mirror of
https://github.com/NixOS/nixpkgs.git
synced 2024-11-25 07:00:43 +00:00
82 lines
2.3 KiB
Nix
82 lines
2.3 KiB
Nix
# Utility functions.
|
|
|
|
let
|
|
|
|
inherit (builtins) head tail isList;
|
|
|
|
in
|
|
|
|
rec {
|
|
|
|
# "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));
|
|
|
|
|
|
# Concatenate a list of strings.
|
|
concatStrings =
|
|
fold (x: y: x + y) "";
|
|
|
|
|
|
# Place an element between each element of a list, e.g.,
|
|
# `intersperse "," ["a" "b" "c"]' returns ["a" "," "b" "," "c"].
|
|
intersperse = separator: list:
|
|
if list == [] || tail list == []
|
|
then list
|
|
else [(head list) separator]
|
|
++ (intersperse separator (tail 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];
|
|
|
|
|
|
# Return an attribute from nested attribute sets. For instance ["x"
|
|
# "y"] applied to some set e returns e.x.y, if it exists. The
|
|
# default value is returned otherwise.
|
|
getAttr = attrPath: default: e:
|
|
let {
|
|
attr = head attrPath;
|
|
body =
|
|
if attrPath == [] then e
|
|
else if builtins ? hasAttr && builtins.hasAttr attr e
|
|
then getAttr (tail attrPath) default (builtins.getAttr attr e)
|
|
else default;
|
|
};
|
|
|
|
|
|
# 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;
|
|
|
|
|
|
# Find the sole element in the list matching the specified
|
|
# predicate, or returns the default value.
|
|
findSingle = pred: default: list:
|
|
let found = filter pred list;
|
|
in if found == [] then default
|
|
else if tail found != [] then
|
|
abort "Multiple elements match predicate in findSingle."
|
|
else head found;
|
|
|
|
|
|
# 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);
|
|
|
|
|
|
}
|