nixpkgs/lib/lists.nix

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# General list operations.
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with import ./trivial.nix;
let
inc = builtins.add 1;
dec = n: builtins.sub n 1;
in rec {
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inherit (builtins) head tail length isList elemAt concatLists filter elem;
# 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:
let
len = length list;
fold' = n:
if n == len
then nul
else op (elemAt list n) (fold' (inc n));
in fold' 0;
# 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:
let
len = length list;
foldl' = n:
if n == minus1
then nul
else op (foldl' (dec n)) (elemAt list n);
in foldl' (dec (length list));
minus1 = dec 0;
# map with index: `imap (i: v: "${v}-${toString i}") ["a" "b"] ==
# ["a-1" "b-2"]'
imap = f: list:
let
len = length list;
imap' = n:
if n == len
then []
else [ (f (inc n) (elemAt list n)) ] ++ imap' (inc n);
in imap' 0;
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# 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];
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# Remove elements equal to 'e' from a list. Useful for buildInputs.
remove = e: filter (x: x != e);
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# 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; len = length found;
in if len == 0 then default
else if len != 1 then multiple
else head found;
# Find the first element in the list matching the specified
# predicate or returns `default' if no such element exists.
findFirst = pred: default: list:
let found = filter pred list;
in if found == [] then default else head found;
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# Return true iff function `pred' returns true for at least element
# of `list'.
any = pred: fold (x: y: if pred x then true else y) false;
# Return true iff function `pred' returns true for all elements of
# `list'.
all = pred: fold (x: y: if pred x then y else false) true;
# Count how many times function `pred' returns true for the elements
# of `list'.
count = pred: fold (x: c: if pred x then inc c else c) 0;
# 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.
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toList = x: if isList x then x else [x];
# Return a list of integers from `first' up to and including `last'.
range = first: last:
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if lessThan last first
then []
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else [first] ++ range (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 = []; };
zipListsWith = f: fst: snd:
let
len1 = length fst;
len2 = length snd;
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len = if lessThan len1 len2 then len1 else len2;
zipListsWith' = n:
if n != len then
[ (f (elemAt fst n) (elemAt snd n)) ]
++ zipListsWith' (inc n)
else [];
in zipListsWith' 0;
zipLists = zipListsWith (fst: snd: { inherit fst snd; });
# Reverse the order of the elements of a list. FIXME: O(n^2)!
reverseList = fold (e: acc: acc ++ [ e ]) [];
# Sort a list based on a comparator function which compares two
# elements and returns true if the first argument is strictly below
# the second argument. The returned list is sorted in an increasing
# order. The implementation does a quick-sort.
sort = strictLess: list:
let
len = length list;
first = head list;
pivot' = n: acc@{ left, right }: let el = elemAt list n; next = pivot' (inc n); in
if n == len
then acc
else if strictLess first el
then next { inherit left; right = [ el ] ++ right; }
else
next { left = [ el ] ++ left; inherit right; };
pivot = pivot' 1 { left = []; right = []; };
in
if lessThan len 2 then list
else (sort strictLess pivot.left) ++ [ first ] ++ (sort strictLess pivot.right);
# Return the first (at most) N elements of a list.
take = count: list:
let
len = length list;
take' = n:
if n == len || n == count
then []
else
[ (elemAt list n) ] ++ take' (inc n);
in take' 0;
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# Remove the first (at most) N elements of a list.
drop = count: list:
let
len = length list;
drop' = n:
if n == minus1 || lessThan n count
then []
else
drop' (dec n) ++ [ (elemAt list n) ];
in drop' (dec len);
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# Return the last element of a list.
last = list:
assert list != []; elemAt list (dec (length list));
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# Zip two lists together.
zipTwoLists = xs: ys:
let
len1 = length xs;
len2 = length ys;
len = if lessThan len1 len2 then len1 else len2;
zipTwoLists' = n:
if n != len then
[ { first = elemAt xs n; second = elemAt ys n; } ]
++ zipTwoLists' (inc n)
else [];
in zipTwoLists' 0;
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deepSeqList = xs: y: if any (x: deepSeq x false) xs then y else y;
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}