docs: write docs, add README

README.md

@@ -0,0 +1,44 @@
+# Rensa
+
+Rensa is a powerful and flexible Nix flake framework designed to organize your Nix projects into logical units called "Cells".
+It is heavily inspired by [divnix/std](https://github.com/divnix/std) but aims to provide a streamlined experience.
+Feel free to check out std's awesome [docs](https://std.divnix.com/reference/std.html) too.
+
+## Features
+
+- **Cells**: Organize your code into functional domains (e.g., `backend`, `frontend`, `devops`).
+- **Blocks**: Define types of outputs within a cell (e.g., `packages`, `devShells`, `nixosConfigurations`).
+- **Actions**: Integrate with the CLI to run commands defined in your cells.
+- **Cell Flakes**: Each cell can be its own flake, allowing for modular dependency management.
+
+## Quick Start
+
+1. **Initialize a Flake**: Create a `flake.nix` that uses `rensa`.
+1. **Define Cells**: Create a `cells/` directory.
+1. **Add Blocks**: Add `.nix` files in your cells to define outputs.
+
+### Example `flake.nix`
+
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    ren.url = "gitlab:rensa-nix/core";
+  };
+
+  outputs = {ren, ...}@inputs: ren.buildWith {
+    inherit inputs;
+    cellsFrom = ./cells;
+    cellBlocks = with ren.blocks; [
+      (simple "packages")
+      (simple "devShells")
+    ];
+  } {
+    packages = ren.select inputs.self [
+      ["my-cell" "packages"]
+    ];
+  };
+}
+```
+
+See the [docs](https://rensa.projects.tf) for more.

cells/repo/docs.nix

@@ -31,6 +31,11 @@ in
         };
         nav = [
           {"Introduction" = "index.md";}
+          {"Concepts" = "concepts.md";}
+          {"Tutorial" = "tutorial.md";}
+          {"API Reference" = "api.md";}
+          {"Related Libraries" = "libraries.md";}
+          {"Direnv Integration" = "direnv.md";}
         ];
         markdown_extensions = [
           {
@@ -41,6 +46,7 @@ in
           "pymdownx.superfences"
           "pymdownx.escapeall"
           "fenced_code"
+          "admonition"
         ];
       };
     };

docs/api.md

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+# API Reference
+
+## `rensa.buildWith`
+
+The main entry point for creating a Rensa flake.
+
+### Arguments
+
+- `inputs`: The flake inputs.
+- `cellsFrom`: Path to the directory containing your cells.
+- `cellBlocks`: A list of blocks to load for each cell.
+- `transformInputs` (optional): A function to transform inputs before they are passed to cells.
+
+### Returns
+
+A standard Nix flake output set.
+
+## `rensa.build`
+
+The underlying builder function used by `buildWith`. It returns the raw Rensa output structure without the recursive update functor.
+
+### Arguments
+
+- `inputs`: The flake inputs.
+- `cellsFrom`: Path to the directory containing your cells.
+- `cellBlocks`: A list of blocks to load for each cell.
+- `transformInputs` (optional): A function to transform inputs.
+
+### Returns
+
+An attribute set containing:
+
+- `output`: The generated flake outputs.
+- `__ren`: Internal metadata about the cells and blocks.
+
+## `rensa.blocks`
+
+Helper functions to define blocks.
+
+### `simple`
+
+```nix
+simple "name"
+```
+
+Creates a block definition where the type matches the name.
+
+### `dynamic`
+
+```nix
+dynamic "name"
+```
+
+Creates a block definition for dynamic content. This allows integration of the `std` cli
+by passing through actions etc. from a cell.
+
+Currently not really used.
+
+## `rensa.select`
+
+Helper to select specific outputs from the generated flake.
+
+```nix
+select inputs.self [
+  ["cellName" "blockName" "outputName"]
+]
+```
+
+## `rensa.filter`
+
+Filters the generated flake outputs based on a predicate and paths.
+
+```nix
+filter predicate inputs.self [
+  ["cellName" "blockName" "outputName"]
+]
+```
+
+### Arguments
+
+- `predicate`: A function or boolean. If `true`, returns the attribute as is. If a function, it's used to filter the attributes of the target.
+- `target`: The flake outputs (usually `inputs.self`).
+- `paths`: A list of paths to select. Supports wildcards (`*`).
+
+## `rensa.get`
+
+Retrieves a single attribute from the flake outputs.
+
+```nix
+get inputs.self ["cellName" "blockName" "outputName"]
+```
+
+### Arguments
+
+- `target`: The flake outputs.
+- `path`: The path to the attribute to retrieve.

docs/concepts.md

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+# Concepts
+
+Rensa is built around a few core concepts that help structure your Nix projects.
+
+## Cells
+
+A **Cell** is the fundamental unit of organization in Rensa.
+It represents a functional domain or a component of your system.
+For example, you might have cells for:
+
+- `backend`: Your backend services.
+- `frontend`: Your frontend applications.
+
+Cells are directories located in the `cellsFrom` directory (usually `cells/` or `nix/`).
+
+### Cell Flakes
+
+A unique feature of Rensa is that each cell can be a self-contained Flake.
+If a cell directory contains a `flake.nix`, Rensa will treat it as a flake and pass its outputs to the cell's blocks.
+This allows you to manage dependencies at the cell level, keeping your top-level `flake.nix` clean.
+
+!!! success "Advantage"
+
+    These nested flakes are also evaluated lazily, so inputs are only fetched if they are actually used.
+
+## Blocks
+
+A **Block** defines a specific type of output within a cell. Blocks are typically defined as `.nix` files within a cell directory.
+Rensa provides different block types to handle various use cases.
+
+### Simple Blocks
+
+`rensa.blocks.simple` is the most common block type.
+Use it to tell Rensa which blocks you have/it should load.
+
+**Example**:
+
+```nix
+blocks = with ren.blocks; [
+  (simple "hello")
+]
+```
+
+This will tell Rensa to load any `hello.nix` or `hello/default.nix` in the cells.
+It's required to be able to map these to flake outputs (eg. using `ren.select`).
+
+### Dynamic Blocks
+
+`rensa.blocks.dynamic` allows for more complex logic and can generate actions.
+
+## Actions
+
+**Actions** are commands that can be executed via the Std CLI.
+They are defined within blocks and allow you to operationalize your Nix code.
+For example, a `deploy` action in an `infra` cell could run the necessary commands to deploy your infrastructure.
+
+!!! warning
+
+    Actions are also heavily inspired by Std, I added them to Rensa too, but nowadays
+    I don't see any real use cases for them. If you do, feel free to use and/or extend them :)
+
+## Performance & Best Practices
+
+### Single Instantiation of Nixpkgs
+
+Instantiating `nixpkgs` (e.g., `import inputs.nixpkgs { inherit system; }`) is a computationally expensive operation. In a large project with many cells, doing this repeatedly for every cell or block can significantly slow down evaluation.
+
+The recommended pattern in Rensa is to instantiate `nixpkgs` **once** in your top-level `flake.nix` and pass it down to your cells.
+
+1. **Instantiate in `transformInputs`**: In your `flake.nix`, use the `transformInputs` argument of `buildWith` to add the instantiated `pkgs` to the inputs passed to cells.
+1. **Use `i.parent.pkgs`**: In your cell flakes, access this pre-instantiated package set via `inputs.parent.pkgs`.
+
+This ensures that `nixpkgs` is evaluated only once per system, drastically improving performance.
+
+See [Related Libraries](./libraries.md) for examples of this pattern.

docs/direnv.md

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+# Direnv Integration
+
+Rensa provides a custom `direnv` integration to make your development experience smoother.
+It allows you to automatically load development shells defined in your cells.
+
+!!! note
+
+    This integration is based on [nix-direnv](https://github.com/nix-community/nix-direnv)
+    but has been improved and optimized specifically for Rensa usage.
+
+## Setup
+
+1. **Create `.envrc`**: Add the following to your `.envrc` file:
+
+    ```bash
+    source $(fetchurl https://gitlab.com/rensa-nix/direnv/-/raw/v0.3.0/direnvrc "sha256-u7+KEz684NnIZ+Vh5x5qLrt8rKdnUNexewBoeTcEVHQ=")
+    use ren //backend/devShells/default
+    ```
+
+    This will automatically load the devShell `default` defined in `cells/backend/devShells.nix`.
+
+1. **Allow Direnv**: Run `direnv allow` to trust the configuration.
+
+## Updating
+
+To update the `direnvrc` to the latest version (or a specific version), you can use `direnv fetchurl`.
+
+1. **Run fetchurl**:
+
+    ```bash
+    direnv fetchurl https://gitlab.com/rensa-nix/direnv/-/raw/v0.3.0/direnvrc
+    ```
+
+    *Note: Replace `v0.3.0` with the desired version tag.*
+
+1. **Update `.envrc`**: The command above will output a new hash.
+    Replace the previous version and hash in your `.envrc` with the new values.

docs/index.md

@@ -1 +1,30 @@
 # Rensa Core
+
+Welcome to the documentation for **Rensa**, a powerful and flexible Nix flake framework.
+
+Rensa is designed to help you organize your Nix projects into logical units called **Cells**,
+making your code more modular, maintainable, and scalable.
+
+## Getting Started
+
+If you are new to Rensa, we recommend starting with the [Tutorial](tutorial.md) to build your first project.
+
+## Core Concepts
+
+Understand the philosophy behind Rensa by reading about [Concepts](concepts.md), including:
+
+- **Cells**: Functional domains of your project.
+- **Blocks**: Typed outputs within cells.
+- **Actions**: CLI commands for your cells.
+
+## API Reference
+
+For detailed information on Rensa's functions and helpers, check the [API Reference](api.md).
+
+## Ecosystem
+
+See [Related Libraries](libraries.md) to see how to extend Rensa with pre-built solutions for development shells, documentation, CI, and more.
+
+## Further Reading
+
+Rensa is inspired by `std`. For more in-depth information on the underlying concepts, you can refer to the [std Reference](https://std.divnix.com/reference/std.html).

docs/libraries.md

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+# Related Libraries
+
+Rensa has a growing ecosystem of compatible libraries designed to work seamlessly with its cell structure.
+These libraries typically expose a `lib` output that accepts `pkgs` as an argument.
+
+## Available Libraries
+
+Here are some common libraries you might want to use:
+
+- [**Rensa Devshell**](https://devshell.rensa.projects.tf): _Minimal devshell implementation using Modules._
+- [**Rensa Devtools**](https://devtools.rensa.projects.tf): _Utils and dev tools for the Rensa ecosystem._
+- [**Rensa Utils**](https://utils.rensa.projects.tf): _Utilities for NixOS/darwin systems, home & disko configurations etc._
+- [**Nix-GitLab-CI**](https://nix-gitlab-ci.projects.tf): _Allows (advanced) configuration of GitLab CI using Nix._
+- [**Nixible**](https://nixible.projects.tf): _Ansible but with Nix._
+- [**Tofunix**](https://tofunix.projects.tf): _Combining Nix and Terraform for reproducibility and developer experience._
+- [**Nixlets**](https://nixlets.projects.tf): _Nixlets - like Helm Charts or Grafana Tanka but instead using Nix._
+- [**Nixtest**](https://nixtest.projects.tf): _Test runner for Nix code._
+- [**Soonix**](https://soonix.projects.tf): _Auto generated project files from Nix code, with gitignore handling and many generators._
+- [**NixMkDocs**](https://nix-mkdocs.projects.tf): _Nix library for easy mkdocs integration into projects._
+- [**Torikae**](https://torikae.projects.tf): _Simple CLI to replace versions in files._
+
+## Usage Pattern
+
+When using these libraries within a Cell Flake, you can instantiate them using the `pkgs` passed down from the parent flake.
+This follows the "Single Instantiation" best practice.
+
+### Example: `cells/mycell/flake.nix`
+
+```nix
+{
+  inputs = {
+    # Add the library to your inputs
+    devshell.url = "gitlab:rensa-nix/devshell?dir=lib";
+    
+    # ... other inputs
+  };
+
+  outputs = i:
+    i
+    // {
+      # Instantiate the library using the parent's pkgs
+      dslib = i.devshell.lib { inherit (i.parent) pkgs; };
+    };
+}
+```
+
+Now you can use `inputs.dslib` in your blocks!

docs/tutorial.md

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+# Tutorial
+
+This tutorial will guide you through creating a new project using Rensa.
+
+## Prerequisites
+
+- Nix installed with flakes enabled.
+- Optionally `direnv`.
+
+## Step 1: Initialize the Flake
+
+Create a new directory for your project and initialize a `flake.nix`:
+
+```nix title="flake.nix"
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    ren.url = "gitlab:rensa-nix/core";
+  };
+
+  outputs = {ren, self, ...}@inputs: ren.buildWith {
+    inherit inputs;
+    cellsFrom = ./nix;
+    cellBlocks = with ren.blocks; [
+      (simple "packages")
+    ];
+    transformInputs = system: inputs: inputs // {
+      pkgs = import inputs.nixpkgs { inherit system; };
+    };
+  } {
+    packages = ren.select self [
+      ["backend" "packages"]
+    ];
+  };
+}
+```
+
+## Step 2: Create a Cell
+
+Create a directory named `nix` and inside it, create a directory for your first cell, e.g., `backend`.
+
+```bash
+mkdir -p nix/backend
+```
+
+## Step 3: Add a Block
+
+Inside `nix/backend`, create a file named `packages.nix`.
+This will correspond to the `packages` block we defined in `flake.nix`.
+
+Since we instantiated `pkgs` in the top-level `flake.nix`, we can access it via `inputs.pkgs`
+(because we added it to inputs in `transformInputs`).
+
+```nix
+{
+  inputs,
+  cell,
+}: {
+  default = inputs.pkgs.hello;
+}
+```
+
+### Using Cell Flakes (Advanced)
+
+If your cell is a flake (has a `flake.nix`), you can access the parent inputs via `inputs.parent`.
+
+**`nix/backend/flake.nix`**:
+
+```nix
+{
+  inputs.common.url = "github:some/common-lib";
+  outputs = inputs: inputs // {
+    # inputs just for the current cell, for example:
+    common = inputs.common.lib {inherit (inputs.parent) pkgs;};
+  };
+}
+```
+
+See [here](./libraries.md) for more information about this syntax.
+
+**`cells/backend/packages.nix`**:
+
+```nix
+{
+  inputs,
+  cell,
+}: {
+  default = inputs.common.mkPackage { ... };
+}
+```
+
+## Step 4: Build
+
+Now you can build your package using the standard Nix commands:
+
+```bash
+nix build .#backend.packages.default
+```
+
+Or, if you mapped it in `flake.nix`:
+
+```bash
+nix build .#default
+```
+
+## Step 5: Setting up Direnv
+
+Rensa provides a custom `direnv` integration. See the [Direnv Integration](direnv.md) guide for setup instructions.