March 2021 - Beginners

Problem with Graphics.SOE
by Vitaly Dolgov 30 Mar '21

30 Mar '21

Hi, everybody! I'm trying to do an example from the book "The Haskell School of Expression", which originally uses `SOEGraphics`, I use `Graphics.SOE` instead, but I get the following error on each run: `user error (loadQueryFont)` I have no idea how to approach the problem... `:trace` outputs an empty exception :( Could you please help me to solve this? My environment is: Linux, ghc 8.10.4, HGL 3.2.3.2 The code is quite basic: ``` import Graphics.SOE main = runGraphics $ do w <- openWindow "Hello World!" (300, 300) drawInWindow w (text (100, 200) "Hello World!") k <- getKey w closeWindow w ``` Thank you, Vitaly

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How to include {-# LANGUAGE LambdaCase #-} at the ghci prompt
by Galaxy Being 28 Mar '21

28 Mar '21

I've got this code I've entered into the ghci by hand between :{ and :} :{ {-# LANGUAGE LambdaCase #-} data List a = Empty | Cons a (List a) deriving (Eq, Ord, Show) infixr 5 `Cons` subst_c :: (a -> Bool) -> (a, MyList a) -> MyList a subst_c pred = \ case (_, Empty) -> Empty (n, Cons e t) | pred e -> Cons n $ subst_c pred (n, t) | otherwise -> Cons e $ subst_c pred (n, t) :} but I keep getting the error Illegal lambda-case (use -XLambdaCase) This works fine inside an .hs file and :load and run at the prompt. However, I'm using Emacs org-mode and need to keep everything limited to a version of hand-entered :{ ... :}. Any suggestions? LB

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Closure: exact wording
by Galaxy Being 27 Mar '21

27 Mar '21

I've got this addTwo x y = x + y aT5 = addTwo 5 > aT5 3 8 Is it right to say aT5 is a closure over 5, or a closure over addTwo 5? LB

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Product/tuple type vs. ...
by Galaxy Being 17 Mar '21

17 Mar '21

Reading this <https://www.cefns.nau.edu/~edo/Classes/CS396_WWW/Misc_docs/Haskell.html> I'm a bit confused but think I understand what is meant by a *product *or *tuple *type constructor such as data Point a b = Pt a b Because Point has no "logical or," (|) of more than one possible value set such as data Color = Red | Green | Blue it is automatically a tuple type? Also, it has no possible "consing" -- does that play a role? data Things a = T1 a (Things a) | T2 a (Things a) | LastT a Things is also parameterized just like Point, but, again, Things is not a tuple type for the reasons above, e.g., just having one parameterized constructor means automatically a tuple type, right? LB

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Type * and * -> *
by Galaxy Being 17 Mar '21

17 Mar '21

I found this interesting page <https://wiki.haskell.org/Peano_numbers> at Wiki Haskell. Confusing, however, is how it first establishes data Peano = Zero | Succ Peano It says Here Zero and Succ are values (constructors). Zero has type Peano, and Succ has type Peano -> Peano. but then it breaks down each member further a few lines later data Zero data Succ a and then says Zero has kind *, and Succ has kind * -> *. The natural numbers are represented by types (of kind *) Zero, Succ Zero, Succ (Succ Zero) etc. Why is it giving two separate treatments and what is meant by the * and * -> * ? There's something fundamental I'm missing. If anyone knows of a really thorough and definitive *and *understandable treatment of Haskell types, I'd appreciate it. LB

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Recursion with a self-defined type
by Galaxy Being 13 Mar '21

13 Mar '21

I'm trying to translate *The Little MLer *into Haskell. I've got this data Shishkebab = Skewer | Onion Shishkebab | Lamb Shishkebab | Tomato Shishkebab deriving Show Then I have this which works veggieKebab :: Shishkebab -> Bool veggieKebab Skewer = True veggieKebab (Onion (shk)) = veggieKebab shk veggieKebab (Tomato (shk)) = veggieKebab shk veggieKebab (Lamb (shk)) = False > veggieKebab (Tomato (Onion (Tomato (Onion Skewer)))) True but I'm wondering if I could do something like this veggieKebab :: Shishkebab -> Bool veggieKebab Skewer = True veggieKebab (shkb (sk)) | (shkb == Onion) || (shkb == Tomato) = veggieKebab sk | otherwise = False This doesn't work, giving a "Parse error in pattern: shkb". I've been advised that I'm trying to treat what is a data constructor like a variable, but I can't fathom what that means in this case. What I'm trying to leverage is what I've learned from dealing with lists and recursion through the consed list. So if effect I'm trying to recurse through a consed Shishkebab object. It works in the first case, but hyow could I do this in this more generic way like the second try does? LB

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Maybe problems converting back to number
by Galaxy Being 07 Mar '21

07 Mar '21

I've got this example from the Internet import Data.List import Data.Maybe firstFactorOf x | m == Nothing = x | otherwise = fromJust m where m =(find p [2..x-1]) p y = mod x y == 0 and this as a crude return the nth element of a list import Data.List import Data.Maybe -- myIndex :: [a] -> Int -> Maybe a myIndex [] _ = Nothing myIndex (x:xs) 0 = Just x myIndex (x:xs) n = myIndex xs (n-1) I would like the Just x in the second block to actually be fromJust x as in the first block, i.e., I want a number returned, not a Just typed object. I've tried changing Just x to fromJust x but get the error when I try to use it > myIndex [1,2,3,4,5] 3 * Non type-variable argument : in the constraint: Num (Maybe (Maybe a)) : (Use FlexibleContexts to permit this) : * When checking the inferred type : it :: forall a. Num (Maybe (Maybe a)) => Maybe a What am I missing here? Also, my type declaration seems to be wrong too, but I don't see why. LB

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Alphanumerical input
by A. Mc. 06 Mar '21

06 Mar '21

Hello, I was wondering what the best way was for tranforming a list of alphanumerical characters from main IO() such as: Enter String: A 101 E 182 and tranforming it into an Int list of [0, 101, 4, 182] Which converting A and E to 0 and 4 is easy enough with toEnum/fromEnum and simple subtraction, and using read can convert to a [Char], but recognizing 101 as a single Int from a Char string in main, and getting both converted together as part of the same function, I'm a bit less sure of without resorting to much lower level imperative methods. Thanks in advance and thank you for your time.

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Creating a Triple List from a List
by A. Mc. 03 Mar '21

03 Mar '21

Hello, What is the best way to take: [1, 2, 3, 4 ] And convert it to: [ [ [ 1 ], [ 2 ] ], [ [3]. [4] ] ] so that each member pair is: [ [1], [2] ] roughly analogous to a 1x2 vector? Thanks in advance and thank you for your time.

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