Re: [Haskell-cafe] Monad of no `return` Proposal (MRP): Moving `return` out of `Monad`

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 - -- I am going to do some logging, yay! data Logs a = Logs a [a] deriving (Eq, Show) - -- one log message singlelog :: a -> Logs a singlelog a = Logs a [] - -- two log messages twologs :: a -> a -> Logs a twologs a1 a2 = Logs a1 [a2] class Semigroup a where (<>) :: a -> a -> a - -- I can append logs instance Semigroup (Logs a) where Logs h1 t1 <> Logs h2 t2 = Logs h1 (t1 ++ h2 : t2) - -- I can map on Logs instance Functor Logs where fmap f (Logs h t) = Logs (f h) (fmap f t) - -- I will collect logs with a value data WriteLogs l a = WriteLogs (Logs l) a deriving (Eq, Show) - -- I can map the pair of logs and a value instance Functor (WriteLogs l) where fmap f (WriteLogs l a) = WriteLogs l (f a) singlewritelog :: l -> a -> WriteLogs l a singlewritelog l a = WriteLogs (singlelog l) a - -- Monad without return class Bind f where (-<<) :: (a -> f b) -> f a -> f b - -- Can I Applicativate WriteLogs? Let's see. instance Applicative (WriteLogs l) where -- Well that was easy. WriteLogs l1 f <*> WriteLogs l2 a = WriteLogs (l1 <> l2) (f a) pure a = WriteLogs (error "wait, what goes here?") a -- Oh I guess I cannot Applicativate WriteLogs, but I can Apply them! - -- Well there goes that idea. - -- instance Monad (WriteLogs l) where - -- Wait a minute, can I bind WriteLogs? instance Bind (WriteLogs l) where -- Of course I can! f -<< WriteLogs l1 a = let WriteLogs l2 b = f a in WriteLogs (l1 <> l2) b - -- OK here goes ... myprogram :: WriteLogs String Int myprogram = -- No instance for (Monad (WriteLogs String)) -- RAR!, why does do-notation require extraneous constraints?! -- Oh that's right, Haskell is broken. -- Oh well, I guess I need to leave Prelude turned off and rewrite the base libraries. do a <- singlewritelog "message" 18 b <- WriteLogs (twologs "hi" "bye") 73 WriteLogs (singlelog "exit") (a * b) - -- One day, one day soon, I can move on. On 06/10/15 11:20, amindfv@gmail.com wrote:

IMO, the "tech debt" you're talking about feels very small. We've already made the change that return = pure by default. The historical baggage that this proposal cleans up is just the fact that legacy code is able to define its own "return" without breaking (which must be the same as the definition of pure anyway). I am also moving from +0.5 to +0 on this.

Tom

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El 5 oct 2015, a las 18:29, Alexander Berntsen escribió:

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On 05/10/15 20:50, Nathan Bouscal wrote: There have been a lot of objections based on the idea that learners will consult books that are out of date, but the number of learners affected by this is dwarfed by the number of learners who will use updated materials and be confused by this strange historical artifact. Permanently-enshrined historical artifacts accrete forever and cause linear confusion, whereas outdated materials are inevitably replaced such that the amount of confusion remains constant. Thank you for making this point

I would be very saddened if the appeal to history (i.e. technical debt) would halt our momentum. That's what happens to most things both in and out of computer science. And it's honestly depressing.

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