- Beginners - Haskell.org

Linking C++ in Cabal
by Leonhard Applis 24 May '20

24 May '20

Hello, I want to use a foreign C++ Function in a Haskell Library, and the Library in a Haskell Program. The C++ function is separate in a file and I have a header file. I made a minimal reproducing repository. The closes I have found is this repository which wraps the c++ in c and makes a full library, needing a makefile etc. So while I think I'd get it working like that, it looks to be quite a lot to do for my simple task. the repository structure looks as follows: Projekt - cpp-src - some.cpp - some.h - lib-src - CPPLib.hs - program-src - program.hs - project.cabal And my (currently failing) project.cabal the following: cabal-version: 3.0 name: cpp_lib [...] library exposed-modules: CPPLib other-extensions: ForeignFunctionInterface build-depends: base hs-source-dirs: lib-src default-language: Haskell2010 include-dirs: ./cpp-src cxx-sources: ./cpp-src/some.cpp install-includes: ./cpp-src/some.h executable CPPApp main-is: program.hs default-language: Haskell2010 build-depends: HaskellForeignCPP, base hs-source-dirs: program-src The some.cpp is trivial for this case lets assume its a function multiply :: Int -> Int -> Int, and both .cpp and .h are well-formed and don't have further dependencies. The library compiles fine, but when building the executable it fails telling me collect2: error: ld returned 1 exit status `gcc' failed in phase `Linker'. (Exit code: 1) Can someone help me out? I find very little documentation on this topic. I know there is Inline-CPP which would help as well, but I just very much like the idea of having a nice separate file and interface for modularity. best Leonhard

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Re: [Haskell-beginners] MaximumBy (Alexander Chen)
by nowsnow 23 May '20

23 May '20

maxinumBy::(Foldable t, Ord a) => t a -> a maxinumBy = foldr1 max > Hi, > I want to re-write a function maximumBy (its an assignment). > However, I don't get how it works. >>maximumBy compare [1,53,9001, 10] > 9001 > but what does it actually do to get there? > thanks in advance. > best, >

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MaximumBy
by Alexander Chen 22 May '20

22 May '20

Hi, I want to re-write a function maximumBy (its an assignment). However, I don't get how it works. >maximumBy compare [1,53,9001, 10] 9001 but what does it actually do to get there? thanks in advance. best,

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Understanding types and constructors more thoroughly
by Josh Diamond 22 May '20

22 May '20

Ok so I have an assessment, first part is: Consider the following declaration for a parameterised type of lists in which Nil represents the empty list, and Cons x xs represent a non-empty list with first element x and remaining list of elements xs: data List a = Nil | Cons a (List a) Am I right in reading it like: Declare a List type which can take any parameter and is constructed by Nil or Cons which itself takes anything and a List of anything? Also it seems strange to define a List without [] or : like in the prelude but I assume it’s just for the purposes of the question. Is the (List a) used just to make the order of evaluation explicit or is it some weird thing with tuples? Ok so the actual question is: Q) Define a recursive function append :: List a -> List a -> List a that appends two lists together to give a single list. So I’d have to do something similar to how append is done in the standard prelude I.e. (++) :: [a] -> [a] -> [a] [] ++ ys = ys (x:xs) ++ ys = x : (xs ++ ys) How do I do pattern matching on a user defined type? I was thinking something like: Nil append List a = List a For the first line but the second line is confusing me. I’ve read some stuff on pattern matching for data constructors but I don’t know how relevant it is here. Maybe something like this would work: Cons a (List a) append List b = a : (List a append List b) But I’m getting ‘Data constructor not in scope’ for everything to do with List...it thinks it’s a data constructor although I’ve defined it as a type above. Any help would be great 😊 Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10 Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10

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Re: [Haskell-beginners] Code Review of Caesar-Cipher
by 鲍凯文 21 May '20

21 May '20

Hi, For what it's worth, Daniel's suggestion of: ```haskell isAlphabetic char = elem char ['A'..'Z'] ``` does read nicer but would have to traverse `['A'..'Z']` every time. I think what you have is fine, and although relational and boolean operators are also in imperative languages, they behave as pure functions even in imperative languages if subexpressions don't cause side effects. I don't think it's unidiomatic, and especially in this case, "between A and Z" means the same thing as "is one of the letters A, B, C...Z", so the intent of the function is clear as written. Best of all would probably be using `isAlpha` from `Data.Char` (in `base`). Best, toz On Wed, May 20, 2020 at 2:41 PM <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Code Review of Caesar-Cipher (chrysaetos99) > 2. Re: Code Review of Caesar-Cipher (Daniel van de Ghinste) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Wed, 20 May 2020 20:26:37 +0200 > From: chrysaetos99 <chrysaetos99(a)posteo.de> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] Code Review of Caesar-Cipher > Message-ID: <cd08aa1e-1c9b-a609-15df-5bf37508cb96(a)posteo.de> > Content-Type: text/plain; charset="utf-8"; Format="flowed" > > Background > ---------- > I am a total beginner in Haskell, so after reading the "Starting > out"-chapter of "Learn you a Haskell", I wanted to create my first > program that actually does something. > > I decided to do the famous Caesar-Cipher. > > > Code > ---- > See attachment. > > > Question(s) > > ----------- > > - How can this code be improved in general? > - Do I follow the style guide of Haskell (indentation, etc.)? > - I have a background in imperative languages. Did I do something that > is untypical for functional programming languages? > > I would appreciate any suggestions. > > --- > > Please note: I also asked this question on > > https://codereview.stackexchange.com/questions/242529/caesar-cipher-impleme…, > > but didn't receive an answer that really answered all my questions. > > > Kind regards > > chrysaetos99 > > >

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Code Review of Caesar-Cipher
by chrysaetos99 20 May '20

20 May '20

Background ---------- I am a total beginner in Haskell, so after reading the "Starting out"-chapter of "Learn you a Haskell", I wanted to create my first program that actually does something. I decided to do the famous Caesar-Cipher. Code ---- See attachment. Question(s) ----------- - How can this code be improved in general? - Do I follow the style guide of Haskell (indentation, etc.)? - I have a background in imperative languages. Did I do something that is untypical for functional programming languages? I would appreciate any suggestions. --- Please note: I also asked this question on https://codereview.stackexchange.com/questions/242529/caesar-cipher-impleme…, but didn't receive an answer that really answered all my questions. Kind regards chrysaetos99

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Maybe?
by Alexander Chen 13 May '20

13 May '20

Hi, safeHead.hs safeHead :: [a] -> Maybe a safeHead [] = Nothing safeHead (x:_) = Just x prelude> safeHead [23,1,4,2,4,2] Just 23 Question: what am i doing wrong? best, Alexander

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formatting?
by Alexander Chen 09 May '20

09 May '20

Hi, tensDigit.hs tensDigit :: Integral a => a ->a tensDigit x = d where xLast = x `div` 10 d = xLast `mod` 10 prelude> load tensDigit.hs parse error on input ‘=’ Perhaps you need a 'let' in a 'do' block? e.g. 'let x = 5' instead of 'x = 5' 4 | d = xLast `mod` 10 | ^ [1 of 1] Compiling Main ( chapter7_8.hs, interpreted ) Failed, no modules loaded. I suspect this has something to do with the spacing (since this is example code) if so could someone explain to me what the rules on spacing are so I can get past this. I am using a simple text editor so if someone has a recommendation for an IDE that simply takes care of this, much obliged!! best,

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Re: [Haskell-beginners] formatting? (Alexander Chen)
by nowsnow 08 May '20

08 May '20

> I am using a simple text editor > so if someone has a recommendation for an IDE that simply takes care of this, much obliged!! Geany is a powerful, stable and lightweight programmer's text editor that provides tons of useful features without bogging down your workflow. It runs on Linux, Windows and MacOS is translated into over 40 languages, and has built-in support for more than 50 programming languages.

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why does the superclass not work?
by Alexander Chen 29 Apr '20

29 Apr '20

Hi, superclass.hs myX = 1 :: Int sigmund :: Int -> Int sigmund x = myX prelude> :l superclass.hs [1 of 1] Compiling Main Ok, one module loaded. ================================================================================== superclass.hs myX = 1 :: Int sigmund :: Num -> Num sigmund x = myX prelude> :l superclass.hs typed_checked.hs:3:13: error: • Expecting one more argument to ‘Num’ Expected a type, but ‘Num’ has kind ‘* -> Constraint’ • In the type signature: sigmund :: Num -> Num | 3 | sigmund :: Num -> Num | ^^^ typed_checked.hs:3:20: error: • Expecting one more argument to ‘Num’ Expected a type, but ‘Num’ has kind ‘* -> Constraint’ • In the type signature: sigmund :: Num -> Num =================================================================================== I would think since Num is a superclass of Int defining the type in the superclass would be OK, also in the error message it refers to (see black) what does that refer to? thanks!

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