nixpkgs/pkgs/lib/lists.nix

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# General list operations.
rec {
inherit (builtins) elemAt head tail length isList add sub lessThan;
# 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 =
if builtins ? elemAt
then op: nul: list:
let
len = length list;
fold' = n:
if n == len
then nul
else op (elemAt list n) (fold' (add n 1));
in fold' 0
else op: nul:
let fold' = list:
if list == []
then nul
else op (head list) (fold' (tail list));
in fold';
# Left fold: `fold op nul [x_1 x_2 ... x_n] == op (... (op (op nul
# x_1) x_2) ... x_n)'.
foldl =
if builtins ? elemAt
then op: nul: list:
let
len = length list;
foldl' = n:
if n == minus1
then nul
else op (foldl' (sub n 1)) (elemAt list n);
in foldl' (sub (length list) 1)
else op:
let foldl' = nul: list:
if list == []
then nul
else foldl' (op nul (head list)) (tail list);
in foldl';
minus1 = sub 0 1;
# map with index: `imap (i: v: "${v}-${toString i}") ["a" "b"] ==
# ["a-1" "b-2"]'
imap = f: list:
zipListsWith f (range 1 (length list)) list;
# Concatenate a list of lists.
concatLists = builtins.concatLists or (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 =
builtins.filter or
(pred: list:
fold (x: y: if pred x then [x] ++ y else y) [] list);
# Remove elements equal to 'e' from a list. Useful for buildInputs.
remove = e: filter (x: x != e);
# Given two lists, removes all elements of the first list from the second list
removeList = l: filter (x: elem x l);
# Return true if `list' has an element `x'.
elem =
builtins.elem or
(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;
# 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;
# 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;
# 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 = []; };
zipListsWith = f: fst: snd:
if fst != [] && snd != [] then
[ (f (head fst) (head snd)) ]
++ zipListsWith f (tail fst) (tail snd)
else [];
zipLists = zipListsWith (fst: snd: { inherit fst snd; });
# Reverse the order of the elements of a list.
reverseList = l:
let reverse_ = accu: l:
if l == [] then accu
else reverse_ ([(head l)] ++ accu) (tail l);
in reverse_ [] l;
# 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
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# This implementation only has one element list on the left hand
# side of the concatenation operator.
qs = l: concat:
if l == [] then concat
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else if length l == 1 then l ++ concat
else let
part = partition (strictLess (head l)) (tail l);
in
qs part.wrong ([(head l)] ++ qs part.right []);
in
qs list [];
# Return the first (at most) N elements of a list.
take = count: list:
if list == [] || count == 0 then []
else [ (head list) ] ++ take (builtins.sub count 1) (tail list);
# Remove the first (at most) N elements of a list.
drop = count: list:
if count == 0 then list
else drop (builtins.sub count 1) (tail list);
last = list:
assert list != [];
let loop = l: if tail l == [] then head l else loop (tail l); in
loop list;
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# Zip two lists together.
zipTwoLists = xs: ys:
if xs != [] && ys != [] then
[ {first = head xs; second = head ys;} ]
++ zipTwoLists (tail xs) (tail ys)
else [];
}