nhc allows to execute arbitrary commands inside a build environment
that is created from the cabal file. nhc allows you to do things like:
$ nhc cabal build
$ nhc hdevtools check src/Main.hs
Interactive commands are also allowed:
$ nhc ghci
$ nhc zsh
nhc provides basic development tools (ghc, ghc-pkg, cabal), but also
some convenient additional development tools. (Currently only hdevtools in
a version that tries to understand cabal files.)
nhc is a thin wrapper around nix-build and some nix-expressions heavily
inspired by http://ocharles.org.uk/blog/posts/2014-02-04-how-i-develop-with-nixos.html.
So you need a working nix installation for this to work.
Invoking nhc will not alter your user's nix profile (but it will install
software to the nix store). It creates two nix files defaults.nix and
nhc.nix. Then nix-build is invoked to create a script that sets up the wanted
environment. nix-build will put a link to the derivation containing the
script in ./result. This script will normally drop you in a newly created
bash inside the created environment. nhc invokes this script to run its commands.
On subsequent invocations of nhc it will check whether the cabal file changed.
If it didn't, nhc will just execute the given command inside of the created
environment. If it did change, the new environment will be built (with nix-build),
and the given command will be executed in the new environment.
hdevtools is a little special since it spawns a background process to make
module checking fast. This background process runs in the environment that it
was started in, which only works as long as you don't rebuild your environment.
nhc therefore invokes hdevtools --stop-server when the environment is rebuilt.