5 ms·

> Particularly, if we could assume that nested functions automatically and properly capture the arguments to the parent function. well, how would you do that ?

doteka 6y ago
I don’t know any other language with lambdas which requires me to spell out explicitly which variables it should close over... so you tell me how Java, Scala, Clojure, Kotlin, Python, JavaScript, Rust, Go, C# and tens of others managed this clearly impossible feat of engineering.
- gpderetta 6y ago
  Forced heap allocation and GC of course that would be unacceptable in C++. IIRC Rust uses capture modifiers similar to C++.
  - steveklabnik 6y ago
    Rust is not really the same as C++ here, you get "the compiler infers how to capture things" as the default behavior, and "take everything by value", which happens with the "move" keyword. We've found that these two cases cover the vast, vast, vast majority of usage, and so C++-style specific capture modifiers aren't necessary. You can emulate C++ style stuff by taking references outside of the closure and using move to capture only the reference.
- DCoder 6y ago
  > I don’t know any other language with lambdas which requires me to spell out explicitly which variables it should close over PHP closures have to specify which variables they close over, and whether it's done by-val or by-ref [0]. Nowadays there's also a shorthand single-expression-closure syntax that closes implicitly and by value [1]. [0]: https://www.php.net/manual/en/functions.anonymous.php#example-167 https://www.php.net/manual/en/functions.anonymous.php#exampl... [1]: https://www.php.net/manual/en/functions.arrow.php https://www.php.net/manual/en/functions.arrow.php
TeMPOraL 6y ago
By having the compiler generate the equivalent of [&args...], where args are names of arguments in the parent function's header.
- jcelerier 6y ago
  why [&args...] and not [args...] ? do you really want to capture by reference when you're setting a lambda as a callback ? (likewise in the other direction - tons of arguments for every default, already debated ad nauseam)
  - gpderetta 6y ago
    And why specifically args instead of all locals?
- nwallin 6y ago
  You mean like this? https://godbolt.org/z/jvG3hY https://godbolt.org/z/jvG3hY

doteka 6y ago

I don’t know any other language with lambdas which requires me to spell out explicitly which variables it should close over... so you tell me how Java, Scala, Clojure, Kotlin, Python, JavaScript, Rust, Go, C# and tens of others managed this clearly impossible feat of engineering.

gpderetta 6y ago
Forced heap allocation and GC of course that would be unacceptable in C++. IIRC Rust uses capture modifiers similar to C++.
- steveklabnik 6y ago
  Rust is not really the same as C++ here, you get "the compiler infers how to capture things" as the default behavior, and "take everything by value", which happens with the "move" keyword. We've found that these two cases cover the vast, vast, vast majority of usage, and so C++-style specific capture modifiers aren't necessary. You can emulate C++ style stuff by taking references outside of the closure and using move to capture only the reference.
DCoder 6y ago
> I don’t know any other language with lambdas which requires me to spell out explicitly which variables it should close over PHP closures have to specify which variables they close over, and whether it's done by-val or by-ref [0]. Nowadays there's also a shorthand single-expression-closure syntax that closes implicitly and by value [1]. [0]: https://www.php.net/manual/en/functions.anonymous.php#example-167 https://www.php.net/manual/en/functions.anonymous.php#exampl... [1]: https://www.php.net/manual/en/functions.arrow.php https://www.php.net/manual/en/functions.arrow.php

TeMPOraL 6y ago

By having the compiler generate the equivalent of [&args...], where args are names of arguments in the parent function's header.

jcelerier 6y ago
why [&args...] and not [args...] ? do you really want to capture by reference when you're setting a lambda as a callback ? (likewise in the other direction - tons of arguments for every default, already debated ad nauseam)
- gpderetta 6y ago
  And why specifically args instead of all locals?
nwallin 6y ago
You mean like this? https://godbolt.org/z/jvG3hY https://godbolt.org/z/jvG3hY