"use strict";

const unreachable = require("@joepie91/unreachable");
const assert = require("assert");
const splitFilter = require("split-filter");
const _nixTypes = require("./util/nix-types");

function isValidBodyAttribute(binding) {
	assert(binding.attrpath.type === "NixAttributePath");
	assert(binding.attrpath.attr.length > 0);
	assert(binding.attrpath.attr[0].type === "NixAttributeIdentifier");

	if (binding.attrpath.attr[0].name === "body") {
		if (binding.attrpath.attr.length > 1) {
			throw unreachable("attribute paths should have been desugared");
		} else {
			return true;
		}
	} else {
		return false;
	}
}

module.exports = {
	name: "desugar-let-attribute-set",
	visitors: {
		NixLetAttributeSet: (node) => {
			// We save a bunch of complexity here by directly translating to a recursive attribute set instead of `let..in`; our JS representation of `let..in` is *functionally* identical to how a LetAttributeSet works. We just use a different attribute name to represent the returned evaluation.
			
			let [ bodyBindings, actualBindings ] = splitFilter(node.bind, (binding) => isValidBodyAttribute(binding));

			if (bodyBindings.length === 0) {
				// TODO: Display source code position + snippet
				// FIXME: It's possible to specify the `body` with a dynamic key; this means that we can't actually do a compile-time check here, at least not reliably, and we need to insert a runtime guard if the compile-time check fails and dynamic attributes are specified
				throw new Error(`Missing required 'body' attribute in LetAttributeSet`);
			} else {
				return _nixTypes.JSNixLet(
					actualBindings,
					bodyBindings[0].expression
				);
			}
		}
	}
};