build a parser for JSON, the JavaScript Object Notation. Input Input will be pro

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build a parser for JSON, the JavaScript Object Notation. Input Input will be provided in the form of a file containing a single JSON object or array. The syntax for both structures is defined on the front page of http://www.json.org/ using railroad diagrams. Processing Build a recursive-descent parser for JSON. While the specification includes syntax diagrams for tokens, you may wish to build a separate scanner rather than creating procedures for string, number, true, false, and null. As you parse the JSON object, construct an Abstract Syntax Tree using the following definitions: Abstract Syntax Tree ast.h #ifndef AST_H_ #define AST_H_ class JsonValue { public: virtual void Print() = 0; }; class JsonObject : public JsonValue { private: map pairs; public: virtual void Print(); void Add(string name, JsonValue* value); }; class JsonArray : public JsonValue { private: list values; public: virtual void Print(); void Add(JsonValue *v); }; class JsonString : public JsonValue { private: string value; public: JsonString(string s) : value(s) { }; virtual void Print(); }; class JsonNumber : public JsonValue { private: double value; public: JsonNumber(double d) : value(d) { }; virtual void Print(); }; class JsonBoolean : public JsonValue { private: bool value; public: JsonBoolean(bool b) : value(b) { }; virtual void Print(); }; class JsonNull : public JsonValue { public: virtual void Print(); }; #endif // AST_H_ Usage An AST for the following JSON object: { "str": "foo", "num": 123, "bool": true, "nul": null, "arr": [ "bar", 456, false ] } might be constructed with the following C++ code: JsonObject* o = new JsonObject(); o->Add("str", new JsonString("foo")); o->Add("num", new JsonNumber(123)); o->Add("bool", new JsonBoolean(true)); o->Add("nul", new JsonNull()); JsonArray *a = new JsonArray(); a->Add(new JsonString("bar")); a->Add(new JsonNumber(456)); a->Add(new JsonBoolean(false)); o->Add("arr", a); Output If the file is parsed successfully, call the Print() method to traverse the AST and print the structure of the the tree as a Scheme list with one or more of the following elements: ( string value ) ( number value ) ( boolean value ) ( null ) ( array {value} ) ( object {name value} ) As in the footnote on p. 49 of the textbook, curly braces ({ }) are used to indicate zero or more instances of the values inside. Calling o->Print() on the object defined above should produce the following output: ( object arr ( array ( string bar ) ( number 456 ) ( boolean false ) ) bool ( boolean true ) nul ( null ) num ( number 123 ) str ( string foo ) ) Note that the order of the object’s name/value pairs has changed since the C++ std::map type sorts its keys. If the file is not parsed successfully, output the line number and a clear error message describing what has gone wrong.

Explanation / Answer

from pyrser import grammar, meta from pyrser.directives import ignore class JSON(grammar.Grammar): """Pyrser JSON parser""" entry = "json" grammar = """ json =[ object:>_ eof ] object = [ '{' #is_dict(_) [pair:p #add_kv(_, p) [',' pair:p #add_kv(_, p) ]*]? '}' ] pair = [ string:s ':' value:v #is_pair(_, s, v) ] value = [ [number | object | array]:>_ | [ string:s #is_str(_, s) | "true":t #is_bool(_, t) | "false":f #is_bool(_, f) | "null" #is_none(_) ] ] array = [ '[' #is_array(_) [value:v #add_item(_, v) [',' value:v #add_item(_, v)] *]? ']' ] number = [ @ignore("null") [int frac? exp?]:n #is_num(_, n) ] int = [ '-'? [ digit1_9s | digit ] ] frac = [ '.' digits ] exp = [ e digits ] digit = [ '0'..'9' ] digit1_9 = [ '1'..'9' ] digits = [ digit+ ] digit1_9s = [ digit1_9 digits] e = [ ['e'|'E'] ['+'|'-']? ] """ @meta.hook(JSON) def is_num(self, ast, n): ast.node = float(self.value(n)) return True @meta.hook(JSON) def is_str(self, ast, s): ast.node = self.value(s).strip('"') return True @meta.hook(JSON) def is_bool(self, ast, b): bval = self.value(b) if bval == "true": ast.node = True if bval == "false": ast.node = False return True @meta.hook(JSON) def is_none(self, ast): ast.node = None return True @meta.hook(JSON) def is_pair(self, ast, s, v): ast.node = (self.value(s).strip('"'), v.node) return True @meta.hook(JSON) def is_array(self, ast): ast.node = [] return True @meta.hook(JSON) def add_item(self, ast, item): ast.node.append(item.node) return True @meta.hook(JSON) def is_dict(self, ast): ast.node = {} return True @meta.hook(JSON) def add_kv(self, ast, item): ast.node[item.node[0]] = item.node[1] return True

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