hashable

I do not understand why Data.HashTable is using Data.Hashable , which has hashWithSalt as the (only/basic) method. This does not fit with the natural optimization of computing the hash value once, and storing it in the object (natural, because Haskell objects are immutable). If I want to use HashTables with that, then I'm forced to implement hashWithSalt . (Going 1.2.0.* to 1.2.1.*, hashable re-introduced hash as a class method, but this does not help?) The actual Table implementations don't seem to make use of hashWithSalt ( HashTable.ST.Linear does not at all, HashTable.ST.Cuckoo uses two

The docs say that a class is hashable as long as it defines __hash__ method and __eq__ method. However: class X(list): # read-only interface of `tuple` and `list` should be the same, so reuse tuple.__hash__ __hash__ = tuple.__hash__ x1 = X() s = {x1} # TypeError: unhashable type: 'X' What makes X unhashable? Note that I must have identical lists (in terms of regular equality) to be hashed to the same value; otherwise, I will violate this requirement on hash functions: The only required property is that objects which compare equal have the same hash value The docs do warn that a hashable object

I was thinking about how Swift ensures uniqueness for Set because I have turned one of my obj from Equatable to Hashable for free and so I came up with this simple Playground struct SimpleStruct: Hashable { let string: String let number: Int static func == (lhs: SimpleStruct, rhs: SimpleStruct) -> Bool { let areEqual = lhs.string == rhs.string print(lhs, rhs, areEqual) return areEqual } } var set = Set<SimpleStruct>() let first = SimpleStruct(string: "a", number: 2) set.insert(first) So my first question was: Will the static func == method be called anytime I insert a new obj inside the set?

Consider this code : a = {...} # a is an dict with arbitrary contents b = a.copy() What role does mutability play in the keys and values of the dicts? How do I ensure changes to keys or values of one dict are not reflected in the other? How does this relate to the hashable constraint of the dict keys? Are there any differences in behaviour between Python 2.x and Python 3.x? How do I check if a type is mutable in Python? 1) Keys must not be mutable, unless you have a user-defined class that is hashable but also mutable. That's all that's forced upon you. However, using a hashable, mutable

I've been facing following issue (it's just a warning) with my iOS project. 'Hashable.hashValue' is deprecated as a protocol requirement; conform type 'ActiveType' to 'Hashable' by implementing 'hash(into:)' instead Xcode 10.2 Swift 5 Source Code: public enum ActiveType { case mention case hashtag case url case custom(pattern: String) var pattern: String { switch self { case .mention: return RegexParser.mentionPattern case .hashtag: return RegexParser.hashtagPattern case .url: return RegexParser.urlPattern case .custom(let regex): return regex } } } extension ActiveType: Hashable, Equatable {