Rework Nix example project & add Nix template

Changes the Nix infrastructure inside of simple-nix to use Nix flakes.
Adds some more explanation how to include dependencies using Nix and
includes a Nix template. All of this was done to make it more user
friendly (...for Nix users) to get started with Clash without having to
have very deep knowledge of Nix or how Cabal works to make Nix and Cabal
work together happily.

People who have Nix installed can now use immediately start using Clash
with the simple-nix directory by running three commands:

```
mkdir my-project
cd my-project
nix flake init -t github:clash-lang/clash-starters
```

After this nix command is executed, it will display a 'welcome' message
which gives a plain overview of what the Nix related commands do and
instructs the user to read the README.md for more information.

The original PR is: clash-lang/clash-starters#6
Which I have closed after realizing that repository is essentially just
a mirror

Co-authored-by: Peter Lebbing <peter@digitalbrains.com>

Author: jaschutte

.ci/publish.sh

@@ -10,14 +10,27 @@ export GIT_SSH_COMMAND="ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChec
 
 cd "${ROOT}"
 
-# Clone clash-starters and rollback to very first commit
-git clone git@github.com:clash-lang/clash-starters.git
-cd clash-starters
-    first_commit=$(git rev-list --max-parents=0 --abbrev-commit HEAD)
-    git reset "${first_commit}" --hard
-cd ..
+# Set up repo for clash-starters
+git init -b main clash-starters
+
+# Everything in clash-starters-files/ is put into the clash-starters
+# repository verbatim.
+cp -a clash-starters-files/* clash-starters/
+
+# Nix projects are copied with symlinks dereferenced
+cp -R --dereference --preserve projects-nix/* clash-starters/
+
+# We also offer archives of Nix projects
+for project_dir in projects-nix/*; do
+    project="${project_dir#*/}"
+
+    cd clash-starters
+        zip -r "${project}.zip" "${project}"
+        tar -czf "${project}.tar.gz" "${project}"
+    cd ..
+done
 
-# Render all projects in rendered/
+# Render all Stack projects for clash-starters
 for hsfile in *.hsfiles; do
     project="${hsfile%.*}"
 
@@ -31,6 +44,7 @@ done
 
 # Commit and push files
 cd clash-starters
+git remote add origin git@github.com:clash-lang/clash-starters.git
 git config --global user.name "Clash DevOps"
 git config --global user.email "devopsXXX@qbaylogic.com"
 git add -A

README.md

@@ -7,6 +7,16 @@ Templates for `stack new` command. To use this template perform the following st
 ## Cabal users
 All starter projects are also available on [clash-lang/clash-starters](https://github.com/clash-lang/clash-starters).
 
+## Nix users
+
+Nix users can immediately get the `simple-nix` template up and running by executing:
+```
+mkdir starter && cd starter
+nix flake init -t github:clash-lang/clash-starters
+```
+
+Read the `README.md` inside of the newly created directory for more information on how to use Nix with Clash.
+
 ## Contributing
 If you wish to contribute to this template, edit them in `projects/` and perform the following steps to test the template:
 1. Edit the template

clash-starters-files/LICENSE

@@ -0,0 +1,121 @@
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clash-starters-files/README.md

@@ -0,0 +1,22 @@
+# Clash starter projects
+This repository houses a number of projects ready to get you going with Clash. You only need [Stack](https://docs.haskellstack.org/en/stable/README/) (recommended) or [GHC](https://www.haskell.org/)+[Cabal](https://www.haskell.org/cabal/) to get going. There is also a [Nix](https://nixos.org/) flake template available. Stack users can instantiate a project by running:
+
+```
+stack new my-clash-project clash-lang/simple
+```
+
+where `simple` is one of the projects in this repository. Stack will modify the project such that it is called `my-clash-project`. Cabal users can download one of the `.zip`s in this repository and change the name manually.
+
+Nix users can immediately get the `simple-nix` template up and running by executing:
+```
+mkdir starter && cd starter
+nix flake init -t github:clash-lang/clash-starters
+```
+
+Read the `README.md` in each project for more information. Happy hacking!
+
+## Issues
+Issues can be reported at [clash-lang/stack-templates](https://github.com/clash-lang/stack-templates).
+
+## Contributing
+Do not submit PRs to this repository directly. Follow the instructions on [clash-lang/stack-templates](https://github.com/clash-lang/stack-templates) instead. Any changes there will be pushed to this repository automatically.

clash-starters-files/flake.nix

@@ -0,0 +1,36 @@
+{
+  description = "A flake for clash-starter templates. For an actual flake containing the project, check the `simple-nix` subdirectory.";
+
+  outputs = { self }: {
+    defaultTemplate = {
+      path = ./simple-nix;
+      description = "A template containing a simple Clash project with a Nix flake for development";
+      welcomeText = ''
+        # Nix development
+        To use Nix to develop your Clash project, simply run `nix develop` in your current directory.
+        This will put you into an environment with all the tools you will need to build the starter
+        project.
+
+        To compile the project and generate HDL, simply run `nix run` and the output Verilog will be
+        under the `verilog` directory. If you changed the HDL language to VHDL, it will be under the
+        `vhdl` directory.
+
+        You can also run `nix build` to build the project using Nix. Although this isn't very
+        useful in the starter project, it is very useful when developing libraries!
+
+        # Cabal & Nix clashes
+        Cabal and Nix work happily together, most of the time. One important thing of note is that
+        you should **not** define sources using `cabal.project` files. This will overwrite Nix's
+        package source and *will* cause problems. To add dependencies, add them via `flake.nix`.
+
+        # TLDR
+        1. run `nix develop` to immediately jump into development
+        2. run `nix run` to build the package and generate HDL with Nix
+        3. run `nix build` to build the package as a library with Nix
+
+        Read the README.md for more information!
+        Happy hacking!
+      '';
+    };
+  };
+}

projects-nix/simple-nix/.envrc

@@ -0,0 +1 @@
+use flake

projects-nix/simple-nix/.vscode

@@ -0,0 +1 @@
+../../projects/simple/.vscode

projects-nix/simple-nix/README.md

@@ -0,0 +1,102 @@
+<!-- omit in toc -->
+# Simple Nix Starter Project
+This starter project contains the scaffolding needed to integrate Clash with the Nix build system. Read [Simple Starter Project](https://github.com/clash-lang/clash-starters/blob/main/simple/README.md) for more information on the various files.
+
+<!-- omit in toc -->
+# Table of Contents
+- [Getting this project](#getting-this-project)
+- [Building and testing this project](#building-and-testing-this-project)
+- [REPL](#repl)
+- [Adding custom dependencies / updating nix](#adding-custom-dependencies--updating-nix)
+
+# Getting this project
+First create a new directory and enter it: `mkdir starter && cd starter`
+
+Then run: `nix flake init -t github:clash-lang/clash-starters`.
+
+# Building and testing this project
+Build the project with:
+
+```bash
+nix run
+```
+
+Verilog code will be available under the `verilog` directory.
+Modify the `hdl` variable in `flake.nix` to configure whether to generate
+SystemVerilog or VHDL.
+
+However development itself is more streamlined by using a Nix shell. Start one 
+by invoking:
+
+```
+nix develop
+```
+
+Then, to run the tests defined in `tests/`:
+
+```bash
+cabal run test-library
+cabal run doctests
+```
+
+To compile the project to VHDL run:
+
+```bash
+cabal run clash -- Example.Project --vhdl
+```
+
+You can find the HDL files in `vhdl/`. The source can be found in `src/Example/Project.hs`.
+
+# VSCode
+
+In order to make VSCode use the right version of HLS (the Haskell language server), you will need to
+install [this](https://marketplace.visualstudio.com/items?itemName=mkhl.direnv) plugin.
+
+Without it, VSCode will attempt to use the wrong version of HLS and (mostly) crash.
+
+# REPL
+Clash offers a [REPL](https://en.wikipedia.org/wiki/Read%E2%80%93eval%E2%80%93print_loop) as a quick way to try things, similar to Python's `python` or Ruby's `irb`. Open the REPL by invoking:
+
+```
+cabal run clashi
+```
+
+# Adding custom dependencies / updating nix
+The nix flake contains the sources of all Haskell and non-Haskell related packages.
+You can add newer/older packages by overwriting them using the `overlay` defined in `flake.nix`, or
+if the package is not available, include it by adding it as a flake input.
+
+Example:
+
+```diff
+package-overlay = final: prev: {
+  # Here you define the project you want to build
+  simple-nix = prev.developPackage {
+    root = ./.;
+    overrides = _: _: final;
+  };
++ # Adds an older version of `hello` package
++ hello = prev.callHackageDirect {
++   pkg = "hello";
++   ver = "1.0";
++   sha256 = "sha256-/oxATGk025R39CkjfWAcX2SrcM+pLY7IqlkPLJYVIIc=";
++ } {};
+};
+```
+
+For adding other Clash dependencies which also have a flake (such as clash-protocols), you can use
+the overlay those projects expose and append them to your own as follows:
+
+```diff
+package-overlay = final: prev: {
+  # Here you define the project you want to build
+  simple-nix = prev.developPackage {
+    root = ./.;
+    overrides = _: _: final;
+  };
+-};
++} // clash-protocols.overlays.${system}.${ghc-version} final prev;
+```
+
+When you include those projects as a flake input, make sure to make their `clash-compiler` input
+follow your own! Aka; set `clash-protocols.inputs.clash-compiler.follows = "clash-compiler";`

projects-nix/simple-nix/bin

@@ -0,0 +1 @@
+../../projects/simple/bin

projects-nix/simple-nix/cabal.project

@@ -0,0 +1,7 @@
+packages:
+  simple-nix.cabal
+
+write-ghc-environment-files: always
+
+-- Eliminates the need for `--enable-tests`, which is needed for HLS.
+tests: true