lib.mergeDefaultOption

A merge function that merges multiple definitions of an option into a single value

Inputs

loc

location of the option in the configuration as a list of strings. e.g. ["boot" "loader "grub" "enable"]

defs

list of definition values and locations. e.g. [ { file = "/foo.nix"; value = 1; } { file = "/bar.nix"; value = 2 } ]

Example

myType = mkOptionType {
  name = "myType";
  merge = mergeDefaultOption; # <- This line is redundant. It is the default aready.
};

Merge behavior

Merging requires all definition values to have the same type.

• If all definitions are booleans, the result of a foldl' with the or operation is returned.
• If all definitions are strings, they are concatenated. (lib.concatStrings)
• If all definitions are integers and all are equal, the first one is returned.
• If all definitions are lists, they are concatenated. (++)
• If all definitions are attribute sets, they are merged. (lib.mergeAttrs)
• If all definitions are functions, the first function is applied to the result of the second function. (f -> x: f x)
• Otherwise, an error is thrown.

Noogle detected

• lib.options.mergeDefaultOption

mergeDefaultOption = loc: defs:
    let list = getValues defs; in
    if length list == 1 then head list
    else if all isFunction list then x: mergeDefaultOption loc (map (f: f x) list)
    else if all isList list then concatLists list
    else if all isAttrs list then foldl' lib.mergeAttrs {} list
    else if all isBool list then foldl' lib.or false list
    else if all isString list then lib.concatStrings list
    else if all isInt list && all (x: x == head list) list then head list
    else throw "Cannot merge definitions of `${showOption loc}'. Definition values:${showDefs defs}";