- Beginners - Haskell.org

How to change base version easily?
by Treee July 29 May '20

29 May '20

Hello, I installed the latest version of ghc, which caused one of my packages does not satisfy the constraint of the version. I just wondering if there exists a convenient way to degrade the version of the base. I use Arch Linux, which is not easy to install an elder version. Thanks, July

3 2

data type
by Alexander Chen 29 May '20

29 May '20

import Data.Time data DatabaseItem = DbString String | DbNumber Integer | DbDate UTCTime deriving (Eq, Ord, Show) theDatabase :: [DatabaseItem] theDatabase = [ DbDate (UTCTime (fromGregorian 1911 5 1) (secondsToDiffTime 34250)) , DbNumber 9001 , DbString "Hello, world!" , DbDate (UTCTime (fromGregorian 1921 5 1) (secondsToDiffTime 34123)) ] My question: I want to get the UTCTime. So, thisFunction :: [DatabaseItem] -> [UTCTime] I don't need the answer but more the intuition (a link to a youtube etc. is also possible) how to handle these types of situations. Because i keep trying to think in terms of key:values (Python,Julia) which doesn't seem the right paradigm to approach this. thanks in advance! best,

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Foldl
by Alexander Chen 29 May '20

29 May '20

Hi, I guess it's time to get acquainted with foldr and foldl. prelude>xs = [1..5] ++ undefined prelude> foldr const 0 xs 1 I assume it goes something like this: ( 1 `const`(2 `const`(3 `const`(4 `const`(5 `const`(undefined `const` 0)))))) ( 5 `const` undefined) (4 `const` 5) (3 `const` 4) ( 2 `const` 3) (1 `const`2) = 1 ======================================================================== What i don't get is the opposite: prelude> foldl const 0 xs error in my mind this should go like this: ((((((0`const`1)`const` 2) `const` 3 )`const` 4)`const` 5) `const` undefined) (0 `const`2) (0`const` 3) ( 0`const`4) (0`const` 5) (0 `const` undefined ) = 0 I have been told that the main difference between foldl and foldr is that foldl needs to evaluate the whole spline before it continues. And i guess that has something to do with it. What I don't understand is WHY foldl need to do this and foldr doesn't. thanks in advance! best,

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Re: [Haskell-beginners] Linking C++ in Cabal
by Leonhard Applis 27 May '20

27 May '20

Thanks Toz! Works like a charm. Repository is updated for anyone interested. Beginners Mailing list is the best <3 ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ On Tuesday, May 26, 2020 10:01 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. Re: Linking C++ in Cabal (鲍凯文) > > > Message: 1 > Date: Tue, 26 May 2020 13:03:13 -0700 > From: 鲍凯文 traqueofziche(a)gmail.com > To: beginners(a)haskell.org > Subject: Re: [Haskell-beginners] Linking C++ in Cabal > Message-ID: > CAMjcG+FisqcazdRV14Hut7QS1-37tyhXptZG2prNH7-90aswmA(a)mail.gmail.com > Content-Type: text/plain; charset="utf-8" > > Hi, > > So I tried it out, and there're a couple of things: > > - Declaring a function as `static` will hide it from anything other than > the current translation unit, AFAIK, which means that you won't be able to > link it to anything else; the symbol won't be visible (you can verify using > `objdump`). > > - For C++ functions, you need to suppress name-mangling since Haskell's FFI > doesn't know about that. With your repo as is, the symbol corresponding to > the `multiply` function is not...`multiply`: > > > dist-newstyle/build/x86_64-linux/ghc-8.6.5/HaskellForeignCPP-0.0.1/build/cpp-src/some.o: > file format elf64-x86-64 > > SYMBOL TABLE: > 0000000000000000 l df *ABS* 0000000000000000 some.cpp > 0000000000000000 l d .text 0000000000000000 .text > 0000000000000000 l d .data 0000000000000000 .data > 0000000000000000 l d .bss 0000000000000000 .bss > 0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack > 0000000000000000 l d .eh_frame 0000000000000000 .eh_frame > 0000000000000000 l d .comment 0000000000000000 .comment > 0000000000000000 g F .text 0000000000000006 _Z8multiplyii > > > so you just need to wrap every exposed function in `extern "C"` blocks, > like it's explained here: https://www.geeksforgeeks.org/extern-c-in-c/. > > With `extern "C"` and removing `static`: > > dist-newstyle/build/x86_64-linux/ghc-8.6.5/HaskellForeignCPP-0.0.1/build/cpp-src/some.o: > file format elf64-x86-64 > > SYMBOL TABLE: > 0000000000000000 l df *ABS* 0000000000000000 some.cpp > 0000000000000000 l d .text 0000000000000000 .text > 0000000000000000 l d .data 0000000000000000 .data > 0000000000000000 l d .bss 0000000000000000 .bss > 0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack > 0000000000000000 l d .eh_frame 0000000000000000 .eh_frame > 0000000000000000 l d .comment 0000000000000000 .comment > 0000000000000000 g F .text 0000000000000006 multiply > > > ...and everything builds fine. > > Best, > > toz > > On Tue, May 26, 2020 at 5:03 AM 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. Re: Linking C++ in Cabal - repository Link (Leonhard Applis) > > 2. Re: Linking C++ in Cabal (鲍凯文) > > > > Message: 1 > > Date: Mon, 25 May 2020 13:00:03 +0000 > > From: Leonhard Applis Leonhard.Applis(a)protonmail.com > > To: "beginners(a)haskell.org" beginners(a)haskell.org > > Subject: Re: [Haskell-beginners] Linking C++ in Cabal - repository > > Link > > Message-ID: > > <iF7eLvkM4HniAVDyO09U-O3y48pPEAciKs5zjKAtajj8lldf1g8Tp5UHi3-HiT2GMc15GBi5JgVgkRXr4KQ8Y8p6i0CuuAU-RLh-g2t57uQ=@ > > protonmail.com> > > Content-Type: text/plain; charset="utf-8" > > Ups, in my mailing list the links have disappeared. > > Just in case: > > The minimal reproducing repository: > > https://github.com/Twonki/HaskellForeingCPPMinimal > > The repository from Aditya Siram with C and makefile: > > https://github.com/deech/CPlusPlusBindings > > best > > Leonhard > > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ > > On Monday, May 25, 2020 2:00 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. Linking C++ in Cabal (Leonhard Applis) > > > > > > > > > > Message: 1 > > > Date: Sun, 24 May 2020 16:30:22 +0000 > > > From: Leonhard Applis Leonhard.Applis(a)protonmail.com > > > To: "beginners(a)haskell.org" beginners(a)haskell.org > > > Subject: [Haskell-beginners] Linking C++ in Cabal > > > Message-ID: > > > > 51wOfngyX74dYjJRw6hibqw07vGHvvSYPD_f3f2-_F7hG0k0GwWf2Xv4g7LyeZNyn_vR28s6GIbd3e-78_TapNIEo_tJZ-2fTViOmkQYOnM=@ > > protonmail.com > > > > > > > > > > Content-Type: text/plain; charset="utf-8" > > > > > 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] Linking C++ in Cabal
by 鲍凯文 26 May '20

26 May '20

Hi, So I tried it out, and there're a couple of things: - Declaring a function as `static` will hide it from anything other than the current translation unit, AFAIK, which means that you won't be able to link it to anything else; the symbol won't be visible (you can verify using `objdump`). - For C++ functions, you need to suppress name-mangling since Haskell's FFI doesn't know about that. With your repo as is, the symbol corresponding to the `multiply` function is not...`multiply`: ``` dist-newstyle/build/x86_64-linux/ghc-8.6.5/HaskellForeignCPP-0.0.1/build/cpp-src/some.o: file format elf64-x86-64 SYMBOL TABLE: 0000000000000000 l df *ABS* 0000000000000000 some.cpp 0000000000000000 l d .text 0000000000000000 .text 0000000000000000 l d .data 0000000000000000 .data 0000000000000000 l d .bss 0000000000000000 .bss 0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack 0000000000000000 l d .eh_frame 0000000000000000 .eh_frame 0000000000000000 l d .comment 0000000000000000 .comment 0000000000000000 g F .text 0000000000000006 _Z8multiplyii ``` so you just need to wrap every exposed function in `extern "C"` blocks, like it's explained here: https://www.geeksforgeeks.org/extern-c-in-c/. With `extern "C"` and removing `static`: ``` dist-newstyle/build/x86_64-linux/ghc-8.6.5/HaskellForeignCPP-0.0.1/build/cpp-src/some.o: file format elf64-x86-64 SYMBOL TABLE: 0000000000000000 l df *ABS* 0000000000000000 some.cpp 0000000000000000 l d .text 0000000000000000 .text 0000000000000000 l d .data 0000000000000000 .data 0000000000000000 l d .bss 0000000000000000 .bss 0000000000000000 l d .note.GNU-stack 0000000000000000 .note.GNU-stack 0000000000000000 l d .eh_frame 0000000000000000 .eh_frame 0000000000000000 l d .comment 0000000000000000 .comment 0000000000000000 g F .text 0000000000000006 multiply ``` ...and everything builds fine. Best, toz On Tue, May 26, 2020 at 5:03 AM <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. Re: Linking C++ in Cabal - repository Link (Leonhard Applis) > 2. Re: Linking C++ in Cabal (鲍凯文) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 25 May 2020 13:00:03 +0000 > From: Leonhard Applis <Leonhard.Applis(a)protonmail.com> > To: "beginners(a)haskell.org" <beginners(a)haskell.org> > Subject: Re: [Haskell-beginners] Linking C++ in Cabal - repository > Link > Message-ID: > > <iF7eLvkM4HniAVDyO09U-O3y48pPEAciKs5zjKAtajj8lldf1g8Tp5UHi3-HiT2GMc15GBi5JgVgkRXr4KQ8Y8p6i0CuuAU-RLh-g2t57uQ=@ > protonmail.com> > > Content-Type: text/plain; charset="utf-8" > > Ups, in my mailing list the links have disappeared. > Just in case: > > > The minimal reproducing repository: > > https://github.com/Twonki/HaskellForeingCPPMinimal > > The repository from Aditya Siram with C and makefile: > > https://github.com/deech/CPlusPlusBindings > > best > Leonhard > > > > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ > On Monday, May 25, 2020 2:00 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. Linking C++ in Cabal (Leonhard Applis) > > > > > > > > Message: 1 > > Date: Sun, 24 May 2020 16:30:22 +0000 > > From: Leonhard Applis Leonhard.Applis(a)protonmail.com > > To: "beginners(a)haskell.org" beginners(a)haskell.org > > Subject: [Haskell-beginners] Linking C++ in Cabal > > Message-ID: > > > 51wOfngyX74dYjJRw6hibqw07vGHvvSYPD_f3f2-_F7hG0k0GwWf2Xv4g7LyeZNyn_vR28s6GIbd3e-78_TapNIEo_tJZ-2fTViOmkQYOnM=@ > protonmail.com > > > > > Content-Type: text/plain; charset="utf-8" > > > > > 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] Linking C++ in Cabal
by 鲍凯文 25 May '20

25 May '20

Hi, Would it be possible to upload your full repo somewhere so people could see everything and possibly reproduce this? Thanks, toz On Mon, May 25, 2020 at 5:03 AM <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. Linking C++ in Cabal (Leonhard Applis) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Sun, 24 May 2020 16:30:22 +0000 > From: Leonhard Applis <Leonhard.Applis(a)protonmail.com> > To: "beginners(a)haskell.org" <beginners(a)haskell.org> > Subject: [Haskell-beginners] Linking C++ in Cabal > Message-ID: > > <51wOfngyX74dYjJRw6hibqw07vGHvvSYPD_f3f2-_F7hG0k0GwWf2Xv4g7LyeZNyn_vR28s6GIbd3e-78_TapNIEo_tJZ-2fTViOmkQYOnM=@ > protonmail.com> > > Content-Type: text/plain; charset="utf-8" > > 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] Linking C++ in Cabal - repository Link
by Leonhard Applis 25 May '20

25 May '20

Ups, in my mailing list the links have disappeared. Just in case: The minimal reproducing repository: https://github.com/Twonki/HaskellForeingCPPMinimal The repository from Aditya Siram with C and makefile: https://github.com/deech/CPlusPlusBindings best Leonhard ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ On Monday, May 25, 2020 2:00 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. Linking C++ in Cabal (Leonhard Applis) > > > Message: 1 > Date: Sun, 24 May 2020 16:30:22 +0000 > From: Leonhard Applis Leonhard.Applis(a)protonmail.com > To: "beginners(a)haskell.org" beginners(a)haskell.org > Subject: [Haskell-beginners] Linking C++ in Cabal > Message-ID: > 51wOfngyX74dYjJRw6hibqw07vGHvvSYPD_f3f2-_F7hG0k0GwWf2Xv4g7LyeZNyn_vR28s6GIbd3e-78_TapNIEo_tJZ-2fTViOmkQYOnM=(a)protonmail.com > > Content-Type: text/plain; charset="utf-8" > > 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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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 22 May '20

22 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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