February 2021 - Beginners

Creating a Triple List from a List
by A. Mc. 02 Mar '21

02 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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Bounded number types?
by stuebinm 27 Feb '21

27 Feb '21

Hi all, I'm wondering: is there any type that represents e.g. a floating point value that is guaranteed to be within some interval (e.g. [0,1]?). My practical use-case would be that I'm reading in input data from json, which may be ill-behaved — obviously I could just manually check, and then keep track of which numbers in which record fields are within which intervals, but coming from less strongly typed programming languages I wonder if there would be a "typed" way to do this, too.

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Closure clear-up?
by Galaxy Being 26 Feb '21

26 Feb '21

>From my moving-target understanding of closures, I'm guessing this has a closure let succ = add 1 add x = xadder where xadder y = x + y in succ 3 Now, can someone explain in words how the closure system is at work here? I see that the add 1 is being "baked in". Do we say this is a closure on add 1? (Closure wording is confusing.) The enclosing scope of add contains 1; however, xadder is what succ ultimately becomes, which has 1 in its scope in that when xadder is defined, the x argument will be the 1 of add 1. Or am I missing something? Researching closures vis-a-vis Haskell, I've seen the argument that, - no, Haskell doesn't have closures, - yes, Haskell is lazy, *everything* can be seen as a closure, as even a value can be seen as a function without argument waiting to be evaluated (and so capturing its environment until it gets evaluated). This was taken from an older Haskell textbook (*Introduction to Functional Programming Systems Using Haskell *by Davie). LB

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Non-exhaustive patterns in function error
by Galaxy Being 26 Feb '21

26 Feb '21

I have this intersect1 :: ([a],[a]) -> [a] intersect1 (s,[]) = [] interesct1 (s,t:ts) | elem t s = t : intersect1 (s,ts) | otherwise = intersect1 (s,ts) and when I try this intersect1 ([1,2],[1,2,3]) I get the error Non-exhaustive patterns in function intersect1 Not sure what's wrong with this. LB

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Using Fractional Type
by A. Mc. 21 Feb '21

21 Feb '21

Hello, I need to create a function that does: mod (recip x) y. However, I am getting all kinds of errors with the type signature. Is there a better way to use Fractional type than needing to enable FlexibleContexts? What do I need to do to still make use of Haskell's type system, but also still be able to perform operations such as reciprocal (and division into fractions for that matter). Thanks in advance and thank you for your time.

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import question
by Galaxy Being 18 Feb '21

18 Feb '21

I'm trying to use Decimal but (I'm guessing) I don't know how to import it. import Data.Decimal dList :: [Decimal] dList = [1.1,1.2..2.0] : <interactive>:70:11-17: error: : Not in scope: type constructor or class `Decimal' Obviously I was asleep the day the teacher told us how to do this. . . . LB

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Add Global Package
by A. Mc. 17 Feb '21

17 Feb '21

Hello, I'm attempting to install and update the .yaml file so I don't have to run the command $ stack ghci --package random every single time I want to load a module that imports System.Random. Or maybe I need to use chocolatey? I don't know, but being able to use System.Random like every other package would be greatly appreciated. Right now, I'm not sure what command I need to use to do that. Thanks in advance and thank you for your time.

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Recursive calls inside lazy data constructors?
by Galaxy Being 13 Feb '21

13 Feb '21

I'm looking at this <https://stackoverflow.com/questions/23768413/building-a-list-from-left-to-r…> from Stackoverflow and wondering what is meant in the accepted answer when he says *In Haskell you can safely do recursive calls inside lazy data constructors, and there will be no risk of stack overflow or divergence. Placing the recursive call inside a constructor eliminates the need for an accumulator, and the order of elements in the list will also correspond to the order in which they are computed*: collatz :: Integer -> [Integer] collatz n | n <= 1 = [] collatz n = n : collatz next where next = if even n then div n 2 else n * 3 + 1 What is meant by lazy data constructor in this context? LB

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Function with mod cases?
by Lawrence Bottorff 06 Feb '21

06 Feb '21

I'm looking through *An Introduction to Functional Programming Systems Using Haskell *by Davie and one of the exercises is *Define a function* f *such that* f(x) = x + 3 (mod 5) *Make a version that elaborates cases, as well as a straightforward way.* I'm not really sure what's being asked, so this is what I've come up with myMods xs = map (\x -> (x + 3) `mod` 5) xs > myMods [1..20] [4,0,1,2,3,4,0,1,2,3,4,0,1,2,3,4,0,1,2,3] but when he says *make a version that elaborates cases *I'm not sure what more there is to do. What am I missing? LB

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Cabal Build
by A. Mc. 06 Feb '21

06 Feb '21

Hello, I've been trying to do a cabal build. I'm using Windows 10. But, after going through cabal init, change diretories, run, etc it keeps telling me I don't have permission. My project also has 2 libraries that I wrote for main, and although I tried editing the .cabal file, I'm not sure I have it right. Thanks in advance and thank you for your time.

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Mixed division
by Lawrence Bottorff 05 Feb '21

05 Feb '21

I can divide a float by an integer > 2.4 / 3 0.7999999999999999 but why can I not do this? < 2.4 / (length [1,2,3]) <interactive>:288:1-22: error: * No instance for (Fractional Int) arising from a use of `/' * In the expression: 2.4 / (length [1, 2, 3]) LB

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type variables - error: 'No instance for'
by Corrado Corrado 05 Feb '21

05 Feb '21

2 1

Data Types and Functions Visualization
by Leduin José Cuenca Macas 03 Feb '21

03 Feb '21

Hey, there! Do you know about any tool to generate a graph that connects data types through functions of my program? I tried stack dot but only I obtained the relationship of dependencies. Greetings! *Leduin José Cuenca Macas* *Student | Estudiante* Tel. (+593) 991 618 273

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Using Functions with Fold
by A. Mc. 01 Feb '21

01 Feb '21

Hello, I feel like I need to go back to basics, so I've made a toy problem that takes a list of Ints, say [10, 20, 30, 40, 50, 60] and, in theory, iterates through the list in successive pairs i.e. [10, 20], [20, 30], [30, 40], [40, 50], [50, 60] and looks for a specific pair set, say 30, 40, and accumulates the number of times that pair appears in the list (for example purpose, there are no repeats, but assuming there were....). What I've done is the following: ---take starting at position 2, in increments of 2 with overlap allowed takeAtTwo :: [Int] -> Int -> [Int] takeAtTwo list position = take (2 + position) list ---expects range of 0 to length list - 2 grabTwo :: [Int] -> Int -> [Int] grabTwo list position = drop position (takeAtTwo list position) -- and this does NOT work, half-pseudocode --- pass in a pair of Int, say [30, 40] and sum up the number of appearances numPair list = foldr (\pair acc -> if grabTwo list iterator == pair then acc + 1 else acc) 0 However, I am unsure how to get this to work properly with fold. Suggestions? Thanks in advance and thank you for your time.

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