What cabal install did when type this command?

[WendaoLee]

🙇‍♂️I'm new to Haskell and cabal.And these are my questions when using with cabal.I do hope these noob questions won't burden you and I would be gratefully appreciate for your answers.

My question is: What cabal install did when type this command?

For the package manager,I'm familiar with npm of nodejs.It seems like cabal install remote-package won't modify the .cabal like the npm install remote-package will modify the package.json.And it not download files into the workspace like npm.

I notice cabal manage its package like that:

1. When enter cabal install remote-package，it will download sourcecode from hackage into local directory ./cabal/packages if not exist.
2. Then try to complie it into .hi files for better performence,which will help for building project afterwards.
3. The last thing is what I get confused:Finished above things,it will try to compile the download-package into executable file no matter it is just a lib?For instance,I try to install a lib named sqilte-simple with cabal install sqlite-simple.When all things done,it throw error:

Error: cabal-3.10.1.0.exe: Cannot build the executables in the package
sqlite-simple because it does not contain any executables. Check the .cabal
file for the package and make sure that it properly declares the components
that you expect.

Is that a useless design?Or has some reasons for it?🙇‍♂️I'm really curious about it.

And for just enter cabal install in a local cabal project.It seem that it will try to compile the project into executeable file and install it on ./cabal/bin.

Is there any wrong about my cognition?🙇‍♂️And it would be delightful for someone could answer me the details of how this command actually work.

[ffaf1]

And for just enter cabal install in a local cabal project.It seem that it will try to compile the project into executeable file and install it on ./cabal/bin.

For now! Once you switch to a more modern cabal, it will be based on XDG paths (e.g. ~/.local/bin).

[ffaf1]

Reading this again, it seems to be that your congnition is spot on (but wait for more experienced people)!

I would notice that cabal install is indeed for executables, while cabal install --lib (provisional) is for libraries (plain is to split it: see #6481).

[WendaoLee]

Wow，🥰thank you for your detailed description.

Well,I have a new question when using cabal.That is,some package (e.g a package named http-client) I used has its build-dependences(e.g has dependence http-types).And it seems like that cabal won't automatically import these dependences unless I specified it in my .cabal file?(e.g I want to use a module in http-types but get modules not found error,though I specified http-client in .cabal file which has build-dependences of http-types).

All in all,sincerely appreciate for your help in today and yesterday!😽

[ffaf1]

That is,some package (e.g a package named http-client) I used has its build-dependences(e.g has dependence http-types).And it seems like that cabal won't automatically import these dependences unless I specified it in my .cabal file?

That is correct. E.g.:

1. package foo (which depends on bar) exports constructor Foo;
2. package bar exports constructor Bar;
3. if you need to use both Foo and Bar in your package, you cannot merely state build-depends: foo, but need a more principled build depends: foo, bar

This might feel awkward at first, but imagine what a headache for you it would be tomorrow foo stopped depending on bar and you had to hunt down missing dependencies.

[WendaoLee]

Thanks a lot！🥰🥰🥰

[fgaz]

For the package manager,I'm familiar with npm of nodejs.It seems like cabal install remote-package won't modify the .cabal like the npm install remote-package will modify the package.json.And it not download files into the workspace like npm.

Yes, the install command of cabal is very different from the npm one. To do the equivalent of npm install you don't have to run cabal install at all. Instead, you add the package you want to use to the build-depends section of the .cabal file of your package. Cabal will take care of fetching and building the dependency as needed when you run cabal build, cabal run, or other similar commands.

cabal install instead is more like npm install --global. It's mostly used by end users to install executables into their ~/.local/bin (or equivalent) directory, or by advanced developers to create custom environments through the --lib flag.

[WendaoLee]

Thanks😘