{ pkgs, config, lib, ... }:

with lib;

let

  cfg = config.programs.vscode.haskell;

  defaultHieNixExe = hie-nix.hies + "/bin/hie-wrapper";
  defaultHieNixExeText =
    literalExpression ''"''${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 = literalExpression ''
        (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;
  };
}