benjamnre·i·fy verb, transitive 
re·i·fied past re·i·fies present re·i·fy·ing participle re·i·fi·ca·tion noun re·i·fi·er noun
- to make (something abstract) more concrete or real
"these instincts are, in humans, reified as verbal constructs" - to regard or treat (an idea, concept, etc.) as if having material existence
- to enable ECMAScript 2015 modules in any version of Node.js
Usage
- Run
npm install --save reifyin your package or app directory. The--saveis important because reification only applies to modules in packages that explicitly depend on thereifypackage. - Call
require("reify")before importing modules that containimportandexportdeclarations.
You can also easily reify the Node REPL:
% node
> require("reify")
{}
> import { strictEqual } from "assert"
> strictEqual(2 + 2, 5)
AssertionError: 4 === 5
at repl:1:1
at REPLServer.defaultEval (repl.js:272:27)
...How it works
Code generated by the reify compiler relies on a simple runtime
API that can be explained through a series of
examples. While you do not have to write this API by hand, it is designed
to be easily human readable and writable, in part because that makes it
easier to explain.
I will explain the Module.prototype.link method first, then the
Module.prototype.export method after that. Note that this Module is
the constructor of the CommonJS module object, and the import and
export methods are custom additions to Module.prototype.
module.link(id, setters)
Here we go:
import a, { b, c as d } from "./module";becomes
// Local symbols are declared as ordinary variables.
let a, b, d;
module.link("./module", {
// The keys of this object literal are the names of exported symbols.
// The values are setter functions that take new values and update the
// local variables.
default(value) { a = value; },
b(value) { b = value; },
c(value) { d = value; },
});All setter functions are called synchronously before module.link returns,
with whatever values are immediately available. However, when there are
import cycles, some setter functions may be called again, when the exported
values change. Calling these setter functions one or more times is the key
to implementing live bindings,
as required by the ECMAScript 2015 specification.
Importing a namespace object is no different from importing a named
export. The name is simply "*" instead of a legal identifier:
import * as utils from "./utils";becomes
let utils;
module.link("./utils", {
"*"(ns) { utils = ns; }
});Note that the ns object exposed here is !== require("./utils"), but
instead a normalized view of the require("./utils") object. This
approach ensures that the actual exports object is never exposed to the
caller of module.link.
Notice that this compilation strategy works equally well no matter where
the import declaration appears:
if (condition) {
import { a as b } from "./c";
console.log(b);
}becomes
if (condition) {
let b;
module.link("./c", {
a(value) { b = value; }
});
console.log(b);
}See WHY_NEST_IMPORTS.md for a much more detailed
discussion of why nested import declarations are worthwhile.
module.export(getters)
What about export declarations? One option would be to transform them into
CommonJS code that updates the exports object, since interoperability
with Node and CommonJS is certainly a goal of this approach.
However, if Module.prototype.link takes an id string and a map of
setter functions, then it seems natural for Module.prototype.export to
be method that registers getter functions. Given these getter functions,
whenever module.link(id, ...) is called by a parent module, the getters
for the id module will run, updating its module.exports object, so
that the module.link method has access to the latest exported values.
The module.export method is called with a single object literal whose
keys are exported symbol names and whose values are getter functions for
those exported symbols. So, for example,
export const a = "a", b = "b", ...;becomes
module.export({
a: () => a,
b: () => b,
...
});
const a = "a", b = "b", ...;This code registers getter functions for the variables a, b, ..., so
that module.link can easily retrieve the latest values of those
variables at any time. It's important that we register getter functions
rather than storing computed values, so that other modules always can
import the newest values.
Export remapping works, too:
let c = 123;
export { c as see }becomes
module.export({ see: () => c });
let c = 123;Note that the module.export call is "hoisted" to the top of the block
where it appears. This is safe because the getter functions work equally
well anywhere in the scope where the exported variable is declared, and
a good idea because the hoisting ensures the getters are registered as
early as possible.
What about export default <expression> declarations? It would be a
mistake to defer evaluation of the default expression until later, so
wrapping it in a hoisted getter function is not exactly what we want.
Instead,
export default computeDefault();gets replaced where it is (without any hoisting) by
module.exportDefault(computeDefault());The module.exportDefault method is just a convenient
wrapper
around module.export:
module.exportDefault = function (value) {
return this.export({
default: function () {
return value;
}
}, true);
};That true argument we're passing to module.export is a hint that the
value returned by this getter function will never change, which enables
some optimizations behind
the scenes.
module.runSetters()
Now, suppose you change the value of an exported local variable after the
module has finished loading. Then you need to let the module system know
about the update, and that's where module.runSetters comes in. The
module system calls this method on your behalf whenever a module finishes
loading, but you can also call it manually, or simply let reify generate
code that calls module.runSetters for you whenever you assign to an
exported local variable.
Calling module.runSetters() with no arguments causes any setters that
depend on the current module to be rerun, but only if the value a setter
would receive is different from the last value passed to the setter.
If you pass an argument to module.runSetters, the value of that argument
will be returned as-is, so that you can easily wrap assignment expressions
with calls to module.runSetters:
export let value = 0;
export function increment(by) {
return value += by;
};should become
module.export({
value: () => value,
increment: () => increment,
});
let value = 0;
function increment(by) {
return module.runSetters(value += by);
};Note that module.runSetters(argument) does not actually use argument.
However, by having module.runSetters(argument) return argument
unmodified, we can run setters immediately after the assignment without
interfering with evaluation of the larger expression.
Because module.runSetters runs any setters that have new values, it's
also useful for potentially risky expressions that are difficult to
analyze statically:
export let value = 0;
function runCommand(command) {
// This picks up any new values of any exported local variables that may
// have been modified by eval.
return module.runSetters(eval(command));
}
runCommand("value = 1234");exports that are really imports
What about export ... from "./module" declarations? The key insight here
is that export declarations with a from "..." clause are really just
import declarations that update the exports object instead of updating
local variables:
export { a, b as c } from "./module";becomes
module.link("./module", {
a(value) { exports.a = value; },
b(value) { exports.c = value; },
});Since this pattern is so common, and no local variables need to be modified by these setter functions, the runtime API supports an alternative shorthand for re-exporting values:
module.link("./module", { a: "a", b: "c" });This strategy cleanly generalizes to export * from "..." declarations:
export * from "./module";becomes
module.link("./module", {
"*"(ns) {
Object.assign(exports, ns);
}
});Though the basic principle is the same, in reality the Reify compiler generates shorthand notation for this pattern as well:
module.link("./module", { "*": "*" });This version is shorter, does not rely on Object.assign (or a polyfill),
can be a little smarter about copying special properties such as getters,
and reliably modifies module.exports instead of the exports variable
(whatever it may be). Win!
Exporting named namespaces (proposal):
export * as ns from "./module";becomes
module.link("./module", {
"*"(ns) { exports.ns = ns; }
});Shorthand:
module.link("./module", { "*": "ns" });Re-exporting default exports (proposal):
export a, { b, c as d } from "./module";becomes
module.link("./module", {
default(value) { exports.a = value; },
b(value) { exports.b = value; },
c(value) { exports.d = value; }
});Shorthand:
module.link("./module", {
default: "a",
b: "b",
c: "d"
});While these examples have not covered every possible syntax for import
and export declarations, I hope they provide the intuition necessary to
imagine how any declaration could be compiled.
When I have some time, I hope to implement a live-compiling text editor to enable experimentation.