May 2018 - Beginners - Haskell.org

Adding 1 to Just 9
by Olumide 14 May '18

14 May '18

Dear List, Chapter 14 of LYH appears to suggest that a Just value can be added to an Int. Quote from http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions "For instance, say we have Just 9 and the function \x -> Just (x+1). If we map this function over Just 9, we're left with Just (Just 10)." I've tried the following in ghci but got the error: <interactive>:12:1: error: • Non type-variable argument in the constraint: Num (Maybe a) (Use FlexibleContexts to permit this) • When checking the inferred type it :: forall a. (Num (Maybe a), Num a) => Maybe a Am I reading the quote wrong? Is Just (Just 10) a hypothetical? Regards, - Olumide

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Re: [Haskell-beginners] EasyPlot on Windows
by Gregory Guthrie 11 May '18

11 May '18

Thanks; good idea - but nope. I'd file a report to EasyPlot, but not sure where to do that. Older discussions on this issue note that there is a "persist" option for gnuplot, but I didn't yet see how to pass that to try it. -----Original Message----- My first hunch was that, since the program ends right after plotting, it cleans up the plot window and exits. But trying it on an X11 system, I got a persistent window despite the program exiting. That may be a difference between Unix and Windows child process handling; I don't have a Windows machine handy right now to test, but you could try this: main = do plot Windows $ Data2D [Title "Demo Title"] [] tData _ <- getLine return () If that doesn't fix it, my only guess is there's either a difference between the Unix and Windows versions of gnuplot, or a bug in easyplot's Windows handling.

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Graph path
by mike h 11 May '18

11 May '18

Does anyone know of a good description of the algorithm to visit (if possible) each node in a graph? I can find a few links that describe the imperative way of doing it but I struggle converting to functional style :) Many Thanks Mike

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EasyPlot on Windows
by Gregory Guthrie 10 May '18

10 May '18

A very simple test program with Graphics.EasyPlot, flashes up a window, but too fast to see it. There are several old posts on this issue with gnuplot, but I haven't seen any current solutions. Example: import Graphics.EasyPlot tData = [ (1,2), (2,3), (3,1), (4,5), (5,3), (6,1) ] main = do plot Windows $ Data2D [Title "Demo Title"] [] tData

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Join'ing a State Monad, example from LYH
by Olumide 09 May '18

09 May '18

Dear List, I've been stumped for a few months on the following example, from chapter 13 of LYH http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions runState (join (State $ \s -> (push 10,1:2:s))) [0,0,0] I find the following implementation of join in the text is hard to understand or apply join :: (Monad m) => m (m a) -> m a join mm = do m <- mm m In contrast, I find the following definition(?) on Haskell Wikibooks https://en.wikibooks.org/wiki/Haskell/Category_theory#Monads join :: Monad m => m (m a) -> m a join x = x >>= id easier to understand, and although I can apply it to the following Writer Monad example, in the same section of LYH, runWriter $ join (Writer (Writer (1,"aaa"),"bbb")) I cannot apply it to the State Monad example. Regards, - Olumide

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