how-i-use-cabal

How I use cabal

– version 3.2.0.0.

I want to download cabal

I use ghcup. Personally I prefer the manual install. I am on a 32-bit Linux system so I do

curl -f https://downloads.haskell.org/~ghcup/0.1.9/i386-linux-ghcup-0.1.9 > ~/ghcup
chmod +x ~/ghcup

Then to install cabal it is a simple matter of

~/ghcup install cabal 3.2.0.0

The binary that it installs will end up at ~/.ghcup/bin/cabal-3.2.0.0 and a symlink to it will probably be created at ~/.ghcup/bin/cabal.

I want to play around with a package in the REPL

For very quick exploration you can specify a dependency on the command line. If you want something that is a little more persistent you can create a temporary project and add it as a dependency.

Specify a dependency on the command line

In a directory that doesn’t contain a Haskell package’s .cabal file (perhaps your home directory) run

cabal v2-repl --build-depends QuickCheck
<it downloads and builds QuickCheck if necessary...>
*Main> import Test.QuickCheck
*Main Test.QuickCheck> let prop h t = (length t + 1) == (length $ h:t)
*Main Test.QuickCheck> quickCheck prop
+++ OK, passed 100 tests.

Create a temporary project and add it as a dependency

$ mkdir my-temporary-package
$ cd my-temporary-package
$ cabal init

Edit my-temporary-package.cabal to change

build-depends:       base >=... && <...

to

build-depends:       base, QuickCheck

(The constraints on base are going to cause you more harm than good for a quick exploratory project. You can add them back later if it becomes necessary.)

$ cabal v2-repl

What about cabal install --lib?

I would avoid it. To me it seems like a fools errand to try to load non-system packages with anything but cabal.

I have a package in local source directory. I want to use it in another local package.

Suppose I have a package called package1 in a local source directory and whose cabal file is at /home/me/package1/package1.cabal; I want to use it in another local package I’m developing whose cabal file is /home/me/package2/package2.cabal.

I achieve this by adding an optional-packages stanza to package2’s cabal.project, as follows.

• If /home/me/package2/cabal.project already exists

  just add the following line

  optional-packages: /home/me/package1/package1.cabal

• If /home/me/package2/cabal.project does not yet exist

  create /home/me/package2/cabal.project containing the following

  packages: *.cabal
  optional-packages: /home/me/package1/package1.cabal

  It is vital that the “packages: *.cabal” line be included otherwise your project defaults to containing no packages at all. This default doesn’t seem to make any sense, but there it is.

cabal install --lib doesn’t seem to work

Although cabal install --lib seems like it ought to support use cases like this, I’ve never been able to get it to work.