lib.attrsets.longestValidPathPrefix: init (72bd4bbb) · Commits · nix / nixpkgs

lib/attrsets.nix

+65 −0

Original line number	Diff line number	Diff line
		@@ -80,6 +80,71 @@ rec {
		in
		hasAttrByPath' 0 e;

		/*
		Return the longest prefix of an attribute path that refers to an existing attribute in a nesting of attribute sets.

		Can be used after [`mapAttrsRecursiveCond`](#function-library-lib.attrsets.mapAttrsRecursiveCond) to apply a condition,
		although this will evaluate the predicate function on sibling attributes as well.

		Note that the empty attribute path is valid for all values, so this function only throws an exception if any of its inputs does.

		Laws:
		1. ```nix
		attrsets.longestValidPathPrefix [] x == []
		```

		2. ```nix
		hasAttrByPath (attrsets.longestValidPathPrefix p x) x == true
		```

		Example:
		x = { a = { b = 3; }; }
		attrsets.longestValidPathPrefix ["a" "b" "c"] x
		=> ["a" "b"]
		attrsets.longestValidPathPrefix ["a"] x
		=> ["a"]
		attrsets.longestValidPathPrefix ["z" "z"] x
		=> []
		attrsets.longestValidPathPrefix ["z" "z"] (throw "no need")
		=> []

		Type:
		attrsets.longestValidPathPrefix :: [String] -> Value -> [String]
		*/
		longestValidPathPrefix =
		# A list of strings representing the longest possible path that may be returned.
		attrPath:
		# The nested attribute set to check.
		v:
		let
		lenAttrPath = length attrPath;
		getPrefixForSetAtIndex =
		# The nested attribute set to check, if it is an attribute set, which
		# is not a given.
		remainingSet:
		# The index of the attribute we're about to check, as well as
		# the length of the prefix we've already checked.
		remainingPathIndex:

		if remainingPathIndex == lenAttrPath then
		# All previously checked attributes exist, and no attr names left,
		# so we return the whole path.
		attrPath
		else
		let
		attr = elemAt attrPath remainingPathIndex;
		in
		if remainingSet ? ${attr} then
		getPrefixForSetAtIndex
		remainingSet.${attr} # advance from the set to the attribute value
		(remainingPathIndex + 1) # advance the path
		else
		# The attribute doesn't exist, so we return the prefix up to the
		# previously checked length.
		take remainingPathIndex attrPath;
		in
		getPrefixForSetAtIndex v 0;

		/* Create a new attribute set with `value` set at the nested attribute location specified in `attrPath`.

		Example:

lib/tests/misc.nix

+40 −0

Original line number	Diff line number	Diff line
		@@ -697,6 +697,46 @@ runTests {
		expected = false;
		};

		testLongestValidPathPrefix_empty_empty = {
		expr = attrsets.longestValidPathPrefix [ ] { };
		expected = [ ];
		};

		testLongestValidPathPrefix_empty_nonStrict = {
		expr = attrsets.longestValidPathPrefix [ ] (throw "do not use");
		expected = [ ];
		};

		testLongestValidPathPrefix_zero = {
		expr = attrsets.longestValidPathPrefix [ "a" (throw "do not use") ] { d = null; };
		expected = [ ];
		};

		testLongestValidPathPrefix_zero_b = {
		expr = attrsets.longestValidPathPrefix [ "z" "z" ] "remarkably harmonious";
		expected = [ ];
		};

		testLongestValidPathPrefix_one = {
		expr = attrsets.longestValidPathPrefix [ "a" "b" "c" ] { a = null; };
		expected = [ "a" ];
		};

		testLongestValidPathPrefix_two = {
		expr = attrsets.longestValidPathPrefix [ "a" "b" "c" ] { a.b = null; };
		expected = [ "a" "b" ];
		};

		testLongestValidPathPrefix_three = {
		expr = attrsets.longestValidPathPrefix [ "a" "b" "c" ] { a.b.c = null; };
		expected = [ "a" "b" "c" ];
		};

		testLongestValidPathPrefix_three_extra = {
		expr = attrsets.longestValidPathPrefix [ "a" "b" "c" ] { a.b.c.d = throw "nope"; };
		expected = [ "a" "b" "c" ];
		};

		testFindFirstIndexExample1 = {
		expr = lists.findFirstIndex (x: x > 3) (abort "index found, so a default must not be evaluated") [ 1 6 4 ];
		expected = 1;