feat(devShells): init

devShells/default.nix

@@ -0,0 +1,80 @@
+# Development shells
+
+/**
+  Isolated environments for convenient software development.
+  Provides shells for all languages I use:
+  - ~~❄️  [Nix]~~  (not yet)
+  - ~~🦀 Rust~~  (not yet)
+  - ~~🐍 Python~~  (not yet)
+
+  Also provides separate shell for configuration development (not yet).
+
+  `devShells/{name}/default.nix` files contain package list (as attrset for documentation comments) and,
+  for some shells, shell hooks. It usually looks like this:
+  ```nix
+  # {language name}
+  /**
+    Some info on shell
+  * /
+  pkgs:
+  {
+    /**
+      description on foo
+    * /
+    packages.foo = pkgs.foo;
+    /**
+      description on bar
+    * /
+    packages.bar = pkgs.bar
+
+    /**
+      comments on shell hook
+    * /
+    shellHook = ''
+      ...
+    '';
+  }
+  ```
+
+  Also, it has templates, so you can use development shell for projects which don't have their own.
+*/
+let
+  dirs =
+    let
+      contents = builtins.readDir ./.;
+    in
+    builtins.filter (name: contents.${name} == "directory") (builtins.attrNames contents);
+in
+{
+  perSystem =
+    {
+      pkgs,
+      ...
+    }:
+
+    {
+      devShells = builtins.listToAttrs (
+        map (name: {
+          inherit name;
+          value =
+            let
+              cfg = import ./${name}/default.nix pkgs;
+            in
+            pkgs.mkShell {
+              inherit name;
+              packages = map (p: p) (builtins.attrValues cfg.packages);
+              shellHook = cfg.shellHook or "";
+            };
+        }) dirs 
+      );
+    };
+  flake.templates = builtins.listToAttrs (
+    map (name: {
+      inherit name;
+      value = {
+        path = ./${name};
+        description = "${name}";
+      };
+    }) dirs
+  );
+}