Re-enable “Nix development cache” workflow

.github/workflows/nix-dev-cache.yaml

@@ -1,15 +1,16 @@
 name: Nix development cache
 
 on:
+  push:
+    branches:
+      # Cache trunk, to keep recent builds in the cache for development.
+      - trunk
+    tags:
+      # Cache releases, so they can be pinned in the cache for downstream consumers of Unison.
+      - 'release/**'
+  # Allow manual caching of branches. This is particularly useful when upgrading the Nix tooling, as we can test the PR
+  # and ensure that the cache is populated before merging into trunk.
   workflow_dispatch:
-  # # Build on every pull request (and new PR commit)
-  # pull_request:
-  # # Build on new pushes to trunk (E.g. Merge commits)
-  # # Without the branch filter, each commit on a branch with a PR is triggered twice.
-  # # See: https://github.community/t/how-to-trigger-an-action-on-push-or-pull-request-but-not-both/16662
-  # push:
-  #   branches:
-  #     - trunk
 
 jobs:
   nix:
@@ -22,7 +23,7 @@ jobs:
         os:
           - ubuntu-24.04
           - macos-13
-          # - macos-14
+          - macos-14
     steps:
     - uses: actions/checkout@v4
     - name: mount Nix store on larger partition

development.markdown

@@ -4,6 +4,10 @@ __General:__ `./scripts/test.sh` compiles and builds the Haskell code and runs a
 
 _Disclaimer_ If you have trouble getting started, please get in touch via [Discord](https://unison-lang.org/discord) so we can help.  If you have any fixes to the process, please send us a PR!
 
+## Development Dependencies
+
+If you are having trouble with a build, please ensure that your tooling matches the versions we expect. Some build mechanisms will guarantee this to some extent (e.g., the Nix build, or [the Haskell extension for VS Code](https://marketplace.visualstudio.com/items?itemName=haskell.haskell)), but you can see versions listed in [our version file](./nix/versions.nix) (some of which is inherited from [this repo’s VS Code settings](./.vscode/settings.json)).
+
 ## Running Unison
 
 To get cracking with Unison:
@@ -113,113 +117,9 @@ More context at: https://stackoverflow.com/a/59761201/310162
 
 Stack doesn't work deterministically in Windows due to mismatched expectations about how file deletion works. If you get this error, you can just retry the build and it will probably make more progress than the last time.
 
-## Building with Nix
-
-__NB__: It is important that the Unison Nix cache is trusted when building, otherwise you will likely end up building hundreds of packages, including GHC itself.
-
-The recommended way to do this is to add the public key and URL for the cache to your system’s Nix configuration. /etc/nix/nix.conf should have lines similar to
-```conf
-trusted-public-keys = unison.cachix.org-1:i1DUFkisRPVOyLp/vblDsbsObmyCviq/zs6eRuzth3k=
-trusted-substituters = https://unison.cachix.org
-```
-these lines could be prefixed with `extra-` and they may have additional entries besides the ones for our cache.
-
-This command should work if you don’t want to edit the file manually:
-```shell
-sudo sh -c 'echo "extra-trusted-public-keys = unison.cachix.org-1:i1DUFkisRPVOyLp/vblDsbsObmyCviq/zs6eRuzth3k=
-extra-trusted-substituters = https://unison.cachix.org" >>/etc/nix/nix.conf'
-```
-After updating /etc/nix/nix.conf, you need to restart the Nix daemon. To do this on
-- Ubuntu: `sudo systemctl restart nix-daemon`
-- MacOS:
-    ```shell
-    sudo launchctl unload /Library/LaunchDaemons/org.nixos.nix-daemon.plist
-    sudo launchctl load /Library/LaunchDaemons/org.nixos.nix-daemon.plist
-    ```
-
-If you use NixOS, you may instead add this via your configuration.nix with
-```nix
-nix.settings.trusted-public-keys = ["unison.cachix.org-1:i1DUFkisRPVOyLp/vblDsbsObmyCviq/zs6eRuzth3k="];
-nix.settings.trusted-substituters = ["https://unison.cachix.org"];
-```
-and run `sudo nixos-rebuild switch` afterward.
-
-It is _not_ recommended to add your user to `trusted-users`. This _can_ make enabling flake configurations simpler (like the Unison Nix cache here), but [it is equivalent to giving that user root access (without need for sudo)](https://nix.dev/manual/nix/2.23/command-ref/conf-file.html#conf-trusted-users).
-
-## Building package components with nix
-
-### Build the unison executable
-```shell
-nix build
-```
-
-### Build a specific component
-This is specified with the normal
-`<package>:<component-type>:<component-name>` triple.
-
-Some examples:
-```shell
-nix build '.#component-unison-cli:lib:unison-cli'
-nix build '.#component-unison-syntax:test:syntax-tests'
-nix build '.#component-unison-cli:exe:transcripts'
-```
-
-### Development environments
-
-#### Get into a development environment for building with stack
-This gets you into a development environment with the preferred
-versions of the compiler and other development tools. These
-include:
-
-- ghc
-- stack
-- ormolu
-- haskell-language-server
+## Nix support
 
-```shell
-nix develop
-```
-
-#### Get into a development environment for building with cabal
-This gets you into a development environment with the preferred
-versions of the compiler and other development tools. Additionally,
-all non-local haskell dependencies (including profiling dependencies)
-are provided in the nix shell.
-
-```shell
-nix develop '.#cabal-local'
-```
-
-#### Get into a development environment for building a specific package
-This gets you into a development environment with the preferred
-versions of the compiler and other development tools. Additionally,
-all haskell dependencies of this package are provided by the nix shell
-(including profiling dependencies).
-
-```shell
-nix develop '.#cabal-<package-name>'
-```
-
-for example:
-
-```shell
-nix develop '.#cabal-unison-cli'
-```
-or
-```shell
-nix develop '.#cabal-unison-parser-typechecker'
-```
-
-This is useful if you wanted to profile a package. For example, if you
-want to profile `unison-cli-main:exe:unison` then you could get into one of these
-shells, cd into its directory, then run the program with
-profiling.
-
-```shell
-nix develop '.#cabal-unison-parser-typechecker'
-cd unison-cli
-cabal run --enable-profiling unison-cli-main:exe:unison -- +RTS -p
-```
+See the [readme](./nix/README.md).
 
 ## Native compilation
 

flake.nix

@@ -26,16 +26,7 @@
       "aarch64-darwin"
     ]
     (system: let
-      ## It’s much easier to read from a JSON file than to have JSON import from some other file, so we extract some
-      ## configuration from the VS Code settings to avoid duplication.
-      vscodeSettings = nixpkgs-release.lib.importJSON ./.vscode/settings.json;
-      versions =
-        vscodeSettings."haskell.toolchain"
-        ## There are some things we want to pin that the VS Code Haskell extension doesn’t let us control.
-        // {
-        hpack = "0.35.2";
-        ormolu = "0.7.2.0";
-      };
+      versions = import ./nix/versions.nix {inherit (nixpkgs-haskellNix) lib;};
       pkgs = import nixpkgs-haskellNix {
         inherit system;
         inherit (haskellNix) config;

nix/README.md

@@ -0,0 +1,107 @@
+## Building with Nix
+
+__NB__: It is important that the Unison Nix cache is trusted when building, otherwise you will likely end up building hundreds of packages, including GHC itself.
+
+The recommended way to do this is to add the public key and URL for the cache to your system’s Nix configuration. /etc/nix/nix.conf should have lines similar to
+```conf
+trusted-public-keys = unison.cachix.org-1:i1DUFkisRPVOyLp/vblDsbsObmyCviq/zs6eRuzth3k=
+trusted-substituters = https://unison.cachix.org
+```
+these lines could be prefixed with `extra-` and they may have additional entries besides the ones for our cache.
+
+This command should work if you don’t want to edit the file manually:
+```shell
+sudo sh -c 'echo "extra-trusted-public-keys = unison.cachix.org-1:i1DUFkisRPVOyLp/vblDsbsObmyCviq/zs6eRuzth3k=
+extra-trusted-substituters = https://unison.cachix.org" >>/etc/nix/nix.conf'
+```
+After updating /etc/nix/nix.conf, you need to restart the Nix daemon. To do this on
+- Ubuntu: `sudo systemctl restart nix-daemon`
+- MacOS:
+    ```shell
+    sudo launchctl unload /Library/LaunchDaemons/org.nixos.nix-daemon.plist
+    sudo launchctl load /Library/LaunchDaemons/org.nixos.nix-daemon.plist
+    ```
+
+If you use NixOS, you may instead add this via your configuration.nix with
+```nix
+nix.settings.trusted-public-keys = ["unison.cachix.org-1:i1DUFkisRPVOyLp/vblDsbsObmyCviq/zs6eRuzth3k="];
+nix.settings.trusted-substituters = ["https://unison.cachix.org"];
+```
+and run `sudo nixos-rebuild switch` afterward.
+
+It is _not_ recommended to add your user to `trusted-users`. This _can_ make enabling flake configurations simpler (like the Unison Nix cache here), but [it is equivalent to giving that user root access (without need for sudo)](https://nix.dev/manual/nix/2.23/command-ref/conf-file.html#conf-trusted-users).
+
+## Building package components with nix
+
+### Build the unison executable
+```shell
+nix build
+```
+
+### Build a specific component
+This is specified with the normal
+`<package>:<component-type>:<component-name>` triple.
+
+Some examples:
+```shell
+nix build '.#component-unison-cli:lib:unison-cli'
+nix build '.#component-unison-syntax:test:syntax-tests'
+nix build '.#component-unison-cli:exe:transcripts'
+```
+
+### Development environments
+
+#### Get into a development environment for building with stack
+This gets you into a development environment with the preferred
+versions of the compiler and other development tools. These
+include:
+
+- ghc
+- stack
+- ormolu
+- haskell-language-server
+
+```shell
+nix develop
+```
+
+#### Get into a development environment for building with cabal
+This gets you into a development environment with the preferred
+versions of the compiler and other development tools. Additionally,
+all non-local haskell dependencies (including profiling dependencies)
+are provided in the nix shell.
+
+```shell
+nix develop '.#cabal-local'
+```
+
+#### Get into a development environment for building a specific package
+This gets you into a development environment with the preferred
+versions of the compiler and other development tools. Additionally,
+all haskell dependencies of this package are provided by the nix shell
+(including profiling dependencies).
+
+```shell
+nix develop '.#cabal-<package-name>'
+```
+
+for example:
+
+```shell
+nix develop '.#cabal-unison-cli'
+```
+or
+```shell
+nix develop '.#cabal-unison-parser-typechecker'
+```
+
+This is useful if you wanted to profile a package. For example, if you
+want to profile `unison-cli-main:exe:unison` then you could get into one of these
+shells, cd into its directory, then run the program with
+profiling.
+
+```shell
+nix develop '.#cabal-unison-parser-typechecker'
+cd unison-cli
+cabal run --enable-profiling unison-cli-main:exe:unison -- +RTS -p
+```

nix/versions.nix

@@ -0,0 +1,11 @@
+{lib}: let
+  ## It’s much easier to read from a JSON file than to have JSON import from some other file, so we extract some
+  ## configuration from the VS Code settings to avoid duplication.
+  vscodeSettings = lib.importJSON ../.vscode/settings.json;
+in
+  vscodeSettings."haskell.toolchain"
+  ## There are some things we want to pin that the VS Code Haskell extension doesn’t let us control.
+  // {
+    hpack = "0.35.2";
+    ormolu = "0.7.2.0";
+  }