feat: add a devshell for running incremental cabal build testgen-hs, editor LSP etc.

.envrc

@@ -0,0 +1,3 @@
+use flake
+
+source_env_if_exists .envrc.local

.gitignore

@@ -1,2 +1,14 @@
 # Nix
 result
+result-*
+
+# Haskell / Cabal
+dist-newstyle/
+/cabal.project
+/cabal.project.*
+/.cabal/
+.ghc.environment.*
+
+# direnv
+.direnv/
+.envrc.local

README.md

@@ -50,6 +50,7 @@ Available commands:
 ```
 ❯ testgen-hs generate -g1 -n1 ApplyTxErr_Conway
 ```
+
 ```json
 {
   "seed": 1731690796,
@@ -266,11 +267,55 @@ And then easily reproduce it by providing the same seed that was found:
 
 ```
 ❯ ( echo 8182068182028200a0
-    echo 81820681820481581cc0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456 
+    echo 81820681820481581cc0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456
   ) | testgen-hs deserialize-stream
 ```
 
-``` json
+```json
 {"cbor":"8182068182028200a0","haskellRepr":"ApplyTxError (ConwayCertsFailure (WithdrawalsNotInRewardsCERTS (fromList [])) :| [])","json":{"contents":{"contents":{"contents":{"era":"ShelleyBasedEraConway","error":["ConwayCertsFailure (WithdrawalsNotInRewardsCERTS (fromList []))"],"kind":"ShelleyTxValidationError"},"tag":"TxValidationErrorInCardanoMode"},"tag":"TxCmdTxSubmitValidationError"},"tag":"TxSubmitFail"},"typeTag":"ApplyTxError (ConwayEra StandardCrypto)"}
 {"cbor":"81820681820481581cc0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456","haskellRepr":"ApplyTxError (ConwayWdrlNotDelegatedToDRep (KeyHash {unKeyHash = \"c0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456\"} :| []) :| [])","json":{"contents":{"contents":{"contents":{"era":"ShelleyBasedEraConway","error":["ConwayWdrlNotDelegatedToDRep (KeyHash {unKeyHash = \"c0231342a5c66b25d652a7116559d02cbe9515ef890fd698de38d456\"} :| [])"],"kind":"ShelleyTxValidationError"},"tag":"TxValidationErrorInCardanoMode"},"tag":"TxCmdTxSubmitValidationError"},"tag":"TxSubmitFail"},"typeTag":"ApplyTxError (ConwayEra StandardCrypto)"}
 ```
+
+## Development – HLS setup
+
+This project provides a Nix devshell with GHC and HLS. To get full IDE
+support you need [`direnv`](https://direnv.net/) with
+[`nix-direnv`](https://github.com/nix-community/nix-direnv) so that the
+`.envrc` is picked up automatically. See their official documentation for
+installation instructions.
+
+The instructions below use VS Code as an example, but any editor with LSP and
+`direnv` support (Neovim, Emacs, etc.) works the same way. The devshell puts
+both `haskell-language-server` and `haskell-language-server-wrapper` on
+`PATH`, so most editors should pick up HLS automatically.
+
+### VS Code example
+
+1. Install the extensions **Haskell** (`haskell.haskell`) and **direnv**
+   (`mkhl.direnv`).
+
+2. Tell the Haskell extension to use the HLS provided by the devshell
+   instead of managing its own. Add this to your VS Code settings:
+
+   ```json
+   { "haskell.manageHLS": "PATH" }
+   ```
+
+   Optionally, enable semantic highlighting (richer colours for types,
+   type variables, etc.):
+
+   ```json
+   { "haskell.plugin.semanticTokens.globalOn": true }
+   ```
+
+3. Allow direnv for this project and restart HLS:
+
+   ```
+   direnv allow
+   ```
+
+   Then in VS Code: `Ctrl+Shift+P` → _Haskell: Restart Haskell LSP Server_.
+
+> **Note:** The first HLS session takes roughly 10 minutes while `hie-bios`
+> builds all dependencies via `cabal v2-repl`. Subsequent loads use the
+> cache at `~/.cache/hie-bios/`.

flake.nix

@@ -2,14 +2,26 @@
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
     flake-parts.url = "github:hercules-ci/flake-parts";
-    treefmt-nix.url = "github:numtide/treefmt-nix";
-    treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
-    flake-compat.url = "github:input-output-hk/flake-compat";
-    flake-compat.flake = false;
-    cardano-node.url = "github:IntersectMBO/cardano-node/10.6.2";
-    cardano-node.flake = false; # otherwise, +2k dependencies we don’t really use
-    nix-bundle-exe.url = "github:3noch/nix-bundle-exe";
-    nix-bundle-exe.flake = false;
+    treefmt-nix = {
+      url = "github:numtide/treefmt-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    flake-compat = {
+      url = "github:input-output-hk/flake-compat";
+      flake = false;
+    };
+    devshell = {
+      url = "github:numtide/devshell";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    cardano-node = {
+      url = "github:IntersectMBO/cardano-node/10.6.2";
+      flake = false; # otherwise, +2k dependencies we don’t really use
+    };
+    nix-bundle-exe = {
+      url = "github:3noch/nix-bundle-exe";
+      flake = false;
+    };
   };
 
   outputs = inputs: let
@@ -18,6 +30,7 @@
     inputs.flake-parts.lib.mkFlake {inherit inputs;} ({config, ...}: {
       imports = [
         inputs.treefmt-nix.flakeModule
+        inputs.devshell.flakeModule
       ];
 
       flake.internal =
@@ -29,29 +42,31 @@
         );
 
       systems = ["x86_64-linux" "aarch64-linux" "aarch64-darwin" "x86_64-darwin"];
-      perSystem = {
-        config,
-        system,
-        pkgs,
-        ...
-      }: {
-        packages = let
-          internal = inputs.self.internal.${system};
-        in
+      perSystem = {system, ...}: let
+        internal = inputs.self.internal.${system};
+      in {
+        packages =
           {
             default = internal.defaultPackage;
           }
           // (lib.optionalAttrs (system == "x86_64-linux") {
             default-x86_64-windows = inputs.self.internal.x86_64-windows.defaultPackage;
           });
 
+        devshells.default = internal.devShell.new;
+
         treefmt = {pkgs, ...}: {
           projectRootFile = "flake.nix";
-          programs.alejandra.enable = true; # Nix
-          programs.ormolu.enable = true; # Haskell
-          programs.cabal-fmt.enable = true;
-          programs.shfmt.enable = true;
-          programs.yamlfmt.enable = pkgs.system != "x86_64-darwin"; # a treefmt-nix+yamlfmt bug on Intel Macs
+          programs = {
+            alejandra.enable = true; # Nix
+            ormolu.enable = true; # Haskell
+            cabal-fmt.enable = true;
+            prettier.enable = true; # Markdown, JSON, …
+            shfmt.enable = true;
+            ruff-check.enable = true; # Python
+            ruff-format.enable = true; # Python
+            yamlfmt.enable = pkgs.system != "x86_64-darwin"; # a treefmt-nix+yamlfmt bug on Intel Macs
+          };
         };
       };
 
@@ -60,7 +75,7 @@
           testgen-hs = lib.genAttrs (config.systems ++ ["x86_64-windows"]) (
             targetSystem: inputs.self.internal.${targetSystem}.hydraPackage
           );
-          inherit (inputs.self) checks;
+          inherit (inputs.self) checks devShells;
         };
       in
         allJobs