Modern nix setup with flakes (#2474)

Modern nix setup with flakes

.gitignore

@@ -62,3 +62,6 @@ docs/env
 
 # build_clash_dev.sh output
 clash-dev.result
+
+# direnv
+.envrc

README.md

@@ -103,7 +103,43 @@ stack run -- clash
 Or [use Nix](https://nixos.org/nix/download.html) to get a shell with the `clash` and `clashi` binaries on your PATH:
 
 ```bash
-nix-shell
+# Start the default dev shell
+nix develop .
+
+# Start a dev shell with a specific GHC version
+nix develop .#ghc961
+```
+
+You will need a modern version of nix with support for the new-style `nix`
+command and flakes (2.4 or newer). Support for these must still be manually
+enabled, this can be done by setting
+
+```
+experimental-features = nix-command flakes
+```
+
+in your `nix.conf`.
+
+To automatically enter and exit the `nix` environment on directory change, you
+can install [`direnv`](https://direnv.net/) and
+[`nix-direnv`](https://github.com/nix-community/nix-direnv) and write the
+following to a `.envrc` file in the root of this repository:
+
+```
+use flake
+
+watch_file nix/*
+```
+
+Upon adding or changing this file you must `direnv allow` in order for the file
+to be automatically loaded / reloaded on project changes.
+
+Individual packages / applications can also be built or run using the `nix
+build` and `nix run` commands, i.e.
+
+```
+nix build .#clash-ghc
+nix run .#clashi
 ```
 
 # Related libraries and initiatives

clash-cores/clash-cores.cabal

@@ -33,6 +33,12 @@ flag haddock
   default: False
   manual: True
 
+flag nix
+  description:
+    Are we building this package from nix
+  default: False
+  manual: True
+
 common basic-config
   default-language: Haskell2010
 
@@ -63,6 +69,22 @@ common basic-config
   if flag(haddock)
     ghc-options:
       -fdefer-type-errors
+  elif flag(nix)
+    -- It's absolutely grimy, but if we don't specify the `-package` flags here
+    -- then when nix eventually calls `./Setup register` on the package it will
+    -- still pass the `-fplugin` options to GHC and fail to create the package
+    -- registation file needed to finish the `installPhase`.
+    --
+    -- These flags are something we NEVER want to manually specify in a normal
+    -- build, since correctly configuring this kind of thing should be solely
+    -- left to cabal as a build tool.
+    ghc-options:
+      -package ghc-typelits-extra
+      -fplugin GHC.TypeLits.Extra.Solver
+      -package ghc-typelits-knownnat
+      -fplugin GHC.TypeLits.Normalise
+      -package ghc-typelits-natnormalise
+      -fplugin GHC.TypeLits.KnownNat.Solver
   else
     ghc-options:
       -fplugin GHC.TypeLits.Extra.Solver

default.nix

@@ -1,5 +1,11 @@
-{ nixpkgs ? import ./nix/nixpkgs.nix {} }:
-
-with nixpkgs.pkgs.haskellPackages;
-
-{ inherit clash-ghc clash-lib clash-prelude clash-cores; }
+(import
+  (
+    let lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+    in
+    fetchTarball {
+      url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
+      sha256 = lock.nodes.flake-compat.locked.narHash;
+    }
+  )
+  { src = ./.; }
+).defaultNix