4 ms·

> The one key difference is that any object can be a key in a Lua table How does this work? Internally it is implemented using a hashmap so somehow they must g

by pkaye 8y ago

> The one key difference is that any object can be a key in a Lua table How does this work? Internally it is implemented using a hashmap so somehow they must generate a hash value of the object? What if the object has member variables that are themselves objects? What if those things point to each other in a cycle?

nikki93 8y ago
Equivalence is by whether that name refers to the same object (like “pointers” in C).
masklinn 8y ago
In that case the object's is hashed by identity (location in memory). That's how Javascript's Map (separate from a bare object) works for instance: > let m = new Map > let o = {} > m.set({}, 1) Map { {} => 1 } > m.set(o, 2) Map { {} => 1, {} => 2 } > m.get({}) undefined > m.get(o) 2 or Python's dicts: >>> class A: pass ... >>> a1 = A() >>> a2 = A() >>> {a1: 1, a2: 2, A(): 3} >>> d = {a1: 1, a2: 2, A(): 3} >>> d {<__main__.A object at 0x107a9f1d0>: 1, <__main__.A object at 0x107a9f208>: 2, <__main__.A object at 0x107a9f2b0>: 3} >>> d[A()] Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: <__main__.A object at 0x107a9f240> >>> d[a1] 1 >>> d[a2] 2 or Ruby's: > class A; end > a1 = A.new > a2 = A.new > m = { a1 => 1, a2 => 2, A.new => 3} => {#<A:0x007fbe691ecdf8>=>1, #<A:0x007fbe691f4f08>=>2, #<A:0x007fbe6920efc0>=>3} > m[A.new] => nil > m[a1] => 1 > m[a2] => 2 Statically typed languages can more easily constrain keys[0], but even then the lure of all objects being hashable and equatable is strong e.g. I'm pretty sure all Java and C# objects are hashable and equatable (and can be used as hashmap keys out of the box). [0] Rust's hashmap requires that its keys implement Hash and Eq, neither of which are implemented by default; Haskell's Data.Map that its key be Ord; ...

by pkaye 8y ago

nikki93 8y ago

Equivalence is by whether that name refers to the same object (like “pointers” in C).

masklinn 8y ago

In that case the object's is hashed by identity (location in memory). That's how Javascript's Map (separate from a bare object) works for instance: > let m = new Map > let o = {} > m.set({}, 1) Map { {} => 1 } > m.set(o, 2) Map { {} => 1, {} => 2 } > m.get({}) undefined > m.get(o) 2 or Python's dicts: >>> class A: pass ... >>> a1 = A() >>> a2 = A() >>> {a1: 1, a2: 2, A(): 3} >>> d = {a1: 1, a2: 2, A(): 3} >>> d {<__main__.A object at 0x107a9f1d0>: 1, <__main__.A object at 0x107a9f208>: 2, <__main__.A object at 0x107a9f2b0>: 3} >>> d[A()] Traceback (most recent call last): File "<stdin>", line 1, in <module> KeyError: <__main__.A object at 0x107a9f240> >>> d[a1] 1 >>> d[a2] 2 or Ruby's: > class A; end > a1 = A.new > a2 = A.new > m = { a1 => 1, a2 => 2, A.new => 3} => {#<A:0x007fbe691ecdf8>=>1, #<A:0x007fbe691f4f08>=>2, #<A:0x007fbe6920efc0>=>3} > m[A.new] => nil > m[a1] => 1 > m[a2] => 2 Statically typed languages can more easily constrain keys[0], but even then the lure of all objects being hashable and equatable is strong e.g. I'm pretty sure all Java and C# objects are hashable and equatable (and can be used as hashmap keys out of the box). [0] Rust's hashmap requires that its keys implement Hash and Eq, neither of which are implemented by default; Haskell's Data.Map that its key be Ord; ...