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Logs: liberachat/#haskell

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2021-08-08 15:12:47 <fgaz> tomsmeding dsal: thanks!
2021-08-08 15:13:17 <tomsmeding> it can get fairly large :p
2021-08-08 15:13:43 <Cajun> lechner: given the type signature, both are `[String]` . and by convention, anything that ends with _ disregards results and returns `()` (at least as far as im aware)
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2021-08-08 15:15:46 <lechner> Cajun: thanks! i was actually confused by the statement "each call to putStrLn returns a unit" but my question did not reflect that very well. What is a unit, please?
2021-08-08 15:16:00 <tromp> thx, @lechner. changed to "game of Go"
2021-08-08 15:16:06 <Cajun> "unit" means `()`
2021-08-08 15:16:37 <lechner> Cajun: thank you!
2021-08-08 15:16:38 <Cajun> idk how to use lambda bot but `:t ()` === `() :: ()`
2021-08-08 15:17:03 <tomsmeding> :t ()
2021-08-08 15:17:04 <lambdabot> ()
2021-08-08 15:17:08 <Cajun> and you can see this in mapM_'s type
2021-08-08 15:17:11 <Cajun> :t mapM_
2021-08-08 15:17:12 <lambdabot> (Foldable t, Monad m) => (a -> m b) -> t a -> m ()
2021-08-08 15:17:28 <Cajun> well isnt that simple... :P
2021-08-08 15:17:54 <tomsmeding> if confused by the Foldable, imagine t = [] so that mapM_ :: Monad m => (a -> m b) -> [a] -> m ()
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2021-08-08 15:19:02 <Cajun> and in this instance the monad is IO, so you can replace the m with IO for `(a -> IO b) -> [a] -> IO ()` or even more specifically `(String -> IO b) -> [String] -> IO ()`
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2021-08-08 15:19:27 <lechner> i was confused that anyone would use mapM (without the underscore) but then also do not understand how a list of units is different from a plain unit
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2021-08-08 15:20:01 <lechner> it's just about the signature!
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2021-08-08 15:20:13 <Cajun> sometimes you just want a list of monadic things *shrug*
2021-08-08 15:20:25 <geekosaur> you don't normally use mapM when you're getting a list of units. but consider something that returns IO String
2021-08-08 15:20:36 <lechner> right
2021-08-08 15:21:00 <Cajun> but yeah `mapM` and `mapM_` function identically except with return values
2021-08-08 15:21:01 <tomsmeding> with an imaginary 'performRequest :: Request -> IO Result', you might have 'mapM performRequests :: [Request] -> IO [Result]'
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2021-08-08 15:23:00 <lechner> so i'm making baby steps here. is there a way to restyle mapM_ putStrLn x into something involving the >>= putStrLn operator?
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2021-08-08 15:23:50 <lechner> or is that a bogus thought
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2021-08-08 15:24:39 <Cajun> yeah, heres a small example `foo = getLine >>= \line -> putStrLn line` although this can be more optimized
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2021-08-08 15:25:40 <Cajun> well hlint doesnt yell at me for the code so its fine lol
2021-08-08 15:25:49 <tomsmeding> lechner: if you have 'mapM_ putStrLn x', then x is a list of strings. So somehow, you're going to have to loop over the list x
2021-08-08 15:26:01 <tomsmeding> mapM_ does this for you; you can also do it manually
2021-08-08 15:26:49 <tomsmeding> :t let loop [] = return () ; loop (x:xs) = putStrLn x >> loop xs in loop
2021-08-08 15:26:50 <lambdabot> [String] -> IO ()
2021-08-08 15:27:31 <tomsmeding> which you can express using standard functions like foldr in a number of ways :)
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2021-08-08 15:28:00 <tomsmeding> (or traverse, which happens to be the same as mapM)
2021-08-08 15:28:42 <lechner> i see. thanks!
2021-08-08 15:28:46 <Cajun> i was thinking of doing something with traverse until i realized `mapM` already puts the monad on the outside of the list :p
2021-08-08 15:29:26 <tomsmeding> traverse = mapM
2021-08-08 15:29:49 <Cajun> oh yeah.. i guess it does wind up like that huh
2021-08-08 15:30:02 <tomsmeding> well, with different typeclass constraints, and technically traverse is implemented using the Applicative combinators whereas mapM uses the monad methods
2021-08-08 15:30:13 <tomsmeding> but if your instances are lawful, that should be exactly the same :)
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2021-08-08 15:30:46 <Cajun> "but if your instance are lawful" that sounds like it could be a headache and a half lol
2021-08-08 15:30:49 <tomsmeding> also fmap = liftM
2021-08-08 15:31:07 <Lycurgus> is there a quicklaw?
2021-08-08 15:31:34 <tomsmeding> Lycurgus: https://hackage.haskell.org/package/tasty-quickcheck-laws
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2021-08-08 15:31:44 <Lycurgus> tomsmeding, ty!
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2021-08-08 16:15:32 <dsal> Oh interesting. I was using checkers for that.
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2021-08-08 16:17:38 <dsal> testApplicativeLaws :: (Applicative f, Eq a, Eq b, Eq c, Show a, Show t, Show (f a), Show (f (a -> b)), Show (f (b -> c)), Arbitrary a, Arbitrary b, Arbitrary t, Arbitrary (f a), Arbitrary (f (a -> b)), Arbitrary (f (b -> c)), CoArbitrary a, Typeable f, Typeable a, Typeable b, Typeable c) => Proxy f -> Proxy t -> Proxy a -> Proxy b -> Proxy c -> (forall u. Eq u => t -> f u -> f u -> Bool) -> TestTree
2021-08-08 16:17:45 <dsal> Well that doesn't look significantly easier to use.
2021-08-08 16:18:06 <dsal> (not that I ever remember how to use checkers)
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