vscode.haskell: add module

- Haskell IDE Engine integration
- Syntax highlighting

modules/misc/news.nix

@@ -1065,6 +1065,17 @@ in
           A new module is available: 'programs.alacritty'.
         '';
       }
+
+      {
+        time = "2019-04-26T22:53:48+00:00";
+        condition = config.programs.vscode.enable;
+        message = ''
+          A new module is available: 'programs.vscode.haskell'.
+
+          Enable to add Haskell IDE Engine and syntax highlighting
+          support to your VSCode.
+        '';
+      }
     ];
   };
 }

modules/modules.nix

@@ -85,6 +85,7 @@ let
     (loadModule ./programs/urxvt.nix { })
     (loadModule ./programs/vim.nix { })
     (loadModule ./programs/vscode.nix { })
+    (loadModule ./programs/vscode/haskell.nix { })
     (loadModule ./programs/zathura.nix { })
     (loadModule ./programs/zsh.nix { })
     (loadModule ./services/blueman-applet.nix { })

modules/programs/vscode/haskell.nix

@@ -0,0 +1,66 @@
+{ pkgs, config, lib, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.vscode.haskell;
+
+  defaultHieNixExe = hie-nix.hies + "/bin/hie-wrapper";
+  defaultHieNixExeText = literalExample
+    "\"\${pkgs.hie-nix.hies}/bin/hie-wrapper\"";
+
+  hie-nix = pkgs.hie-nix or (abort ''
+    vscode.haskell: pkgs.hie-nix missing. Please add an overlay such as:
+    ${exampleOverlay}
+  '');
+
+  exampleOverlay = ''
+    nixpkgs.overlays = [
+      (self: super: { hie-nix = import ~/src/hie-nix {}; })
+    ]
+  '';
+
+in
+
+{
+  options.programs.vscode.haskell = {
+    enable = mkEnableOption "Haskell integration for Visual Studio Code";
+
+    hie.enable = mkOption {
+      type = types.bool;
+      default = true;
+      description = "Whether to enable Haskell IDE engine integration.";
+    };
+
+    hie.executablePath = mkOption {
+      type = types.path;
+      default = defaultHieNixExe;
+      defaultText = defaultHieNixExeText;
+      description = ''
+        The path to the Haskell IDE Engine executable.
+        </para><para>
+        Because hie-nix is not packaged in Nixpkgs, you need to add it as an
+        overlay or set this option. Example overlay configuration:
+        <programlisting language="nix">${exampleOverlay}</programlisting>
+      '';
+      example = literalExample ''
+        (import ~/src/haskell-ide-engine {}).hies + "/bin/hie-wrapper";
+      '';
+    };
+  };
+
+  config = mkIf cfg.enable {
+    programs.vscode.userSettings = mkIf cfg.hie.enable {
+      "languageServerHaskell.enableHIE" = true;
+      "languageServerHaskell.hieExecutablePath" = cfg.hie.executablePath;
+    };
+
+    programs.vscode.extensions =
+      [
+        pkgs.vscode-extensions.justusadam.language-haskell
+      ]
+      ++ lib.optional cfg.hie.enable
+        pkgs.vscode-extensions.alanz.vscode-hie-server;
+  };
+}