How to integrate dependency into existing project in Haskell?

Question

I am currently trying to get started with Haskell because I want to use parts of the code base of Pandoc for a different project. Since I am new to Haskell I need proper IDE features like code completion and jump to definition AND type information and documentation on hover. I chose VSCode with the Haskell extension for the job. Now comes my problem: Pandoc depends on pandoc-types which is an integral part of the code, which I need to understand and modify. But using the ghc-option "$everything": -haddock (which should be the right way to achieve my goal according to this) does not seem to give me proper type information and documentation on hover. Since I copied the entire repo and do not intend to pull or push from the original repos I would like to add the code from pandoc-types to the existing Haskell code in the main pandoc repo.

So part of what I have tried was downloading pandoc-types moving the .hs files into the according dir in pandoc, adding the modules to the .cabal file while removing the pandoc-<version> dependency from the .cabal file and the stack.yaml. But all I got where compatibility errors when building:

➜  pandoc git:(master) ✗ stack build

Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for citeproc-0.1.0.1:
    pandoc-types-1.17.6 from stack configuration does not match >=1.22 && <1.23  (latest matching version is 1.22)
needed due to pandoc-2.11.0.1 -> citeproc-0.1.0.1

In the dependencies for commonmark-pandoc-0.2.0.1:
    pandoc-types-1.17.6 from stack configuration does not match >=1.21 && <1.23  (latest matching version is 1.22)
needed due to pandoc-2.11.0.1 -> commonmark-pandoc-0.2.0.1

In the dependencies for texmath-0.12.0.3:
    pandoc-types-1.17.6 from stack configuration does not match >=1.20 && <1.23  (latest matching version is 1.22)
needed due to pandoc-2.11.0.1 -> texmath-0.12.0.3

Some different approaches to resolving this:

  * Set 'allow-newer: true' in /Users/johannes/.stack/config.yaml to ignore all version constraints and build anyway.

  * Recommended action: try adding the following to your extra-deps in /Users/johannes/Programmieren/GITGOV/Pandocs/pandoc/stack.yaml:

- pandoc-types-1.22@sha256:15512ce011555ee720820f11cac0598317293406da5812337cbb1550d144e3bd,4071

Plan construction failed.

How can I change a repo from a dependency be part of the code base. I have tried a few different things, but nothing seemed to work out. I am not really familiar with GHC, stack and cabal or even Haskell itself for that matter. Or is there another way to get the type information and documentation on hover working? Especially as a Haskell beginner I really need this functionality to properly grasp the code base.

Maybe also relevant:

Both repos seem to generate Paths_*.hs files in the building process. As far as I understand it, they also need to be copied into the src/ dir like mentioned here.

Answer 1

Depending on the tool one uses there are different ways to go about it.

If stack is used:

Following the accepted answer in this question here allows me to compile the code via stack build with pandoc-types as a local dependency.

If cabal is used:

As with the solution above one needs to add the local dependency into the root folder of the repo. Furthermore one should add a reference to the dependencies cabal file to the cabal.project file in the packages: section as follows (which tells cabal to also compile the contents of this folder):

packages: pandoc-types/pandoc-types.cabal pandoc.cabal

package pandoc
  flags: +embed_data_files -trypandoc
  ghc-options: -j +RTS -A64m -RTS

source-repository-package
    type: git
    location: https://github.com/jgm/citeproc
    tag: 0.1.0.1

Also the dependencies in the <projectname>.cabal need the version restrictions removed. So the file is changed from this:

library
  build-depends: pandoc-types          >= 1.22     && < 1.23

... to this:

library
  build-depends: pandoc-types

Now my code compiles with cabal build.

However one part of my problem remains. When following both approaches the Haskell extension in VSCode still does not properly autocomplete. Using the stack approach gives warnings like A do-notation statement discarded a result of type ... and errors like Could not deduce ... arising from a use of .... The first warning should actually already be suppressed with the -fno-warn-unused-do-bind flag in ghc-options within the pandoc.cabal file (assuming this is what the extension reads in order to print warnings/errors). So I don't know what is responsible for these errors. They are not present when the repo was downloaded from Hackage during the build process. I will probably need to ask another question on stack overflow regarding this issue.

Anyway since the question in headline is answered, I hope this helps someone at some point in the future.

Answer 2

Avoid the complications of trying to unite what has always been split. Why not just keep the pandoc-types source code in a separate (local) library project --which you can also modify and refer to from your main project-- and load it in a separate editor instance with its own context? You can switch between editors where applicable, when browsing through the source code.