# Example of Scope Analysis in Egg
This code was developed by David Afonso Dorta.
See https://github.com/ULL-ESIT-PL-1920/TFA-davafons (opens new window) and https://github.com/ULL-ESIT-PL-1920/TFA-davafons (opens new window)
const estraverse = require("estraverse");
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const { TopEnv, SpecialForms } = require("../interp/environment.js");
const { Apply, Word } = require("../interp/ast.js");
const SCOPE_OPERATORS = ["do", "if", "while", "for", "foreach", "fun", "->", "object"];
const DEFINE_OPERATORS = [":=", "define", "def"];
const SET_OPERATORS = ["set", "<-"];
const FUNCTION_OPERATORS = ["fun", "->"];
const OBJECT_OPERATORS = ["object", "obj"];
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class Semantic {
constructor(symboltable) {
if (symboltable === undefined) {
this.symboltable = Object.create(null);
} else {
this.symboltable = symboltable;
}
}
check(tree) {
const methods = this;
tree = estraverse.replace(tree, {
enter: function(node, parent) {
if (node.type === "apply") {
node = methods.checkApplyEnter(node);
} else if (node.type === "word") {
node = methods.checkWordEnter(node, parent);
}
return node;
},
leave: function(node) {
if (node.type === "apply") {
node = methods.checkApplyLeave(node);
}
return node;
},
keys: {
apply: ["args"],
word: [],
value: [],
regex: []
},
fallback: "iteration"
});
return tree;
}
static check(tree) {
return new Semantic().check(tree);
}
checkApplyEnter(node) {
if (node.operator.type == "word") {
const operator_name = node.operator.name;
// Assert that the arguments passed to the apply are correct
// this.assertApplyArgs(node);
// If entered on a new scope, create a child symboltable
if (SCOPE_OPERATORS.includes(operator_name)) {
this.symboltable = Object.create(this.symboltable);
}
// If the operator is a "define" operator, add the new symbol to the symboltable
if (DEFINE_OPERATORS.includes(operator_name)) {
node.args[0] = this.addToSymboltable(node.args[0]);
}
if (SET_OPERATORS.includes(operator_name)) {
const symbol = this.findInSymboltable(node.args[0].name);
if (symbol && symbol.const) {
throw new TypeError(`${node.args[0].name} is const and can't be reassigned!`);
}
}
// Add function arguments to function symboltable
if (FUNCTION_OPERATORS.includes(operator_name)) {
for (let i = 0; i < node.args.length - 1; ++i) {
this.addToSymboltable(node.args[i]);
}
}
// Add "this" symbol, and object properties, to object symboltable
if (OBJECT_OPERATORS.includes(operator_name)) {
const apply = new Apply(new Word({ value: "const" }));
apply.args = [new Word({ value: "this" })];
this.addToSymboltable(apply);
for (let i = 0; i < node.args.length; i += 2) {
this.addToSymboltable(new Word({ value: node.args[i].value }));
}
}
// Add the "iterator" symbol to foreach symboltable
if (operator_name === "foreach") {
this.addToSymboltable(node.args[0]);
}
}
return node;
}
assertApplyArgs(node) {
/* ... */
}
addToSymboltable(node) {
/* ... */
}
findInSymboltable(name) {
// Find an element on the symboltable or the parent ones
for (let table = this.symboltable; table; table = Object.getPrototypeOf(table)) {
if (Object.prototype.hasOwnProperty.call(table, name)) {
return table[name];
}
}
return undefined;
}
checkWordEnter(node, parent) {
/* ... */
}
checkApplyLeave(node) {
if (node.operator.type == "word") {
const operator_name = node.operator.name;
// If left the scope, remove the last symboltable
if (SCOPE_OPERATORS.includes(operator_name)) {
this.symboltable = Object.getPrototypeOf(this.symboltable);
}
}
return node;
}
}
module.exports = {
Semantic
};
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See also davafons TFA