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Remove global

GlobalsImplementedThe file on GitHub →

Summary

global leaves the language. A global local, global const, global function, global struct, global type, global namespace, and global attribute each become an error that names the import to write. The side rules that exist only for globals leave with it: --@alloy-side, --@alloy-file-side, the [contexts] table, the shared-context detection, and the mutable_global lint.

Motivation

A global is a name every file sees with no import. Luau has modules for that, and Roblox warns against _G for the same reasons this proposal gives.

The cost showed up as bugs. In one week the feature drew seven: a global local that each file copied at require time, so a write in one file reached no other; an inferred type that did not cross files; a const member of a namespace that any file could assign; a global in a typeof slot that the rewrite missed; emit temps _g1 and _gs in the completion list; a path comparison that reported a global as its own duplicate; and the side rules interacting with each of these. Every feature that reads names across files paid a tax for globals, and every future one would too.

What a global buys is one line. The emit already lowers a global to a module every file requires, so global local counter = 0 in a.aly and counter in b.aly is import { counter } from "./a" with the line left out. Auto-import writes that line.

The user's own read, on the day of the decision: modules exist for this; _G is warned against; a global is to a module what goto is to a loop.

Design

What a user writes

Nothing. The forms that remain cover every use:

-- a.aly
export local counter = 0
export const LIMIT = 5
export function bump() counter += 1 end

-- b.aly
import { counter, LIMIT, bump } from "./a"

A value that several files share is an export local in one module, read and written through the functions that module exports. A bare imported name is a copy taken at require time, as in Luau, so a write to it in another file reaches no one; bump() and read() above are the shape that shares. The global local emit rewrote every use to the module's slot; nothing replaces that, and the doc for global says so.

The error

A global declaration reports at the keyword:

error(3.x): ImportError: `global` is removed; declare `counter` with
`export local` and write `import { counter } from "./a"` where it is
read

A use of a name that was global in another file reports as an unknown name, which it is, with the auto-import action that already exists for any unresolved name.

The code action

On the declaration: "replace global with export" rewrites the one word. On a use in another file: the existing auto-import action writes the import line. Together they migrate a project in two clicks per name.

What leaves

What the emit does

Nothing new. A file with no global compiles as it does today. The line count holds because no line changes.

The language server

An import completes, hovers, and navigates as it does today. A global keyword completes to nothing and hovers as the removal notice with the two forms that replace it.

Drawbacks

A project that uses global stops compiling until it is migrated. The error names the fix and the code action applies it, and the feature is in a release candidate, so the cost lands on few files.

A cross-cutting constant now needs one import per file that reads it. Auto-import writes it, and the line says where the value lives, which the global form hid.

Alternatives

Keep `global` as sugar for an implicit import. That is what it is now, and it still needs its own resolution path in the compiler and the server, which is where the bugs were.

Keep `global const` and `global type`, drop `global local`. The mutable form drew the worst bugs, but the resolution tax is the same for every kind, and a rule with an exception is harder to teach than none.

Keep the side rules for imports. --@alloy-side and [contexts] were written for globals. An import already says which module it reads, so a side rule on top says nothing the path does not.

Unresolved questions