Add shellHook into devShells.default and document how to init and use the flake

- This shows how to control the classpath locations that `clojure` uses
- Make explicit that $HOME is read-only

Author: danieroux

README.md

@@ -1,10 +1,13 @@
 # clojure-nix-locker
+
 Simple and flexible tool to build clojure projects with Nix.
 
 ## Usage
 
 The [example/](example) directory has a small clojure program and the nix code required to build it.
 
+### Stable non-flake way
+
 To generate/update the lockfile:
 ```sh
 nix-shell --run clojure-nix-locker
@@ -15,6 +18,16 @@ To build:
 nix-build -A uberjar
 ```
 
+### With flakes
+
+You can generate a flake example with:
+
+```sh
+mkdir play-with-clojure-nix-locker && cd play-with-clojure-nix-locker && nix flake init -t github:bevuta/clojure-nix-locker
+```
+
+The [example README](example/README.md) has some next steps.
+
 ## Why another tool?
 
 There are two existing projects with a similar goal already, [clj2nix](https://github.com/hlolli/clj2nix) and [clj-nix](https://github.com/jlesquembre/clj-nix).
@@ -35,6 +48,7 @@ This approach results in a pretty simple implementation and loose coupling to th
 As a consequence, things like aliases "just work" without requiring `clojure-nix-locker` to know about them.
 
 Of course, this has its downsides too:
+
 - If the directory layout of these caches changes, this tool breaks.
 - Whatever classpath(s) your clojure tools compute at build-time will only work for the duration of that build.
 

example/README.md

@@ -0,0 +1,29 @@
+# Generating the lockfile
+
+To generate the lockfile of all the deps:
+
+```sh
+nix run .#locker
+```
+
+Run this after modifying `deps.edn`.
+
+# Building uberjar using the lockfile classpath
+
+```sh
+nix build .#uberjar
+```
+
+Run uberjar:
+
+```sh
+./result/bin/simple
+```
+
+# Starting a devshell with the locked classpath
+
+```
+nix develop
+```
+
+It will print out the current locked classpath.

example/flake.nix

@@ -54,6 +54,21 @@
           drv = my-clojure-nix-locker.locker;
         };
         devShells.default = pkgs.mkShell {
+          shellHook = ''
+            # Trace all Bash executions
+            set -o xtrace
+
+            source ${my-clojure-nix-locker.shellEnv}
+
+            echo "Current locked classpath:"
+            ${pkgs.clojure}/bin/clojure -Spath
+
+            set +o xtrace
+
+            echo
+            echo "Note that \$HOME is overridden and read-only: $HOME"
+            echo
+          '';
           inputsFrom = [
             packages.uberjar
           ];
@@ -63,6 +78,9 @@
             clojure
             clj-kondo
             coreutils
+            # This provides the standalone `clojure-nix-locker` script in the shell
+            # You can use it, or `nix run .#locker`
+            # Both does the same
             my-clojure-nix-locker.locker
           ];
         };