Extract the pinned tool versions from flake.nix

And reference them in the development docs, so contributors can more
easily discover what versions we use.

Author: sellout

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:

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/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";
+  }