add nix devshells

default.nix

@@ -4,7 +4,7 @@
 }:
 stdenvNoCC.mkDerivation {
   pname = "wiki";
-  version = "2023-11-26";
+  version = "2023-12-04";
   src = ./.;
   buildInputs = [ mdbook ];
   buildPhase = ''

src/SUMMARY.md

@@ -4,4 +4,5 @@
 
 # Nix/NixOS
  - [Firefox + Nix](./nix/firefox.md)
+ - [Adhoc development environments with Nix](./nix/nix-devshells.md)
 

src/nix/nix-devshells.md

@@ -0,0 +1,50 @@
+# Adhoc development environments with Nix
+
+I've recently started exploring Python and Haskell. When working with projects in these, I need to have a bunch of dependencies installed. For example, with Python, I'd need a version of the Python interpreter itself, Pip of Pipenv or something similar for installing external modules, maybe a language server like pyright etc.
+
+But I don't need these installed and available all the time, polluting my `$PATH`
+
+## That's where Nix comes in.
+
+[Nix](https://nixos.org/download#nix-install-linux) has a way to define development shells and activate them only when needed. When activated, it will update variables like `$PATH` and make any extra dependencies available. Once done working on something, it can be deactivated.
+
+For example, below is a simple dev shell which installs `go 1.20`.
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  };
+
+  outputs = { self, nixpkgs, }:
+  let
+    system = "x86_64-linux";
+    pkgs = import nixpkgs {
+      inherit system;
+    };
+  in
+  {
+    devShells.${system}.default = pkgs.mkShell {
+      packages = [
+        pkgs.go_1_20
+      ];
+    };
+  };
+}
+```
+After creating a file called `flake.nix` in the project directory, it can be activated with the `nix develop` command. And once done, the shell can be closed with `exit` or <kbd>Ctrl+d</kbd>.
+
+The problem with this is that, it starts a new `bash` shell with a really generic prompt. So any customizations made in `.bashrc` or `.zshrc` etc. gets lost.
+
+## Direnv to the rescue
+
+[Direnv](https://direnv.net/) is a tool that, once hooked up with the shell, will execute a set of commands mentioned in a per project `.envrc` file. It will execute these commands automatically when cd'd into the directory and the changes will be reverted when cd'ing out.
+
+Direnv has something they call the [`stdlib`](https://github.com/direnv/direnv/blob/master/stdlib.sh), which is a set of sane defaults in the form of functions that can be put into the `envrc`. One such function is the `use_flake` function.
+
+By creating a file called `.envrc` in the project directory with the content
+```
+use_flake
+```
+direnv will automatically append the environment created by the devShell defined in flake.nix to the existing shell env, without starting a new shell or losing any of the customizations from `.bashrc` or `.zshrc`
+
+