132 lines
3.9 KiB
Nix
132 lines
3.9 KiB
Nix
# General list operations.
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rec {
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inherit (builtins) head tail isList;
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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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if list == []
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then nul
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else op (head list) (fold op nul (tail list));
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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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if list == []
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then nul
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else foldl op (op nul (head list)) (tail list);
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# Concatenate a list of lists.
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concatLists = fold (x: y: x ++ y) [];
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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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# Filter a list using a predicate; that is, return a list containing
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# every element from `list' for which `pred' returns true.
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filter = pred: list:
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fold (x: y: if pred x then [x] ++ y else y) [] list;
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# Return true if `list' has an element `x':
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elem = x: list: fold (a: bs: x == a || bs) false list;
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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;
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in if found == [] then default
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else if tail found != [] 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: list:
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if list == [] then false
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else if pred (head list) then true
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else any pred (tail list);
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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: list:
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if list == [] then true
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else if pred (head list) then all pred (tail list)
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else false;
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# Return true if each element of a list is equal, false otherwise.
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eqLists = xs: ys:
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if xs == [] && ys == [] then true
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else if xs == [] || ys == [] then false
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else head xs == head ys && eqLists (tail xs) (tail ys);
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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 builtins.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 builtins.lessThan last first
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then []
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else [first] ++ range (builtins.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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}
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