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230 lines
6.3 KiB
Nix
230 lines
6.3 KiB
Nix
# General list operations.
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with import ./trivial.nix;
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let
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inc = builtins.add 1;
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dec = n: builtins.sub n 1;
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in rec {
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inherit (builtins) head tail length isList elemAt concatLists filter elem;
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# Create a list consisting of a single element. `singleton x' is
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# sometimes more convenient with respect to indentation than `[x]'
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# when x spans multiple lines.
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singleton = x: [x];
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# "Fold" a binary function `op' between successive elements of
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# `list' with `nul' as the starting value, i.e., `fold op nul [x_1
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# x_2 ... x_n] == op x_1 (op x_2 ... (op x_n nul))'. (This is
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# Haskell's foldr).
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fold = op: nul: list:
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let
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len = length list;
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fold' = n:
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if n == len
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then nul
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else op (elemAt list n) (fold' (inc n));
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in fold' 0;
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# Left fold: `fold op nul [x_1 x_2 ... x_n] == op (... (op (op nul
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# x_1) x_2) ... x_n)'.
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foldl = op: nul: list:
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let
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len = length list;
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foldl' = n:
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if n == minus1
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then nul
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else op (foldl' (dec n)) (elemAt list n);
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in foldl' (dec (length list));
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minus1 = dec 0;
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# map with index: `imap (i: v: "${v}-${toString i}") ["a" "b"] ==
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# ["a-1" "b-2"]'
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imap = f: list:
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let
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len = length list;
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imap' = n:
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if n == len
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then []
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else [ (f (inc n) (elemAt list n)) ] ++ imap' (inc n);
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in imap' 0;
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# Map and concatenate the result.
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concatMap = f: list: concatLists (map f list);
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# Flatten the argument into a single list; that is, nested lists are
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# spliced into the top-level lists. E.g., `flatten [1 [2 [3] 4] 5]
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# == [1 2 3 4 5]' and `flatten 1 == [1]'.
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flatten = x:
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if isList x
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then fold (x: y: (flatten x) ++ y) [] x
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else [x];
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# Remove elements equal to 'e' from a list. Useful for buildInputs.
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remove = e: filter (x: x != e);
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# Find the sole element in the list matching the specified
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# predicate, returns `default' if no such element exists, or
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# `multiple' if there are multiple matching elements.
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findSingle = pred: default: multiple: list:
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let found = filter pred list; len = length found;
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in if len == 0 then default
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else if len != 1 then multiple
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else head found;
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# Find the first element in the list matching the specified
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# predicate or returns `default' if no such element exists.
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findFirst = pred: default: list:
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let found = filter pred list;
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in if found == [] then default else head found;
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# Return true iff function `pred' returns true for at least element
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# of `list'.
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any = pred: fold (x: y: if pred x then true else y) false;
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# Return true iff function `pred' returns true for all elements of
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# `list'.
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all = pred: fold (x: y: if pred x then y else false) true;
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# Count how many times function `pred' returns true for the elements
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# of `list'.
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count = pred: fold (x: c: if pred x then inc c else c) 0;
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# Return a singleton list or an empty list, depending on a boolean
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# value. Useful when building lists with optional elements
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# (e.g. `++ optional (system == "i686-linux") flashplayer').
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optional = cond: elem: if cond then [elem] else [];
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# Return a list or an empty list, dependening on a boolean value.
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optionals = cond: elems: if cond then elems else [];
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# If argument is a list, return it; else, wrap it in a singleton
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# list. If you're using this, you should almost certainly
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# reconsider if there isn't a more "well-typed" approach.
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toList = x: if isList x then x else [x];
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# Return a list of integers from `first' up to and including `last'.
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range = first: last:
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if lessThan last first
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then []
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else [first] ++ range (add first 1) last;
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# Partition the elements of a list in two lists, `right' and
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# `wrong', depending on the evaluation of a predicate.
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partition = pred:
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fold (h: t:
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if pred h
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then { right = [h] ++ t.right; wrong = t.wrong; }
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else { right = t.right; wrong = [h] ++ t.wrong; }
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) { right = []; wrong = []; };
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zipListsWith = f: fst: snd:
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let
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len1 = length fst;
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len2 = length snd;
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len = if lessThan len1 len2 then len1 else len2;
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zipListsWith' = n:
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if n != len then
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[ (f (elemAt fst n) (elemAt snd n)) ]
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++ zipListsWith' (inc n)
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else [];
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in zipListsWith' 0;
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zipLists = zipListsWith (fst: snd: { inherit fst snd; });
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# Reverse the order of the elements of a list. FIXME: O(n^2)!
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reverseList = fold (e: acc: acc ++ [ e ]) [];
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# Sort a list based on a comparator function which compares two
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# elements and returns true if the first argument is strictly below
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# the second argument. The returned list is sorted in an increasing
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# order. The implementation does a quick-sort.
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sort = strictLess: list:
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let
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len = length list;
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first = head list;
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pivot' = n: acc@{ left, right }: let el = elemAt list n; next = pivot' (inc n); in
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if n == len
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then acc
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else if strictLess first el
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then next { inherit left; right = [ el ] ++ right; }
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else
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next { left = [ el ] ++ left; inherit right; };
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pivot = pivot' 1 { left = []; right = []; };
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in
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if lessThan len 2 then list
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else (sort strictLess pivot.left) ++ [ first ] ++ (sort strictLess pivot.right);
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# Return the first (at most) N elements of a list.
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take = count: list:
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let
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len = length list;
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take' = n:
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if n == len || n == count
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then []
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else
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[ (elemAt list n) ] ++ take' (inc n);
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in take' 0;
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# Remove the first (at most) N elements of a list.
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drop = count: list:
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let
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len = length list;
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drop' = n:
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if n == minus1 || lessThan n count
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then []
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else
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drop' (dec n) ++ [ (elemAt list n) ];
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in drop' (dec len);
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# Return the last element of a list.
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last = list:
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assert list != []; elemAt list (dec (length list));
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# Zip two lists together.
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zipTwoLists = xs: ys:
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let
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len1 = length xs;
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len2 = length ys;
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len = if lessThan len1 len2 then len1 else len2;
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zipTwoLists' = n:
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if n != len then
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[ { first = elemAt xs n; second = elemAt ys n; } ]
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++ zipTwoLists' (inc n)
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else [];
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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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crossLists = f: foldl (fs: args: concatMap (f: map f args) fs) [f];
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}
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