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458 lines
13 KiB
Nix
458 lines
13 KiB
Nix
# General list operations.
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with import ./trivial.nix;
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rec {
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inherit (builtins) head tail length isList elemAt concatLists filter elem genList;
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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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Example:
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singleton "foo"
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=> [ "foo" ]
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*/
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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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Example:
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concat = fold (a: b: a + b) "z"
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concat [ "a" "b" "c" ]
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=> "abcz"
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*/
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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' (n + 1));
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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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Example:
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lconcat = foldl (a: b: a + b) "z"
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lconcat [ "a" "b" "c" ]
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=> "zabc"
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*/
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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 == -1
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then nul
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else op (foldl' (n - 1)) (elemAt list n);
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in foldl' (length list - 1);
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/* Strict version of foldl.
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The difference is that evaluation is forced upon access. Usually used
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with small whole results (in contract with lazily-generated list or large
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lists where only a part is consumed.)
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*/
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foldl' = builtins.foldl' or foldl;
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/* Map with index
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FIXME(zimbatm): why does this start to count at 1?
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Example:
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imap (i: v: "${v}-${toString i}") ["a" "b"]
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=> [ "a-1" "b-2" ]
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*/
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imap = f: list: genList (n: f (n + 1) (elemAt list n)) (length list);
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/* Map and concatenate the result.
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Example:
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concatMap (x: [x] ++ ["z"]) ["a" "b"]
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=> [ "a" "z" "b" "z" ]
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*/
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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.
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Example:
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flatten [1 [2 [3] 4] 5]
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=> [1 2 3 4 5]
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flatten 1
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=> [1]
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*/
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flatten = x:
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if isList x
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then concatMap (y: flatten 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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Example:
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remove 3 [ 1 3 4 3 ]
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=> [ 1 4 ]
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*/
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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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Example:
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findSingle (x: x == 3) "none" "multiple" [ 1 3 3 ]
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=> "multiple"
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findSingle (x: x == 3) "none" "multiple" [ 1 3 ]
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=> 3
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findSingle (x: x == 3) "none" "multiple" [ 1 9 ]
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=> "none"
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*/
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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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Example:
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findFirst (x: x > 3) 7 [ 1 6 4 ]
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=> 6
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findFirst (x: x > 9) 7 [ 1 6 4 ]
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=> 7
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*/
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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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Example:
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any isString [ 1 "a" { } ]
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=> true
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any isString [ 1 { } ]
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=> false
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*/
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any = builtins.any or (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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Example:
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all (x: x < 3) [ 1 2 ]
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=> true
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all (x: x < 3) [ 1 2 3 ]
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=> false
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*/
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all = builtins.all or (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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Example:
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count (x: x == 3) [ 3 2 3 4 6 ]
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=> 2
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*/
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count = pred: foldl' (c: x: if pred x then c + 1 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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Example:
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optional true "foo"
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=> [ "foo" ]
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optional false "foo"
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=> [ ]
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*/
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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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Example:
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optionals true [ 2 3 ]
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=> [ 2 3 ]
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optionals false [ 2 3 ]
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=> [ ]
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*/
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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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Example:
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toList [ 1 2 ]
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=> [ 1 2 ]
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toList "hi"
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=> [ "hi "]
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*/
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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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Example:
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range 2 4
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=> [ 2 3 4 ]
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range 3 2
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=> [ ]
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*/
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range = first: last:
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if first > last then
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[]
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else
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genList (n: first + n) (last - first + 1);
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/* Splits 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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Example:
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partition (x: x > 2) [ 5 1 2 3 4 ]
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=> { right = [ 5 3 4 ]; wrong = [ 1 2 ]; }
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*/
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partition = builtins.partition or (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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/* Merges two lists of the same size together. If the sizes aren't the same
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the merging stops at the shortest. How both lists are merged is defined
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by the first argument.
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Example:
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zipListsWith (a: b: a + b) ["h" "l"] ["e" "o"]
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=> ["he" "lo"]
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*/
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zipListsWith = f: fst: snd:
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genList
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(n: f (elemAt fst n) (elemAt snd n)) (min (length fst) (length snd));
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/* Merges two lists of the same size together. If the sizes aren't the same
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the merging stops at the shortest.
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Example:
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zipLists [ 1 2 ] [ "a" "b" ]
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=> [ { fst = 1; snd = "a"; } { fst = 2; snd = "b"; } ]
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*/
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zipLists = zipListsWith (fst: snd: { inherit fst snd; });
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/* Reverse the order of the elements of a list.
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Example:
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reverseList [ "b" "o" "j" ]
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=> [ "j" "o" "b" ]
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*/
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reverseList = xs:
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let l = length xs; in genList (n: elemAt xs (l - n - 1)) l;
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/* Depth-First Search (DFS) for lists `list != []`.
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`before a b == true` means that `b` depends on `a` (there's an
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edge from `b` to `a`).
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Examples:
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listDfs true hasPrefix [ "/home/user" "other" "/" "/home" ]
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== { minimal = "/"; # minimal element
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visited = [ "/home/user" ]; # seen elements (in reverse order)
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rest = [ "/home" "other" ]; # everything else
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}
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listDfs true hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
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== { cycle = "/"; # cycle encountered at this element
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loops = [ "/" ]; # and continues to these elements
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visited = [ "/" "/home/user" ]; # elements leading to the cycle (in reverse order)
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rest = [ "/home" "other" ]; # everything else
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*/
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listDfs = stopOnCycles: before: list:
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let
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dfs' = us: visited: rest:
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let
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c = filter (x: before x us) visited;
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b = partition (x: before x us) rest;
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in if stopOnCycles && (length c > 0)
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then { cycle = us; loops = c; inherit visited rest; }
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else if length b.right == 0
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then # nothing is before us
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{ minimal = us; inherit visited rest; }
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else # grab the first one before us and continue
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dfs' (head b.right)
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([ us ] ++ visited)
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(tail b.right ++ b.wrong);
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in dfs' (head list) [] (tail list);
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/* Sort a list based on a partial ordering using DFS. This
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implementation is O(N^2), if your ordering is linear, use `sort`
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instead.
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`before a b == true` means that `b` should be after `a`
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in the result.
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Examples:
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toposort hasPrefix [ "/home/user" "other" "/" "/home" ]
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== { result = [ "/" "/home" "/home/user" "other" ]; }
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toposort hasPrefix [ "/home/user" "other" "/" "/home" "/" ]
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== { cycle = [ "/home/user" "/" "/" ]; # path leading to a cycle
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loops = [ "/" ]; } # loops back to these elements
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toposort hasPrefix [ "other" "/home/user" "/home" "/" ]
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== { result = [ "other" "/" "/home" "/home/user" ]; }
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toposort (a: b: a < b) [ 3 2 1 ] == { result = [ 1 2 3 ]; }
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*/
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toposort = before: list:
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let
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dfsthis = listDfs true before list;
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toporest = toposort before (dfsthis.visited ++ dfsthis.rest);
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in
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if length list < 2
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then # finish
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{ result = list; }
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else if dfsthis ? "cycle"
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then # there's a cycle, starting from the current vertex, return it
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{ cycle = reverseList ([ dfsthis.cycle ] ++ dfsthis.visited);
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inherit (dfsthis) loops; }
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else if toporest ? "cycle"
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then # there's a cycle somewhere else in the graph, return it
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toporest
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# Slow, but short. Can be made a bit faster with an explicit stack.
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else # there are no cycles
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{ result = [ dfsthis.minimal ] ++ toporest.result; };
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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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Example:
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sort (a: b: a < b) [ 5 3 7 ]
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=> [ 3 5 7 ]
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*/
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sort = builtins.sort or (
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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' (n + 1); 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 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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Example:
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take 2 [ "a" "b" "c" "d" ]
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=> [ "a" "b" ]
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take 2 [ ]
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=> [ ]
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*/
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take = count: sublist 0 count;
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/* Remove the first (at most) N elements of a list.
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Example:
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drop 2 [ "a" "b" "c" "d" ]
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=> [ "c" "d" ]
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drop 2 [ ]
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=> [ ]
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*/
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drop = count: list: sublist count (length list) list;
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/* Return a list consisting of at most ‘count’ elements of ‘list’,
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starting at index ‘start’.
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Example:
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sublist 1 3 [ "a" "b" "c" "d" "e" ]
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=> [ "b" "c" "d" ]
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sublist 1 3 [ ]
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=> [ ]
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*/
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sublist = start: count: list:
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let len = length list; in
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genList
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(n: elemAt list (n + start))
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(if start >= len then 0
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else if start + count > len then len - start
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else count);
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/* Return the last element of a list.
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Example:
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last [ 1 2 3 ]
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=> 3
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*/
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last = list:
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assert list != []; elemAt list (length list - 1);
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/* Return all elements but the last
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Example:
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init [ 1 2 3 ]
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=> [ 1 2 ]
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*/
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init = list: assert list != []; take (length list - 1) list;
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/* FIXME(zimbatm) Not used anywhere
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*/
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crossLists = f: foldl (fs: args: concatMap (f: map f args) fs) [f];
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/* Remove duplicate elements from the list. O(n^2) complexity.
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Example:
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unique [ 3 2 3 4 ]
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=> [ 3 2 4 ]
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*/
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unique = list:
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if list == [] then
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[]
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else
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let
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x = head list;
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xs = unique (drop 1 list);
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in [x] ++ remove x xs;
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/* Intersects list 'e' and another list. O(nm) complexity.
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Example:
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intersectLists [ 1 2 3 ] [ 6 3 2 ]
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=> [ 3 2 ]
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*/
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intersectLists = e: filter (x: elem x e);
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/* Subtracts list 'e' from another list. O(nm) complexity.
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Example:
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subtractLists [ 3 2 ] [ 1 2 3 4 5 3 ]
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=> [ 1 4 5 ]
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*/
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subtractLists = e: filter (x: !(elem x e));
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}
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