simplify Nix workflow

srid/haskell-flake#330

Author: peterbecich

README.md

@@ -13,65 +13,28 @@ You can use the Nix package manager to provide these dependencies, or install th
 
 ### Using the [Nix package manager](https://nixos.org/) and provided [Nix Flake](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html)
 
-If you have the Nix package manager installed, you can build and run `hackage-server` without manually installing any dependencies.
+If you have the Nix package manager installed, you can build and run `hackage-server` without manually installing any dependencies:
 
-This uses `flake.nix`, implemented with [`srid/haskell-flake`](https://github.com/srid/haskell-flake).
+    $ nix run
 
-There are at least three ways to use this `flake.nix`. Clone this repository, enter the repository directory, then choose one of these options:
-
-#### [`nix develop`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-develop.html)
-
-    nix develop
-
-    (in develop shell)
-    $ cabal v2-run -- hackage-server init --static-dir=datafiles
-
-    $ cabal v2-run -- hackage-server run --static-dir=datafiles --base-uri=http://127.0.0.1:8080
+    'state' state-dir already exists
     hackage-server: Ready! Point your browser at http://127.0.0.1:8080
+    
+If the required `state` directory does not already exist, `nix run` will create and initialize it.
 
-Note the `init` command will create a new folder `state` in your working directory.
-
-If you have [direnv](https://direnv.net/), `direnv allow` will load this `nix develop` shell automatically.
-
-#### [`nix build`](https://nixos.org/manual/nix/stable/command-ref/nix-build.html)
-
-    nix build
-
-This will produce a `hackage-server` executable in `result/`.
-
-For this executable, Hackage dependencies are not pulled from Hackage directly like usual. Hackage dependencies are provided by the [Nixpkgs](https://search.nixos.org/packages) [`haskell-updates`](https://github.com/NixOS/nixpkgs/tree/haskell-updates) branch, and a few [overrides in `flake.nix`](https://zero-to-flakes.com/haskell-flake/dependency#using-hackage-versions).
-
-#### [`nix run`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-run)
-
-`nix run` is more convenient to use than `nix build`.
-
-As with `nix build`, Hackage dependencies are not pulled from Hackage directly like usual. See caveat above.
-
-List the available [Flake Apps](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-run#apps) with [`nix flake show`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake-show.html):
-
-    $ nix flake show
-    ...
-    ├───apps
-    ...
-    │   │   ├───hackage-build: app
-    │   │   ├───hackage-import: app
-    │   │   ├───hackage-mirror: app
-    │   │   └───hackage-server: app
-    ...
+The `flake.nix` is implemented with [`srid/haskell-flake`](https://github.com/srid/haskell-flake).
 
-Run the `hackage-server` App:
+Alternatively, open the [`nix develop`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-develop.html) shell:
 
-    nix run .#hackage-server -- init --static-dir=datafiles
+    $ nix develop
 
-    nix run .#hackage-server -- run --static-dir=datafiles --base-uri=http://127.0.0.1:8080
+    (in develop shell)
 
-The `.` refers to the `flake.nix` in your working directory. `#hackage-server` refers to the App specified in that `flake.nix`.
-
-`hackage-server` is the default App, so those commands can be shortened:
+    # if state directory does not already exist
+    $ cabal v2-run -- hackage-server init --static-dir=datafiles --state-dir=state
 
-    nix run . -- init --static-dir=datafiles
-
-    nix run . -- run --static-dir=datafiles --base-uri=http://127.0.0.1:8080
+    $ cabal v2-run -- hackage-server run --static-dir=datafiles --state-dir=state --base-uri=http://127.0.0.1:8080
+    hackage-server: Ready! Point your browser at http://127.0.0.1:8080
 
 ##### Not working
 

flake.nix

@@ -15,8 +15,27 @@
         inputs.flake-root.flakeModule
       ];
       perSystem = { self', system, lib, config, pkgs, ... }: {
-        # The "main" project. You can have multiple projects, but this template
-        # has only one.
+        apps.default = {
+          type = "app";
+          program =
+            let
+              run-hackage-server = pkgs.writeShellApplication {
+                name = "run-hackage-server";
+                runtimeInputs = [ config.packages.default ];
+                text = ''
+                  if [ ! -d "state" ]; then
+                    hackage-server init --static-dir=datafiles --state-dir=state
+                  else
+                    echo "'state' state-dir already exists"
+                  fi
+                  hackage-server run \
+                    --static-dir=datafiles \
+                    --state-dir=state \
+                    --base-uri=http://127.0.0.1:8080
+                '';
+              };
+            in "${lib.getExe run-hackage-server}";
+        };
         packages.default = config.packages.hackage-server;
         haskellProjects.default = {
           settings = {