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Maybe I’m misunderstanding, but I’d argue that arrow syntax in js is equally special syntax as lambda function. They’re both native ways of making anonymous fun
by aidos 7y ago
Maybe I’m misunderstanding, but I’d argue that arrow syntax in js is equally special syntax as lambda function. They’re both native ways of making anonymous functions. Python’s version is more clunky, but no more special
- verttii 7y agoAren't Python's lambdas expressions while JS's arrow functions are not?
- jakear 7y agoInteresting, I don’t use python day-to-day, so I had assumed that the delayed capturing of the iteration variable was a lambda thing. Looks like this is still broken: for i in range(10): def mul(x): return i^x powers_of_x.append(mul) That to me coming from a JS background is totally wild
- aidos 7y agoIt’s actually due to python late binding. The variables are looked up at call time, rather than function define time. One could argue that your example shouldn’t work anymore in python3 since variables in list comprehensions go out of scope when they finish now. I haven’t tried it myself. I fill it under, “things I never need to do” :-)
- masklinn 7y agoIt's not "python late binding", closures behave this way in any language with mutable bindings[0]. The entire point of a closure is to close over its lexical context, if the lexical context is mutable and mutated before the invocation of the closure, the closure is going to reflect that change. for(var i=0; i<10; ++i) { powersOfX.push(x => x^i); } will behave the same way because it does the same thing, so will e.g. for i := 0; i<10; i++ { powers_of_x = append(powers_of_x, func(x int) int { return x ^ i }) } in Go. One of the confounding factors is that many languages have block-scoped for loops especially when using iterators instead of low-level C-style loops; that's also the case in Javascript when using `let` bindings (and is in fact one of the major reason to use `let` instead of `var` if that's possible). The underlying concern is still there[1], but this very common failure case is avoided: rather than update a single binding, each iteration creates a brand new binding for the closure to close over. Alternatively, a common mitigation technique is to emulate that using e.g. immediately invoked function expressions. In Python you can also use the "default parameter" trick to shadow the closed-over binding (though most of the time this is used for performance reasons) without the overhead (both syntactic and runtime) of lambdas in lambdas in lambdas: for i in range(10): powers_of_x.append(lambda x, i=i: x^i) [0] excluding the special case of low-level languages with capture clauses, as well as languages like Java where closing over mutable bindings is specifically forbidden (a lambda or anonymous class can only close over `final` bindings) [1] and remains a regular issue in async code fighting over closed-over context
- gpderetta 7y agoC++: std::vector<std::function<int(int)>> powers_of_x; for (int i = 0; i < 10; ++1) power_of_x.push_back([=](int x) { return std::pow(x, i); }); works as one would intuitively expect (a different i is captured for each iteration). The issue is with those languages that conflate values with references.
- masklinn 7y ago… > excluding the special case of low-level languages with capture clauses Of course it works, you're specifically capturing `i` by value. It's not exactly surprising that doing things completely differently yields a different result.
- masklinn 7y agoWhy would it be "totally wild" coming from a JS background when the corresponding JS code: for(var i=0; i<10; ++i) { powers_of_x.push((x) => i^x); } behaves the exact same way for the same reason? By definition, a closures closes over its definition context, it doesn't capture the values at definition time but instead keeps referring to said definition context. If the definition context is mutable and modified, the closure reflects that change when it's finally invoked.
- deleted 7y ago[deleted]