= Access a Haskell library package in the nixpkgs repo (without using a `.cabal` file)

////
$ mkdir tempwork
$ cd tempwork
$ cp ../Main.hs ../flake.nix .
$ git add Main.hs flake.nix
$ nix develop
$ git add flake.lock
$ git commit -a -m temp
////

Occasionally you might want to run a short Haskell program that depends on a Haskell library,
but you don't want to bother writing a cabal file.
In this example, we will access the `extra` package from the `haskellPackages` set in the nixpkgs repo.

[NOTE]
====
For non-trivial Haskell development projects,
it's usually more convenient to use `haskell-flake` as described in <<#haskell-flake>>,
together with the _high-level workflow_ described in <<_development_workflows>>.
====

[source,nix,linenums,highlight='11,16']
.flake.nix
....
include::flake.nix[]
....

Line 12 makes a custom GHC that knows about `extra`,
and line 16 makes that custom GHC available in the development environment.

Here's a short Haskell program that uses the new flake.

[source,haskell,linenums]
.Main.hs
....
include::Main.hs[]
....

Here's a demonstration using the program.

....
$ runghc Main.hs      # Fails; dependency not available
$# start-shell nix develop <<EOL
$ runghc Main.hs      # Works in development environemnt
$# EOL
....

////
Good adoc0 scripts clean up after themselves.
$ exit
$ cd .. ; rm -rf tempwork # clean up
////