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Comment by networkimprov | Hacker News Reader

I think there's a bug, tho it might not make a difference to the results:

    for i := 0; i < 10; i++ {
       go func() {
          f(i) // sees whatever value i has when f() is called, usually 10
       }()
    }

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This is a very common newbie mistake. Fix by passing in i.

    for i := 0; i < 10; i++ {
       go func(i int) {
          f(i) // sees whatever value i has when f() is called, usually 10
       }(i)
    }

This looks very much like the mistake in javascript with callbacks in for loops.

Note that using `let` instead of `var` in your for-loop fixes this in JavaScript (ignoring IE's faulty implemention of `let`).

absolutely wild to me that Real Engineers (TM) complained about this in JavaScript for years only for it to be called a "newbie mistake" in Go.

The same is true of most of JavaScript's warts

For the curious reader: this can be solved by simply passing i as a parameter.

    for i := 0; i < 10; i++ {
        go func(i int) {
            f(i)
        }(i)
    }

Or commonly, tho perhaps confusingly:

    for i := 0; i < 10 i++ {
       i := i // new variable i for every iteration
       go func() { f(i) }()
    }

Yea. I do this personally - it's easy copypasta since it infers types for you, and as it shadows the iteration var it's impossible to use the wrong one. Same trick works for other local vars e.g. outside the loop, though at that point it's just as not-copypasta-able as the `func(arg){}(arg)` construct.

Thanks for the correction, that is indeed a bug!

Interesting. Integers are captured by reference in Go?

Closures capture all in-scope variables by reference in most languages.

Same situation in Python for example: https://stackoverflow.com/questions/54288926/python-loops-an...

You can get bitten by a similar problem without closures if you stick a loop variable in an array. The loop variable is one address with contents mutating each iteration.

> You can get bitten by a similar problem without closures if you stick a loop variable in an array.

Surely only if you store it indirectly, otherwise it'll be copied into the array.

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