jq: recursively merge objects and concatenate arrays

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Here is a generic function that recursively combines two composite JSON entities by concatenating arrays at the same position:

# Recursively meld a and b,
# concatenating arrays and
# favoring b when there is a conflict 
def meld(a; b):
  a as $a | b as $b
  | if ($a|type) == "object" and ($b|type) == "object"
    then reduce ([$a,$b]|add|keys_unsorted[]) as $k ({}; 
      .[$k] = meld( $a[$k]; $b[$k]) )
    elif ($a|type) == "array" and ($b|type) == "array"
    then $a+$b
    elif $b == null then $a
    else $b
    end;

Output of meld($orig; $patch)

With $orig set to the contents of orig.json and
$patch set to the contents of patch.json:

{
  "a": {
    "a1": "patch a1",
    "a2": "original a2",
    "list": [
      "baz",
      "bar",
      "foo"
    ]
  },
  "b": "original value B",
  "c": "original c"
}

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