{
  description                  = "sisu parser & document generator";
  inputs = {
    nixpkgs.url                = "github:NixOS/nixpkgs/nixpkgs-unstable"; # "github:nixos/nixpkgs";
    flake-utils.url            = "github:numtide/flake-utils";
    sisu.url                  = "git://git.sisudoc.org/software/sisu";
    #git clone git://git.sisudoc.org/software/sisu
    sisu.inputs.nixpkgs.follows = "nixpkgs";
    sisu.flake                = true;
  };
  outputs = { self, nixpkgs, flake-utils, sisu } @inputs:
    let
      version                  = "0.7.3";
      shell                    = ./shell.nix; # ./default.nix;
      devEnv                   = ./.envrc; # ./shell.nix; # ./default.nix;
      supportedSystems         = [ "x86_64-linux" ]; # [ "x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin" ];
      forAllSystems            = nixpkgs.lib.genAttrs supportedSystems;
      nixpkgsFor               = forAllSystems (system: import nixpkgs { inherit system;  }); # nixpkgs instantiated for supported system types.
    in {
      packages = forAllSystems (system:
        let pkgs               = nixpkgsFor.${system};
        in {
          default              = sisu.packages.${system}.default;
          #vendorSha256         = "sha256-0Q00000000000000000000000000000000000000000=";
      });
      apps = forAllSystems (system: {
        default = {
          type                 = "app";
          program              = "${self.packages.${system}.default}/bin/sisu";
        };
      });
      devShells = forAllSystems (system:
        let
          pkgs = nixpkgsFor.${system};
          shellHook = ''
            export Date=`date "+%Y%m%d"`
            ### set local values in .envrc-local (or modify here if you must)
            # export sisuPROJ=~/grotto/repo/git.repo/projects/doc-reform/code/software/sisu
            # export sisuDOC=~/grotto/repo/git.repo/projects/project-sisu/doc-reform-markup/sisu-markup-samples
            # export sisu=/srv/www/sisu
            # export sisu=/var/www/sqlite
            # export sisu=/srv/www/sisu/sqlite
            export sisu=$sisuPROJ/src
            export sisu=$sisuPROJ/result/lib
            export sisu=$sisuDOC/markup/pod
            export sisu=$sisuOUT/$sisuVER
            export sisu='http://localhost/sisu_search'
            # export sisuSearchActionRemote='https://sisudoc.org/sisu_search'
            export sisuCGIform='sisu_search'
            export sisuSQLdb='sisusearch.db'
            export PROG_VER_GIT="`git describe --long --tags | sed -e 's/^[ a-z_-]\+\([0-9.]\+\)/\1/;s/\([^-]*-g\)/r\1/;s/-/./g'`"
            export PROG_VER_DECLARED="`cat ./views/version.txt | grep --color=never "enum" | sed 's/.\+(\([0-9]\+\),[ \t]\+\([0-9]\+\),[ \t]\+\([0-9]\+\)[ \t]*).\+/\1.\2.\3/g'`"
            export sisuNixHelp="cat ./.env/nix-commands"
            export sisuTest="nix shell .#default --print-build-logs --command sisu --very-verbose --epub --html --html-link-search --html-link-curate --curate --output=\"\$sisuOUT\" $sisuPOD/*"
            export sisuHtml="sisu --very-verbose --html --html-link-search --html-link-curate --curate --sqlite-update --sqlite-db-filename=\"sisu.search.db\" --cgi-sqlite-search-filename=\"sisu_search\" --sqlite-db-path=\"\$sisuDBpath\" --output=\"\$sisuOUT\" $sisuPOD/*"
            export sisuEpub="sisu --very-verbose --epub --output=\"\$sisuOUT\" $sisuPOD/*"
            export sisuLatex="sisu --very-verbose --latex --output=\"\$sisuOUT\" $sisuPOD/*"
            export sisuPdf="sisu --very-verbose --pdf --output=\"\$sisuOUT\" $sisuPOD/*"
            export sisuSqliteCreateDB="sisu --very-verbose --sqlite-db-recreate --sqlite-db-path=\"$sisuDBpath\" --sqlite-db-filename=\"sisu.search.db\""
            export sisuSqlite="sisu --very-verbose --sqlite-discrete --sqlite-db-path=\"$sisuDBpath\" --sqlite-db-filename=\"sisu.search.db\" --output=\"\$sisuOUT\" $sisuPOD/*"
            #export sisuSqlite="sisu --very-verbose --sqlite-update --sqlite-db-path=\"$sisuDBpath\" --sqlite-db-filename=\"sisu.search.db\" --output=\"\$sisuOUT\" $sisuPOD/*"
          '';
        in with pkgs; {
          devShell = mkShell {
            name               = "sisu dev shell default";
            inherit shell;
            inherit devEnv;
            packages = [ sqlite ];
            inherit shellHook;
          };
          devShell-epub = mkShell {
            name               = "sisu dev shell for epub output";
            inherit shell;
            inherit devEnv;
            packages = [
              ruby_3_1
              rubyPackages_3_1.rake
              rubyPackages_3_1.sqlite3
              rubyPackages_3_1.thor
              sqlite
              zip
              unzip
              xz
              libxml2
              html-tidy
              xmlstarlet
              epubcheck
              ebook_tools
              libxml2
              html-tidy
              xmlstarlet
              epubcheck
              ebook_tools
              epr
              sigil
              calibre #(suite includes: ebook-viewer)
              foliate
              nixFlakes
              validatePkgConfig
              jq
              git
            ];
            inherit shellHook;
          };
          devShell-html = mkShell {
            name               = "sisu dev shell for latex & pdf output";
            inherit shell;
            inherit devEnv;
            packages = [
              ruby_3_1
              rubyPackages_3_1.rake
              rubyPackages_3_1.sqlite3
              rubyPackages_3_1.thor
              sqlite
              zip
              unzip
              xz
              nixFlakes
              validatePkgConfig
              jq
              git
            ];
            inherit shellHook;
          };
          devShell-latex-pdf = mkShell {
            name               = "sisu dev shell for latex & pdf output";
            inherit shell;
            inherit devEnv;
            packages     = [
              ruby_3_1
              rubyPackages_3_1.rake
              rubyPackages_3_1.sqlite3
              rubyPackages_3_1.thor
              sqlite
              zip
              unzip
              xz
              source-sans-pro
              source-serif-pro
              source-code-pro
              texlive.combined.scheme-full
              nixFlakes
              validatePkgConfig
              jq
              git
            ];
            inherit shellHook;
          };
          devShell-sqlite = mkShell {
            name               = "sisu dev shell for latex & pdf output";
            inherit shell;
            inherit devEnv;
            packages = [
              ruby_3_1
              rubyPackages_3_1.rake
              rubyPackages_3_1.sqlite3
              rubyPackages_3_1.thor
              sqlite
              zip
              unzip
              xz
              nixFlakes
              validatePkgConfig
              jq
              git
            ];
            inherit shellHook;
          };
          devShell-i18n = mkShell {
            name               = "sisu dev shell internationalization, po4a";
            inherit shell;
            inherit devEnv;
            packages = [
              ruby_3_1
              rubyPackages_3_1.rake
              rubyPackages_3_1.sqlite3
              rubyPackages_3_1.thor
              sqlite
              perl534Packages.Po4a
              nixFlakes
              validatePkgConfig
              jq
              git
            ];
            inherit shellHook;
          };
      });
  };
}