{ lib }:
let
    inherit (builtins) head tail isList isAttrs isInt attrNames;

in

with lib.lists;
with lib.attrsets;
with lib.strings;

rec {

  # returns default if env var is not set
  maybeEnv = name: default:
    let value = builtins.getEnv name; in
    if value == "" then default else value;

  defaultMergeArg = x : y: if builtins.isAttrs y then
    y
  else
    (y x);
  defaultMerge = x: y: x // (defaultMergeArg x y);
  foldArgs = merger: f: init: x:
    let arg = (merger init (defaultMergeArg init x));
        # now add the function with composed args already applied to the final attrs
        base = (setAttrMerge "passthru" {} (f arg)
                        ( z: z // {
                            function = foldArgs merger f arg;
                            args = (lib.attrByPath ["passthru" "args"] {} z) // x;
                          } ));
        withStdOverrides = base // {
          override = base.passthru.function;
        };
        in
          withStdOverrides;


  # shortcut for attrByPath ["name"] default attrs
  maybeAttrNullable = maybeAttr;

  # shortcut for attrByPath ["name"] default attrs
  maybeAttr = name: default: attrs: attrs.${name} or default;


  # Return the second argument if the first one is true or the empty version
  # of the second argument.
  ifEnable = cond: val:
    if cond then val
    else if builtins.isList val then []
    else if builtins.isAttrs val then {}
    # else if builtins.isString val then ""
    else if val == true || val == false then false
    else null;


  # Return true only if there is an attribute and it is true.
  checkFlag = attrSet: name:
        if name == "true" then true else
        if name == "false" then false else
        if (elem name (attrByPath ["flags"] [] attrSet)) then true else
        attrByPath [name] false attrSet ;


  # Input : attrSet, [ [name default] ... ], name
  # Output : its value or default.
  getValue = attrSet: argList: name:
  ( attrByPath [name] (if checkFlag attrSet name then true else
        if argList == [] then null else
        let x = builtins.head argList; in
                if (head x) == name then
                        (head (tail x))
                else (getValue attrSet
                        (tail argList) name)) attrSet );


  # Input : attrSet, [[name default] ...], [ [flagname reqs..] ... ]
  # Output : are reqs satisfied? It's asserted.
  checkReqs = attrSet: argList: condList:
  (
    fold lib.and true
      (map (x: let name = (head x); in

        ((checkFlag attrSet name) ->
        (fold lib.and true
        (map (y: let val=(getValue attrSet argList y); in
                (val!=null) && (val!=false))
        (tail x))))) condList));


  # This function has O(n^2) performance.
  uniqList = { inputList, acc ? [] }:
    let go = xs: acc:
             if xs == []
             then []
             else let x = head xs;
                      y = if elem x acc then [] else [x];
                  in y ++ go (tail xs) (y ++ acc);
    in go inputList acc;

  uniqListExt = { inputList,
                  outputList ? [],
                  getter ? (x: x),
                  compare ? (x: y: x==y) }:
        if inputList == [] then outputList else
        let x = head inputList;
            isX = y: (compare (getter y) (getter x));
            newOutputList = outputList ++
                (if any isX outputList then [] else [x]);
        in uniqListExt { outputList = newOutputList;
                         inputList = (tail inputList);
                         inherit getter compare;
                       };

  condConcat = name: list: checker:
        if list == [] then name else
        if checker (head list) then
                condConcat
                        (name + (head (tail list)))
                        (tail (tail list))
                        checker
        else condConcat
                name (tail (tail list)) checker;

  lazyGenericClosure = {startSet, operator}:
    let
      work = list: doneKeys: result:
        if list == [] then
          result
        else
          let x = head list; key = x.key; in
          if elem key doneKeys then
            work (tail list) doneKeys result
          else
            work (tail list ++ operator x) ([key] ++ doneKeys) ([x] ++ result);
    in
      work startSet [] [];

  innerModifySumArgs = f: x: a: b: if b == null then (f a b) // x else
        innerModifySumArgs f x (a // b);
  modifySumArgs = f: x: innerModifySumArgs f x {};


  innerClosePropagation = acc: xs:
    if xs == []
    then acc
    else let y  = head xs;
             ys = tail xs;
         in if ! isAttrs y
            then innerClosePropagation acc ys
            else let acc' = [y] ++ acc;
                 in innerClosePropagation
                      acc'
                      (uniqList { inputList = (maybeAttrNullable "propagatedBuildInputs" [] y)
                                           ++ (maybeAttrNullable "propagatedNativeBuildInputs" [] y)
                                           ++ ys;
                                  acc = acc';
                                }
                      );

  closePropagation = list: (uniqList {inputList = (innerClosePropagation [] list);});

  # calls a function (f attr value ) for each record item. returns a list
  mapAttrsFlatten = f: r: map (attr: f attr r.${attr}) (attrNames r);

  # attribute set containing one attribute
  nvs = name: value: listToAttrs [ (nameValuePair name value) ];
  # adds / replaces an attribute of an attribute set
  setAttr = set: name: v: set // (nvs name v);

  # setAttrMerge (similar to mergeAttrsWithFunc but only merges the values of a particular name)
  # setAttrMerge "a" [] { a = [2];} (x: x ++ [3]) -> { a = [2 3]; }
  # setAttrMerge "a" [] {         } (x: x ++ [3]) -> { a = [  3]; }
  setAttrMerge = name: default: attrs: f:
    setAttr attrs name (f (maybeAttr name default attrs));

  # Using f = a: b = b the result is similar to //
  # merge attributes with custom function handling the case that the attribute
  # exists in both sets
  mergeAttrsWithFunc = f: set1: set2:
    fold (n: set: if set ? ${n}
                        then setAttr set n (f set.${n} set2.${n})
                        else set )
           (set2 // set1) (attrNames set2);

  # merging two attribute set concatenating the values of same attribute names
  # eg { a = 7; } {  a = [ 2 3 ]; } becomes { a = [ 7 2 3 ]; }
  mergeAttrsConcatenateValues = mergeAttrsWithFunc ( a: b: (toList a) ++ (toList b) );

  # merges attributes using //, if a name exists in both attributes
  # an error will be triggered unless its listed in mergeLists
  # so you can mergeAttrsNoOverride { buildInputs = [a]; } { buildInputs = [a]; } {} to get
  # { buildInputs = [a b]; }
  # merging buildPhase doesn't really make sense. The cases will be rare where appending /prefixing will fit your needs?
  # in these cases the first buildPhase will override the second one
  # ! deprecated, use mergeAttrByFunc instead
  mergeAttrsNoOverride = { mergeLists ? ["buildInputs" "propagatedBuildInputs"],
                           overrideSnd ? [ "buildPhase" ]
                         }: attrs1: attrs2:
    fold (n: set:
        setAttr set n ( if set ? ${n}
            then # merge
              if elem n mergeLists # attribute contains list, merge them by concatenating
                then attrs2.${n} ++ attrs1.${n}
              else if elem n overrideSnd
                then attrs1.${n}
              else throw "error mergeAttrsNoOverride, attribute ${n} given in both attributes - no merge func defined"
            else attrs2.${n} # add attribute not existing in attr1
           )) attrs1 (attrNames attrs2);


  # example usage:
  # mergeAttrByFunc  {
  #   inherit mergeAttrBy; # defined below
  #   buildInputs = [ a b ];
  # } {
  #  buildInputs = [ c d ];
  # };
  # will result in
  # { mergeAttrsBy = [...]; buildInputs = [ a b c d ]; }
  # is used by defaultOverridableDelayableArgs and can be used when composing using
  # foldArgs, composedArgsAndFun or applyAndFun. Example: composableDerivation in all-packages.nix
  mergeAttrByFunc = x: y:
    let
          mergeAttrBy2 = { mergeAttrBy = lib.mergeAttrs; }
                      // (maybeAttr "mergeAttrBy" {} x)
                      // (maybeAttr "mergeAttrBy" {} y); in
    fold lib.mergeAttrs {} [
      x y
      (mapAttrs ( a: v: # merge special names using given functions
          if x ? ${a}
             then if y ? ${a}
               then v x.${a} y.${a} # both have attr, use merge func
               else x.${a} # only x has attr
             else y.${a} # only y has attr)
          ) (removeAttrs mergeAttrBy2
                         # don't merge attrs which are neither in x nor y
                         (filter (a: ! x ? ${a} && ! y ? ${a})
                                 (attrNames mergeAttrBy2))
            )
      )
    ];
  mergeAttrsByFuncDefaults = foldl mergeAttrByFunc { inherit mergeAttrBy; };
  mergeAttrsByFuncDefaultsClean = list: removeAttrs (mergeAttrsByFuncDefaults list) ["mergeAttrBy"];

  # sane defaults (same name as attr name so that inherit can be used)
  mergeAttrBy = # { buildInputs = concatList; [...]; passthru = mergeAttr; [..]; }
    listToAttrs (map (n: nameValuePair n lib.concat)
      [ "nativeBuildInputs" "buildInputs" "propagatedBuildInputs" "configureFlags" "prePhases" "postAll" "patches" ])
    // listToAttrs (map (n: nameValuePair n lib.mergeAttrs) [ "passthru" "meta" "cfg" "flags" ])
    // listToAttrs (map (n: nameValuePair n (a: b: "${a}\n${b}") ) [ "preConfigure" "postInstall" ])
  ;

  nixType = x:
      if isAttrs x then
          if x ? outPath then "derivation"
          else "attrs"
      else if lib.isFunction x then "function"
      else if isList x then "list"
      else if x == true then "bool"
      else if x == false then "bool"
      else if x == null then "null"
      else if isInt x then "int"
      else "string";

  /* deprecated:

     For historical reasons, imap has an index starting at 1.

     But for consistency with the rest of the library we want an index
     starting at zero.
  */
  imap = imap1;

  # Fake hashes. Can be used as hash placeholders, when computing hash ahead isn't trivial
  fakeHash = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
  fakeSha256 = "0000000000000000000000000000000000000000000000000000000000000000";
  fakeSha512 = "00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000";
}