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Comment by magicalhippo | Hacker News Reader

> Most third-party code will be unable to forward a `context` argument to a callback

Having a context argument which is passed to the callback has been par for the course in all cases I've seen, but I've mostly been in C land and related areas.

Though, if you have closures, should you still prefer a context param?

> If JavaScript had this feature, the syntax for defining and calling functions would be the same, no matter whether you're dealing with a normal function, a generator, or an async function.

How would one reason about performance and possibly concurrency in such languages?

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In a language with algebraic effects, questions like "how should I make this `CancellationToken` available to these forty small functions?" simply go away :-)

It's ambient state, just like a `catch` block, and so you can make it ambiently available to a whole tree of function calls. The outcome is the same as passing around a context argument by hand, and it can have equally strong type-checking, but the compiler handles all of the boilerplate for you.

To answer your second question: I agree that, in a dynamic language like JavaScript, making all of these language features implicit could be too costly. I'm more optimistic about effect systems in statically-typed languages.

> ambiently available to a whole tree of function calls

So a "localized global variable" of sorts? Sounds interesting, got any links to examples I could look at?

The term is "dynamic scope": https://en.wikipedia.org/wiki/Scope_(computer_science)#Lexic...

It isn't a very common language feature, but you could take a look at React's `Context` or Common Lisp's `defparameter`.

Ah, like Python objects then. Which is unfortunate, because that's one of my main dislikes with languages like Python.

I was imagining more along the lines of (on-the-spot pseudo-code)

    using [x := foo(...)]
    {
      bar();
      output();
    }

    func bar()
    {
      x = x + 1
    }

where bar() and output(), or something they call in turn, can reference x and it would be a compile-time error to use bar() or output() in cases where they couldn't resolve x. Or something more along those lines.

Anyway, cheers, always fun to ponder these things.

Sorry, the Wikipedia article isn't very helpful. Your example is the correct one.

The killer feature is that you can shadow a dynamic variable, changing its value for the duration of a dynamic scope, in the same way that an inner `catch` handler overrides an outer one.

For example, you could have a dynamic variable which tells the `print` function where to send its text output. The variable's value would probably default to stdout. By temporarily assigning a string-stream to that variable, you could collect an arbitrary function's text output into a string instead. Regardless of where the text is going, any function which produces text could still use the convenient global `print` function.

Right. The closest thing I'm familiar with is the dynamic context used in Serilog[1], where you add log properties to the current scope, which gets added to each log line generated in that scope, and removed once the scope exits.

Of course, no compile-time checking there, so the combination sounds indeed quite useful.

https://github.com/serilog/serilog/wiki/Enrichment#the-logco...

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