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# 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;
# Remove elements '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 = 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: 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 = []; };
zipListsWith = f: fst: snd:
if fst != [] && snd != [] then
[ (f (head fst) (head snd)) ]
++ zipLists (tail fst) (tail snd)
else [];
zipLists = zipListsWith (fst: snd: { inherit fst snd; });
# invert 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 the `strictLess' function which compare the two
# elements and return 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
# This implementation only have one element lists on the left hand
# side of the concatenation operator.
qs = l: concat:
if l == [] then concat
else if tail l == [] then l ++ concat
else let
part = partition (strictLess (head l)) (tail l);
in
qs part.wrong ([(head l)] ++ qs part.right []);
in
qs list [];
# haskell's take: take 2 [1 2 3 4] yields [1 2]
take = count: list:
if list == [] || count == 0 then []
else [ (head list) ] ++ take (builtins.sub count 1) (tail list);
# haskell's drop. drop count elements from head of 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;
# Zip two lists together.
zipTwoLists = xs: ys:
if xs != [] && ys != [] then
[ {first = head xs; second = head ys;} ]
++ zipTwoLists (tail xs) (tail ys)
else [];
}
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