guest719> why haskell would give it another name
2021-07-19 08:58:02 <dminuoso> These are value-level/user-defined exceptions
2021-07-19 08:58:02 <guest719> wouldn't
2021-07-19 08:58:38 <dminuoso> guest719: We also have regular RTS exceptions as you know them inside IO.
2021-07-19 08:58:48 <dminuoso> But in pure computations it might still be useful to have shortcircuiting semantics
2021-07-19 08:58:57 <dminuoso> Like with traverse.
2021-07-19 08:59:18 <dminuoso> Say you have a tree, and you want to process each node - but in a way that if you generate an error at any, that the entire computation is considered failed.
2021-07-19 08:59:46 <dminuoso> Then you can use `Maybe` or `Either s` (depending on whether you want to keep information about the error condition)
2021-07-19 09:00:06 <dminuoso> Then producing `Nothing` is semantically equivalent to throwing an exception, except we can do this in pure code and without any special support from the runtime system.
2021-07-19 09:00:20 <dminuoso> data Maybe a = Nothin | Just a
2021-07-19 09:00:25 <dminuoso> instance Monad Maybe where ...
2021-07-19 09:00:42 <dminuoso> Voila! You have created exception semantics on your own, no exception primitive support needed in the language.
2021-07-19 09:01:23 × Erutuon quits (~Erutuon@user/erutuon) (Ping timeout: 265 seconds)
2021-07-19 09:02:48 × lavaman quits (~lavaman@98.38.249.169) (Ping timeout: 252 seconds)
2021-07-19 09:03:00 <albet70> can we think it as a failure computation?
2021-07-19 09:03:37 <albet70> what about 1/0?
2021-07-19 09:03:47 <albet70> 1/0 is an Exception?
2021-07-19 09:04:05 <albet70> IO failure is an Exception?
2021-07-19 09:05:09 <albet70> fmap (+1) Nothing would be an Exception?
2021-07-19 09:05:30 <dminuoso> `Nothing` acts as an exception, yes.
2021-07-19 09:05:55 <dminuoso> 1/0 is an interesting subject for several reasons
2021-07-19 09:06:46 <dminuoso> % :t (1/0)
2021-07-19 09:06:46 <yahb> dminuoso: ; <interactive>:1:3: error: Variable not in scope: (/) :: t0 -> t1 -> t
2021-07-19 09:06:59 <dminuoso> % :q
2021-07-19 09:06:59 <yahb> dminuoso:
2021-07-19 09:07:00 <dminuoso> % :t (1/0)
2021-07-19 09:07:01 <yahb> dminuoso: Fractional a => a
2021-07-19 09:07:06 <dminuoso> % :i Fractional
2021-07-19 09:07:08 <yahb> dminuoso: type Fractional :: * -> Constraint; class Num a => Fractional a where; (/) :: a -> a -> a; recip :: a -> a; fromRational :: Rational -> a; {-# MINIMAL fromRational, (recip | (/)) #-}; -- Defined in `GHC.Real'; instance Fractional a => Fractional (Identity a) -- Defined in `Data.Functor.Identity'; instance forall a k (b :: k). Fractional a => Fractional (Const a b) -- Defined in `Data.Functor.Co
2021-07-19 09:07:19 <dminuoso> % :t (/)
2021-07-19 09:07:20 <yahb> dminuoso: Fractional a => a -> a -> a
2021-07-19 09:07:34 <dminuoso> albet70: ^- so you see, this is either Float or Double, at which point IEEE 754 semantics apply.
2021-07-19 09:08:05 <dminuoso> Under which 1/0 is defined to be either positive or negative infinity (depending on the sign of 0)
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