nixpkgs/lib/modules.nix

with import ./lists.nix;
with import ./trivial.nix;
with import ./attrsets.nix;
with import ./options.nix;
with import ./debug.nix;
with import ./types.nix;

rec {

  /* Evaluate a set of modules.  The result is a set of two
     attributes: ‘options’: the nested set of all option declarations,
     and ‘config’: the nested set of all option values. */
  evalModules = { modules, prefix ? [], args ? {}, check ? true }:
    let
      args' = args // { lib = import ./.; } // result;
      closed = closeModules modules args';
      # Note: the list of modules is reversed to maintain backward
      # compatibility with the old module system.  Not sure if this is
      # the most sensible policy.
      options = mergeModules prefix (reverseList closed);
      # Traverse options and extract the option values into the final
      # config set.  At the same time, check whether all option
      # definitions have matching declarations.
      config = yieldConfig prefix options;
      yieldConfig = prefix: set:
        let res = removeAttrs (mapAttrs (n: v:
          if isOption v then v.value
          else yieldConfig (prefix ++ [n]) v) set) ["_definedNames"];
        in
        if check && set ? _definedNames then
          fold (m: res:
            fold (name: res:
              if hasAttr name set then res else throw "The option `${showOption (prefix ++ [name])}' defined in `${m.file}' does not exist.")
              res m.names)
            res set._definedNames
        else
          res;
      result = { inherit options config; };
    in result;

  /* Close a set of modules under the ‘imports’ relation. */
  closeModules = modules: args:
    let
      toClosureList = file: parentKey: imap (n: x:
        if isAttrs x || isFunction x then
          unifyModuleSyntax file "${parentKey}:anon-${toString n}" (applyIfFunction x args)
        else
          unifyModuleSyntax (toString x) (toString x) (applyIfFunction (import x) args));
    in
      builtins.genericClosure {
        startSet = toClosureList unknownModule "" modules;
        operator = m: toClosureList m.file m.key m.imports;
      };

  /* Massage a module into canonical form, that is, a set consisting
     of ‘options’, ‘config’ and ‘imports’ attributes. */
  unifyModuleSyntax = file: key: m:
    if m ? config || m ? options then
      let badAttrs = removeAttrs m ["imports" "options" "config" "key" "_file"]; in
      if badAttrs != {} then
        throw "Module `${key}' has an unsupported attribute `${head (attrNames badAttrs)}'."
      else
        { file = m._file or file;
          key = toString m.key or key;
          imports = m.imports or [];
          options = m.options or {};
          config = m.config or {};
        }
    else
      { file = m._file or file;
        key = toString m.key or key;
        imports = m.require or [] ++ m.imports or [];
        options = {};
        config = removeAttrs m ["key" "_file" "require" "imports"];
      };

  applyIfFunction = f: arg: if isFunction f then f arg else f;

  /* Merge a list of modules.  This will recurse over the option
     declarations in all modules, combining them into a single set.
     At the same time, for each option declaration, it will merge the
     corresponding option definitions in all machines, returning them
     in the ‘value’ attribute of each option. */
  mergeModules = prefix: modules:
    mergeModules' prefix modules
      (concatMap (m: map (config: { inherit (m) file; inherit config; }) (pushDownProperties m.config)) modules);

  mergeModules' = prefix: options: configs:
    listToAttrs (map (name: {
      # We're descending into attribute ‘name’.
      inherit name;
      value =
        let
          loc = prefix ++ [name];
          # Get all submodules that declare ‘name’.
          decls = concatLists (map (m:
            if hasAttr name m.options
              then [ { inherit (m) file; options = getAttr name m.options; } ]
              else []
            ) options);
          # Get all submodules that define ‘name’.
          defns = concatLists (map (m:
            if hasAttr name m.config
              then map (config: { inherit (m) file; inherit config; })
                (pushDownProperties (getAttr name m.config))
              else []
            ) configs);
          nrOptions = count (m: isOption m.options) decls;
          # Process mkMerge and mkIf properties.
          defns' = concatMap (m:
            if hasAttr name m.config
              then map (m': { inherit (m) file; value = m'; }) (dischargeProperties (getAttr name m.config))
              else []
            ) configs;
        in
          if nrOptions == length decls then
            let opt = fixupOptionType loc (mergeOptionDecls loc decls);
            in evalOptionValue loc opt defns'
          else if nrOptions != 0 then
            let
              firstOption = findFirst (m: isOption m.options) "" decls;
              firstNonOption = findFirst (m: !isOption m.options) "" decls;
            in
              throw "The option `${showOption loc}' in `${firstOption.file}' is a prefix of options in `${firstNonOption.file}'."
          else
            mergeModules' loc decls defns;
    }) (concatMap (m: attrNames m.options) options))
    // { _definedNames = map (m: { inherit (m) file; names = attrNames m.config; }) configs; };

  /* Merge multiple option declarations into a single declaration.  In
     general, there should be only one declaration of each option.
     The exception is the ‘options’ attribute, which specifies
     sub-options.  These can be specified multiple times to allow one
     module to add sub-options to an option declared somewhere else
     (e.g. multiple modules define sub-options for ‘fileSystems’).

     'loc' is the list of attribute names where the option is located.

     'opts' is a list of modules.  Each module has an options attribute which
     correspond to the definition of 'loc' in 'opt.file'. */
  mergeOptionDecls = loc: opts:
    fold (opt: res:
      if opt.options ? default && res ? default ||
         opt.options ? example && res ? example ||
         opt.options ? description && res ? description ||
         opt.options ? apply && res ? apply ||
         opt.options ? type && res ? type
      then
        throw "The option `${showOption loc}' in `${opt.file}' is already declared in ${showFiles res.declarations}."
      else
        let
          /* Add the modules of the current option to the list of modules
             already collected.  The options attribute except either a list of
             submodules or a submodule. For each submodule, we add the file of the
             current option declaration as the file use for the submodule.  If the
             submodule defines any filename, then we ignore the enclosing option file. */
          options' = toList opt.options.options;
          coerceOption = file: opt:
            if isFunction opt then args: { _file = file; } // (opt args)
            else args: { _file = file; options = opt; };
          submodules =
            if opt.options ? options
            then map (coerceOption opt.file) options' ++ res.options
            else res.options;
        in opt.options // res //
          { declarations = [opt.file] ++ res.declarations;
            options = submodules;
          }
    ) { inherit loc; declarations = []; options = []; } opts;

  /* Merge all the definitions of an option to produce the final
     config value. */
  evalOptionValue = loc: opt: defs:
    let
      # Process mkOverride properties, adding in the default
      # value specified in the option declaration (if any).
      defsFinal' = filterOverrides
        ((if opt ? default then [{ file = head opt.declarations; value = mkOptionDefault opt.default; }] else []) ++ defs);
      # Sort mkOrder properties.
      defsFinal =
        # Avoid sorting if we don't have to.
        if any (def: def.value._type or "" == "order") defsFinal'
        then sortProperties defsFinal'
        else defsFinal';
      files = map (def: def.file) defsFinal;
      # Type-check the remaining definitions, and merge them if
      # possible.
      merged =
        if defsFinal == [] then
          throw "The option `${showOption loc}' is used but not defined."
        else
          fold (def: res:
            if opt.type.check def.value then res
            else throw "The option value `${showOption loc}' in `${def.file}' is not a ${opt.type.name}.")
            (opt.type.merge loc defsFinal) defsFinal;
      # Finally, apply the ‘apply’ function to the merged
      # value.  This allows options to yield a value computed
      # from the definitions.
      value = (opt.apply or id) merged;
    in opt //
      { value = addErrorContext "while evaluating the option `${showOption loc}':" value;
        definitions = map (def: def.value) defsFinal;
        isDefined = defsFinal != [];
        inherit files;
      };

  /* Given a config set, expand mkMerge properties, and push down the
     other properties into the children.  The result is a list of
     config sets that do not have properties at top-level.  For
     example,

       mkMerge [ { boot = set1; } (mkIf cond { boot = set2; services = set3; }) ]

     is transformed into

       [ { boot = set1; } { boot = mkIf cond set2; services = mkIf cond set3; } ].

     This transform is the critical step that allows mkIf conditions
     to refer to the full configuration without creating an infinite
     recursion.
  */
  pushDownProperties = cfg:
    if cfg._type or "" == "merge" then
      concatMap pushDownProperties cfg.contents
    else if cfg._type or "" == "if" then
      map (mapAttrs (n: v: mkIf cfg.condition v)) (pushDownProperties cfg.content)
    else if cfg._type or "" == "override" then
      map (mapAttrs (n: v: mkOverride cfg.priority v)) (pushDownProperties cfg.content)
    else # FIXME: handle mkOrder?
      [ cfg ];

  /* Given a config value, expand mkMerge properties, and discharge
     any mkIf conditions.  That is, this is the place where mkIf
     conditions are actually evaluated.  The result is a list of
     config values.  For example, ‘mkIf false x’ yields ‘[]’,
     ‘mkIf true x’ yields ‘[x]’, and

       mkMerge [ 1 (mkIf true 2) (mkIf true (mkIf false 3)) ]

     yields ‘[ 1 2 ]’.
  */
  dischargeProperties = def:
    if def._type or "" == "merge" then
      concatMap dischargeProperties def.contents
    else if def._type or "" == "if" then
      if def.condition then
        dischargeProperties def.content
      else
        [ ]
    else
      [ def ];

  /* Given a list of config values, process the mkOverride properties,
     that is, return the values that have the highest (that is,
     numerically lowest) priority, and strip the mkOverride
     properties.  For example,

       [ { file = "/1"; value = mkOverride 10 "a"; }
         { file = "/2"; value = mkOverride 20 "b"; }
         { file = "/3"; value = "z"; }
         { file = "/4"; value = mkOverride 10 "d"; }
       ]

     yields

       [ { file = "/1"; value = "a"; }
         { file = "/4"; value = "d"; }
       ]

     Note that "z" has the default priority 100.
  */
  filterOverrides = defs:
    let
      defaultPrio = 100;
      getPrio = def: if def.value._type or "" == "override" then def.value.priority else defaultPrio;
      min = x: y: if builtins.lessThan x y then x else y;
      highestPrio = fold (def: prio: min (getPrio def) prio) 9999 defs;
      strip = def: if def.value._type or "" == "override" then def // { value = def.value.content; } else def;
    in concatMap (def: if getPrio def == highestPrio then [(strip def)] else []) defs;

  /* Sort a list of properties.  The sort priority of a property is
     1000 by default, but can be overriden by wrapping the property
     using mkOrder. */
  sortProperties = defs:
    let
      strip = def:
        if def.value._type or "" == "order"
        then def // { value = def.value.content; inherit (def.value) priority; }
        else def;
      defs' = map strip defs;
      compare = a: b: (a.priority or 1000) < (b.priority or 1000);
    in sort compare defs';

  /* Hack for backward compatibility: convert options of type
     optionSet to configOf.  FIXME: remove eventually. */
  fixupOptionType = loc: opt:
    let
      options = opt.options or
        (throw "Option `${showOption loc'}' has type optionSet but has no option attribute, in ${showFiles opt.declarations}.");
      f = tp:
        if tp.name == "option set" || tp.name == "submodule" then
          throw "The option ${showOption loc} uses submodules without a wrapping type, in ${showFiles opt.declarations}."
        else if tp.name == "attribute set of option sets" then types.attrsOf (types.submodule options)
        else if tp.name == "list or attribute set of option sets" then types.loaOf (types.submodule options)
        else if tp.name == "list of option sets" then types.listOf (types.submodule options)
        else if tp.name == "null or option set" then types.nullOr (types.submodule options)
        else tp;
    in opt // { type = f (opt.type or types.unspecified); };


  /* Properties. */

  mkIf = condition: content:
    { _type = "if";
      inherit condition content;
    };

  mkAssert = assertion: message: content:
    mkIf
      (if assertion then true else throw "\nFailed assertion: ${message}")
      content;

  mkMerge = contents:
    { _type = "merge";
      inherit contents;
    };

  mkOverride = priority: content:
    { _type = "override";
      inherit priority content;
    };

  mkOptionDefault = mkOverride 1001; # priority of option defaults
  mkDefault = mkOverride 1000; # used in config sections of non-user modules to set a default
  mkForce = mkOverride 50;
  mkVMOverride = mkOverride 10; # used by ‘nixos-rebuild build-vm’

  mkStrict = builtins.trace "`mkStrict' is obsolete; use `mkOverride 0' instead." (mkOverride 0);

  mkFixStrictness = id; # obsolete, no-op

  mkOrder = priority: content:
    { _type = "order";
      inherit priority content;
    };

  mkBefore = mkOrder 500;
  mkAfter = mkOrder 1500;


  /* Compatibility. */
  fixMergeModules = modules: args: evalModules { inherit modules args; check = false; };

}