How to deep merge 2 Swift dictionaries - swift

Question

My question is simple, I want to know how to do a deep merge of 2 Swift dictionaries (not NSDictionary).
let dict1 = [
"a": 1,
"b": 2,
"c": [
"d": 3
],
"f": 2
]
let dict2 = [
"b": 4,
"c": [
"e": 5
],
"f": ["g": 6]
]
let dict3 = dict1.merge(dict2)
/* Expected:
dict3 = [
"a": 1,
"b": 4,
"c": [
"d": 3,
"e": 5
],
"f": ["g": 6]
]
*/
When dict1 and dict2 have the same key, I expect the value to be replaced, but if that value is another dictionary, I expect it to be merged recursively.
Here is the solution I'd like:
protocol Mergeable {
mutating func merge(obj: Self)
}
extension Dictionary: Mergeable {
// if they have the same key, the new value is taken
mutating func merge(dictionary: Dictionary) {
for (key, value) in dictionary {
let oldValue = self[key]
if oldValue is Mergeable && value is Mergeable {
var oldValue = oldValue as! Mergeable
let newValue = value as! Mergeable
oldValue.merge(newValue)
self[key] = oldValue
} else {
self[key] = value
}
}
}
}
but it gives me the error Protocol 'Mergeable' can only be used as a generic constraint because it has Self or associated type requirements
EDIT:
My question is different from Swift: how to combine two Dictionary instances? because that one is not a deep merge.
With that solution, it would produce:
dict3 = [
"a": 1,
"b": 4,
"c": [
"e": 5
]
]

Answer 1

In my view the question is incoherent. This is an answer, however:
func deepMerge(d1:[String:AnyObject], _ d2:[String:AnyObject]) -> [String:AnyObject] {
var result = [String:AnyObject]()
for (k1,v1) in d1 {
result[k1] = v1
}
for (k2,v2) in d2 {
if v2 is [String:AnyObject], let v1 = result[k2] where v1 is [String:AnyObject] {
result[k2] = deepMerge(v1 as! [String:AnyObject],v2 as! [String:AnyObject])
} else {
result[k2] = v2
}
}
return result
}
Here is your test case:
let dict1:[String:AnyObject] = [
"a": 1,
"b": 2,
"c": [
"d": 3
]
]
let dict2:[String:AnyObject] = [
"b": 4,
"c": [
"e": 5
]
]
let result = deepMerge(dict1, dict2)
NSLog("%#", result)
/*
{
a = 1;
b = 4;
c = {
d = 3;
e = 5;
};
}
*/
3rd-party edit: Alternate version using variable binding and newer Swift syntax.
func deepMerge(_ d1: [String: Any], _ d2: [String: Any]) -> [String: Any] {
var result = d1
for (k2, v2) in d2 {
if let v1 = result[k2] as? [String: Any], let v2 = v2 as? [String: Any] {
result[k2] = deepMerge(v1, v2)
} else {
result[k2] = v2
}
}
return result
}

Answer 2

How about manually doing it.
func += <KeyType, ValueType> (inout left: Dictionary<KeyType, ValueType>, right: Dictionary<KeyType, ValueType>) {
for (k, v) in right {
left.updateValue(v, forKey: k)
}
}
You can also try ExSwift Library

Answer 3

extension Dictionary {
mutating func deepMerge(_ dict: Dictionary) {
merge(dict) { (current, new) in
if var currentDict = current as? Dictionary, let newDict = new as? Dictionary {
currentDict.deepMerge(newDict)
return currentDict as! Value
}
return new
}
}
}
How to use/test:
var dict1: [String: Any] = [
"a": 1,
"b": 2,
"c": [
"d": 3
],
"f": 2
]
var dict2: [String: Any] = [
"b": 4,
"c": [
"e": 5
],
"f": ["g": 6]
]
dict1.deepMerge(dict2)
print(dict1)