alt Hacker News> React has "Context", SwiftUI has "@Environment", Emacs LISP has dynamic scope (so I heard). C# has AsyncLocal, Node.JS AsyncLocalStorage.
Emacs Lisp retains dynamic scope, but it's no longer a default for some time now, in line in other Lisps that remain in use. Dynamic scope is one of the greatest features in Lisp language family, and it's sad to see it's missing almost everywhere else - where, as you noted, it's being reinvented, but poorly, because it's not a first-class language feature.
On that note, the most common case of dynamic scope that almost everyone is familiar with, are environment variables. That's what they're for. Since most devs these days are not familiar with the idea of dynamic scope, this leads to a lot of peculiar practices and footguns the industry has around environment variables, that all stem from misunderstanding what they are for.
> This is one of those ideas that at first seem really wrong (isn't it just a global variable in disguise?)
It's not. It's about scoping a value to the call stack. Correctly used, rebinding a value to a dynamic variable should only be visible to the block doing the rebinding, and everything below it on the call stack at runtime.
> Implicit context (properly integrated into the type system) is something I would consider in any new language.
That's the problem I believe is currently unsolved, and possibly unsolvable in the overall programming paradigm we work under. One of the main practical benefits of dynamic scope is that place X can set up some value for place Z down on the call stack, while keeping everything in between X and Z oblivious of this fact. Now, this is trivial in dynamically typed language, but it goes against the principles behind statically-typed languages, which all hate implicit things.
(FWIW, I love types, but I also hate having to be explicit about irrelevant things. Since whether something is relevant or not isn't just a property of code, but also a property of a specific programmer at specific time and place, we're in a bit of a pickle. A shorter name for "stuff that's relevant or not depending on what you're doing at the moment" is cross-cutting concerns, and we still suck at managing them.)
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> against the principles behind statically-typed languages, which all hate implicit things
But many statically typed languages allow throwing exceptions of any type. Contexts can be similar: "try catch" becomes "with value", "throw" becomes "get".
raku's take on gradual typing may be to your taste; i likewise prefer to leave irrelevant types out and use maximally-expressive types where it makes sense¹. i feel this is helped by the insistence on sigils because you then know the rough shape of things (and thus a minimal interface they implement: $scalar, @positional, %associative, &callable) even when you lack their specific types. in the same vein, dynamically scoped variables are indicated with the asterisk as a twigil (second level sigil).
@foo
@*foo
it's idiomatic to see
$*db
%*ENV
i would prefer to kick more of the complex things i do with types back to compile time, but a lot of static checks are there. more to the point, raku's type system is quite expressive at runtime (that's what you get when you copy common lisp's homework, after all) and helpful to move orthogonal concerns out into discrete manageable things that feel like types to use even if what they're doing is just a runtime branch that lives in the function signature. doing stuff via subset types or roles or coercion types means whatever you do plays nicely with polymorphic dispatch, method resolution order, pattern matching, what have you.
in fact, i just wrote a little entirely type level... thing? to clean up the body of an http handler that lifts everything into a role mix-in pipeline that runs from the database straight on through to live reloading of client-side elements. processing sensor readings for textual display, generating html, customizing where and when the client fetches the next live update, it's all just the same pipeline applying roles to the raw values from the db with the same infix operator (which just wraps a builtin non-associative operator to be left associative to free myself from all the parentheses).
not getting bogged down in managing types all the time frees you up to do things like this when it's most impactful, or at least that's what i tell myself whenever i step on a rake i should have remembered was there.
¹ or times where raku bubbles types up to the end-user, like the autogenerated help messages generated from the type signature of MAIN. i often write "useless" type declarations such as subset Domain-or-IP; which match anything² so that the help message says --host[=Domain-or-IP] instead of --host[=Str] or whatever
² well, except junctions, which i consider the current implementation of to be somewhat of a misstep since they're not fundamentally also a list plus a context. it's a whole thing. in any case, this acts at the level of the type hierarchy that you want anyway.
> Emacs Lisp retains dynamic scope, but it's no longer a default for some time now
https://www.gnu.org/software/emacs/manual/html_node/elisp/Va...
> By default, the local bindings that Emacs creates are dynamic bindings. Such a binding has dynamic scope, meaning that any part of the program can potentially access the variable binding. It also has dynamic extent, meaning that the binding lasts only while the binding construct (such as the body of a let form) is being executed.
It’s also not really germane to the GP’s comment, as they’re just talking about dynamic scoping being available, which it will almost certainly always be (because it’s useful).