nix-in-node/src/transformers/attribute-sets.js

"use strict";

const assert = require("assert");
const types = require("@babel/types");
const template = require("@babel/template").default;
const unpackExpression = require("./_unpack-expression");
const NoChange = require("../astformer/actions/no-change");
const printAst = require("../print-ast");

// FIXME: Add expression parens!
let tmplCallLazy = template(`
	%%wrapper%%()
`);

let tmplLazyWrapper = template(`(
	$$jsNix$memoize(() => %%expression%%)
)`);

let tmplLazyWrapperRecursive = template(`(
	function() { return %%expression%%; }
)`);

let tmplExtend = template(`(
	$$jsNix$extend(this ?? {}, %%object%%)
)`);

let tmplScopeWrapper = template(`(
	(() => {
		%%bindings%%;

		return %%object%%;
	})()
)`);

// FIXME: Verify that this always works, and that we don't need `var` for hoisting!
let tmplRecursiveBinding = template(`
	const %%name%% = %%expression%%;
`);

function lazyEvaluationWrapper(args) {
	let { recursive, ... rest } = args;

	// printAst(rest.expression);
	let wrapper = (recursive)
		? tmplLazyWrapperRecursive
		: tmplLazyWrapper;

	return unpackExpression(wrapper(rest));
}


function isDynamicBinding(binding) {
	return binding.attrpath.attr[0].type !== "NixAttributeIdentifier";
}

module.exports = {
	name: "attribute-sets",
	visitors: {
		Program: (_node, { setContext }) => {
			setContext(null, "attributeSets_inRecursiveSet", false);
			return NoChange;
		},
		NixAttributeSelection: (_node, { defer }) => {
			// TODO: Optimize dynamic attribute access by translating builtins.getAttr to dynamic property access at compile time? This requires scope analysis!
			return defer((node) => {
				assert(node.attrpath.type === "NixAttributePath");

				return node.attrpath.attr.reduce((last, identifier) => {
					assert(identifier.type === "NixAttributeIdentifier");

					return unpackExpression(tmplCallLazy({
						wrapper: types.memberExpression(last, types.identifier(identifier.name))
					}));
				}, node.expression);
			});
		},
		NixAttributeSet: (node, { defer, setContext, getContext }) => {
			if (node.recursive) {
				setContext(null, "attributeSets_inRecursiveSet", true);
			}

			return defer((node) => {
				let isRecursive = node.recursive;
				let hasDynamicBindings = node.bind.some((binding) => isDynamicBinding(binding));

				let bindings = node.bind.map((binding) => {
					assert(binding.attrpath.attr.length === 1); // Nested attributes should have been desugared by this point

					return {
						name: binding.attrpath.attr[0].name,
						expression: lazyEvaluationWrapper({ recursive: isRecursive, expression: binding.expression })
					};
				});

				if (hasDynamicBindings) {
					throw new Error(`UNIMPLEMENTED: Dynamic bindings are not supported yet`); 
				} else if (isRecursive) {
					return unpackExpression(tmplScopeWrapper({
						bindings: bindings.map(({ name, expression }) => {
							return tmplRecursiveBinding({
								name: name,
								expression: expression
							});
						}),
						object: types.objectExpression(bindings.map(({ name }) => {
							return types.objectProperty(
								types.stringLiteral(name),
								types.identifier(name)
							);
						}))
					}));
				} else {
					let object = types.objectExpression(bindings.map(({ name, expression }) => {
						return types.objectProperty(
							types.stringLiteral(name),
							// TODO: Remove the isRecursive distinction here once heap-of-vars works
							expression
						);
					}));

					// Only do prototypal inheritance for `this` if we're dealing with nested recursive attribute sets
					if (isRecursive && getContext("attributeSets_inRecursiveSet") === true) {
						return unpackExpression(tmplExtend({
							object: object
						}));
					} else {
						return object;
					}
				}
			});
		},
	}
};