nixpkgs/pkgs/lib/lists.nix
Eelco Dolstra 6408a6a6fd * Trivial function to create a singleton list. Can reduce unnecessary
indentation in expressions like

    environent.extraJobs =
      [ { name = "foo";
          job =
            ''
              bla bla
            '';
        }
      ];

  which becomes

    environent.extraJobs = singleton
      { name = "foo";
        job =
          ''
            bla bla
          '';
      };

svn path=/nixpkgs/trunk/; revision=15892
2009-06-08 22:42:42 +00:00

125 lines
3.6 KiB
Nix

# General list operations.
rec {
inherit (builtins) head tail isList;
# Create a list consisting of a single element. `singleton x' is
# sometimes more convenient with respect to indentation than `[x]'
# when x spans multiple lines.
singleton = x: [x];
# "Fold" a binary function `op' between successive elements of
# `list' with `nul' as the starting value, i.e., `fold op nul [x_1
# x_2 ... x_n] == op x_1 (op x_2 ... (op x_n nul))'. (This is
# Haskell's foldr).
fold = op: nul: list:
if list == []
then nul
else op (head list) (fold op nul (tail list));
# Left fold: `fold op nul [x_1 x_2 ... x_n] == op (... (op (op nul
# x_1) x_2) ... x_n)'.
foldl = op: nul: list:
if list == []
then nul
else foldl op (op nul (head list)) (tail list);
# Concatenate a list of lists.
concatLists = fold (x: y: x ++ y) [];
# Map and concatenate the result.
concatMap = f: list: concatLists (map f list);
# Flatten the argument into a single list; that is, nested lists are
# spliced into the top-level lists. E.g., `flatten [1 [2 [3] 4] 5]
# == [1 2 3 4 5]' and `flatten 1 == [1]'.
flatten = x:
if isList x
then fold (x: y: (flatten x) ++ y) [] x
else [x];
# Filter a list using a predicate; that is, return a list containing
# every element from `list' for which `pred' returns true.
filter = pred: list:
fold (x: y: if pred x then [x] ++ y else y) [] list;
# Return true if `list' has an element `x':
elem = x: list: fold (a: bs: x == a || bs) false list;
# Find the sole element in the list matching the specified
# predicate, returns `default' if no such element exists, or
# `multiple' if there are multiple matching elements.
findSingle = pred: default: multiple: list:
let found = filter pred list;
in if found == [] then default
else if tail found != [] then multiple
else head found;
# Return true iff function `pred' returns true for at least element
# of `list'.
any = pred: list:
if list == [] then false
else if pred (head list) then true
else any pred (tail list);
# Return true iff function `pred' returns true for all elements of
# `list'.
all = pred: list:
if list == [] then true
else if pred (head list) then all pred (tail list)
else false;
# Return true if each element of a list is equal, false otherwise.
eqLists = xs: ys:
if xs == [] && ys == [] then true
else if xs == [] || ys == [] then false
else head xs == head ys && eqLists (tail xs) (tail ys);
# Return a singleton list or an empty list, depending on a boolean
# value. Useful when building lists with optional elements
# (e.g. `++ optional (system == "i686-linux") flashplayer').
optional = cond: elem: if cond then [elem] else [];
# Return a list or an empty list, dependening on a boolean value.
optionals = cond: elems: if cond then elems else [];
# If argument is a list, return it; else, wrap it in a singleton
# list. If you're using this, you should almost certainly
# reconsider if there isn't a more "well-typed" approach.
toList = x: if builtins.isList x then x else [x];
# Return a list of integers from `first' up to and including `last'.
range = first: last:
if builtins.lessThan last first
then []
else [first] ++ range (builtins.add first 1) last;
# Partition the elements of a list in two lists, `right' and
# `wrong', depending on the evaluation of a predicate.
partition = pred:
fold (h: t:
if pred h
then { right = [h] ++ t.right; wrong = t.wrong; }
else { right = t.right; wrong = [h] ++ t.wrong; }
) { right = []; wrong = []; };
}