April 2017 - Beginners

Multiline code in GHCi
by Shishir Srivastava 29 Aug '17

29 Aug '17

Hi, I am trying to run this basic multiline code in GHCi. I am not sure if this is even possible without writing it in a file and compiling it in a traditional way. ------- Prelude> :set +m Prelude> let main1 = do Prelude| a <- readLn Prelude| return a Prelude| Prelude> Prelude> main1 <interactive>:75:1: No instance for (Show (IO b0)) arising from a use of `print' In the first argument of `print', namely `it' In a stmt of an interactive GHCi command: print it ------- Could someone please confirm if the function can be defined and called like this in GHCi or point to where am going wrong. Thanks, Shishir Srivastava

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Foldable for (,)
by Jonathon Delgado 03 May '17

03 May '17

I've seen many threads, including the one going on now, about why we need to have: length (2,3) = 1 product (2,3) = 3 sum (2,3) = 3 or (True,False) = False but the justifications all go over my head. Is there a beginner-friendly explanation for why such seemingly unintuitive operations should be allowed by default?

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Cafe first, then beginners
by Kim-Ee Yeoh 25 Apr '17

25 Apr '17

Dear haskeller, Please consider writing to the haskell-cafe mailing list with your questions. Should you get an unsatisfactory response there, then retry your queries here on haskell-beginners. Here's why: Haskell-cafe has about 10x the number of eyeballs. Everyone here in haskell-beginners also reads haskell-cafe. So you'll reach the same folks here but many, many more there. This is especially helpful to you if your questions don't just deal with LYAH-level language fundamentals but overlap specialized domains like music, web dev, and infrastructure issues like OS, cabal, stack, etc. Haskell-beginners started way back when there was much traffic coinciding with an overflow of basic questions on cafe. Some folks, including old-timers, felt overwhelmed. There was a sense that the latter convos should be separated out. But seasons change, and the cafe has dried up, mostly in volume but thankfully not in subscriber count. You will do your questions great justice if you air them in a forum capable of rich and varied responses. Best, -- Kim-Ee -- -- Kim-Ee

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Re: [Haskell-beginners] Foldable for (,)
by Jonathon Delgado 24 Apr '17

24 Apr '17

Tony Morris - please could you give a (practical) example of code where the a tuple could realistically be passed to length, but you don't know what the answer will be at compile time? Michael Orlitzky - everything in .NET has to descend from Object because of it's OO design. Why does tuple have to implement Foldable if it doesn't provide any useful functions? Thank you very much everyone in this thread for helping me understand!

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Re: [Haskell-beginners] Foldable for (,)
by Jonathon Delgado 23 Apr '17

23 Apr '17

If a tuple only has one value, why do functions for operating over sets make sense at all? I can see from your explanations why the answers could be considered correct (if a particular convention is assumed), but why does the operation make sense at all? It seems like we're asking for the length of a single value, its product, etc. Francesco Ariis wrote: > I've seen many threads, including the one going on now, about why we need > to have: > > length (2,3) = 1 > product (2,3) = 3 > sum (2,3) = 3 > or (True,False) = False > > but the justifications all go over my head. Is there a beginner-friendly > explanation for why such seemingly unintuitive operations should be > allowed by default? Hello Jonathon, the proponents of `Foldable (a,)` see `(2,3)` not as a pair of 'equal' values, but as a value *and* an annotation, much like some other folks see Either as having a value (Right a) *or* an annotation (usually an error in the form of Left e). So to go back to your examples: (2,3) ^ ^ | +------------- I am the value | +--------------- I am an annotation (and since tuples arguments can be heterogeneous, I could be a String, a Bool, anything). If you agree with this paradigm, `length`, `sum` and friend become a bit less icky. I would prefer tuples to be unbiased, but this intuition helped me connect with the people on the other side of the line. Does this help?

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Re: [Haskell-beginners] Foldable for (,)
by Doug McIlroy 23 Apr '17

23 Apr '17

> So to go back to your examples: > > (2,3) > ^ ^ > | +------------- I am the value > | > +--------------- I am an annotation (and since tuples arguments can > be heterogeneous, I could be a String, a Bool, > anything). > > If you agree with this paradigm, `length`, `sum` and friend become a > bit less icky. To me, "annotation" connotes auxiliary information--yet it will be the primary key when pairs are sorted. I would be more comfortable with the explanation if the field were referred to as an "identifier", or some similar term. Doug

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Francesco Ariis
by Jonathon Delgado 23 Apr '17

23 Apr '17

So then the question becomes, why do tuples need Foldable if the functions it defines aren't useful?

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Re: [Haskell-beginners] Foldable for (,)
by Jonathon Delgado 23 Apr '17

23 Apr '17

So then the question becomes, why do tuples need Foldable if the functions it defines aren't useful?

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Traversable Instance of the Constant type - Think I got it
by Atrudyjane 21 Apr '17

21 Apr '17

Got confused thinking that the pure function called by Constant's traversable instance was the pure defined in Constant's applicative, which just returns Constant. Was explained to me that the type is inferred by the 'f' passed to it. If you read my OP, thank you. Regards, Andrea Sent with [ProtonMail](https://protonmail.com) Secure Email.

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Traversable Instance of the Constant type
by Atrudyjane 20 Apr '17

20 Apr '17

Pretty sure I'm getting bogged down on something simple, but how does the traverse function actually embed the Constant type in the f context passed to it? Or is it the pure defined in the f context that's being called? Thank you, Andrea newtype Constant a b = Constant { getConstant :: a } deriving (Eq, Show) instance Functor (Constant a) where fmap f (Constant a) = Constant a instance Monoid a => Applicative (Constant a) where pure a = Constant { getConstant = mempty } Constant a <*> Constant b = Constant { getConstant = a `mappend` b } instance Foldable (Constant a) where foldMap f (Constant a) = mempty instance Traversable (Constant a) where traverse f (Constant a) = pure $ Constant a λ> traverse (\x -> Just x) (Constant 1) Just (Constant {getConstant = 1}) Sent with [ProtonMail](https://protonmail.com) Secure Email.

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