# General list operations. with import ./trivial.nix; let inc = builtins.add 1; dec = n: builtins.sub n 1; in rec { 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; # 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]; # Remove elements equal to 'e' from a list. Useful for buildInputs. remove = e: filter (x: x != e); # 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; # 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. toList = x: if isList x then x else [x]; # Return a list of integers from `first' up to and including `last'. range = first: last: if lessThan last first then [] 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; 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; # 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); # Return the last element of a list. last = list: assert list != []; elemAt list (dec (length list)); # Return all elements but the last init = list: assert list != []; take (length list - 1) list; # 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; deepSeqList = xs: y: if any (x: deepSeq x false) xs then y else y; crossLists = f: foldl (fs: args: concatMap (f: map f args) fs) [f]; }