diff options
Diffstat (limited to 'lib/attrsets.nix')
-rw-r--r-- | lib/attrsets.nix | 72 |
1 files changed, 72 insertions, 0 deletions
diff --git a/lib/attrsets.nix b/lib/attrsets.nix index 3d4366ce1814..9b4a8684f9b7 100644 --- a/lib/attrsets.nix +++ b/lib/attrsets.nix @@ -51,12 +51,19 @@ rec { /* Return if an attribute from nested attribute set exists. + **Laws**: + 1. ```nix + hasAttrByPath [] x == true + ``` + Example: x = { a = { b = 3; }; } hasAttrByPath ["a" "b"] x => true hasAttrByPath ["z" "z"] x => false + hasAttrByPath [] (throw "no need") + => true Type: hasAttrByPath :: [String] -> AttrSet -> Bool @@ -80,6 +87,71 @@ 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: |